Generator feed to 2L+1N system

[kevindd992002]

So if my neutral-earth bonding happens on the service panel (house side of c/o switch), where do I need to install an RCD/GFCI?

 

[Simon47]

OK, from your description, I think this is what you have before adding any changeover switch etc :

And after a bit of thought, this is what I think you need :

For your changeover switch, note that the N-E bonding is done BEFORE the switch - that avoids having a switch in your earth connection which (apart from some very specific situations) is expressly prohibited in our UK regs. Just to be clear, your earthing must not be reliant on any switch working properly.

And while you correctly point out that your two boards have their earth terminals connected together via the neutrals, I would explicitly bond them with an earth wire. Actually, I'd consider splitting the earth off the neutral at your meter (so L1, N, L2, and E from meter to each board) and avoid having the shared PEN (protective earth and neutral) internally - again it's something that's expressly prohibited in our regs. The issue is what if someone comes along and starts working on the system, on the assumption that (having pulled the supplier fuses) it's OK to disconnect the neutral. You now have an installation where part of it is supplied by a generator, and the earthing is split across two separate earth electrodes. Under fault conditions, that disconnected neutral could now carry a hazardous voltage relative to other neutral/earth conductors/terminals.
This is either-or - you don't need to run a separate earth wire from the meter location to each board AND also run a dedicated bonding wire between them. Either way, you'd have a solid single earth reference for the whole installation regardless of what's being done to the wiring. Also, this can be done just with conections to the earth bars in each board - you don't need to dig anything out to access the earth electrodes.


Now the fault protection.

As you can see, the neutral from the auto-transformer is connected to the earth bar. This means that (within the ability of the genny and transformer), an L1-E or L2-E fault within the installation will trip it's respective breaker. But the genny and/or transformer may not have the "oomph" to trip anything more than a smaller breaker so you could overload one or the other for an extended time. An RCD (GFI) would trip on the imbalance and disconnect the whole board.
With the assumption that the installation is otherwise adequately protected, this RCD is only to protect the equipment - so it can be of a larger trip current and time delayed (time delayed being the most important part) to give you some discrimination between that and what you have in the distribution board. E.g., if you already have an RCD/GFI on a circuit with a fault, then the one in the board would trip and disconnect the fault, while the time delay on the supply from the transformer will mean that it won't trip as well. But if you have a fault on a non-RCD/GFI protected circuit, then the RCD/GFI in your transformer supply will disconnect the whole system - inconvenient, but better than burning out your equipment.

You may have noticed an additional breaker on the output of the genny. This needs to be a true 2-pole (i.e. senses in both poles) breaker to protect the genny.
The genny only has a single pole breaker in one line, if there is an internal fault to earth, then a large current could flow with nothing to trip - and again there's a risk of burning out the genny or transformer. Consider fault between the genny winding on the side with the internal breaker and earth - then draw the path the fault current would take and you'll see what I mean.
Selecting this breaker could be tricky. Too low a tripping current and you'll get nuisance tripes, too high and the genny won't be able to drive enough current through it to trip. Another RCD/GFI might be more appropriate.

I have to say that this is somewhat outside of my comfort zone. It does really need someone familiar with your local practices and available equipment to do the detail design work - wire sizes, breaker ratings, etc. Hopefully I've explained the "why" of each part and you can see how the pieces fit together.

 

[kevindd992002]

@Simon47

OK, from your description, I think this is what you have before adding any changeover switch etc :
View attachment 63362

Yes, without a changeover switch, this is how it is setup. The N and G are tied in the main panel of both buildings.

And after a bit of thought, this is what I think you need :
View attachment 63363

For your changeover switch, note that the N-E bonding is done BEFORE the switch - that avoids having a switch in your earth connection which (apart from some very specific situations) is expressly prohibited in our UK regs. Just to be clear, your earthing must not be reliant on any switch working properly.

For the most part, I understand but I thought that (at least for the US and the Philippines) the standard is to bond N-E at the main panel alone? The main panel (distribution board as you call it) is AFTER the changeover switch. You also mentioned earlier to bond both B1 and B2 earth rods through the main panel ground bus bars. How does the grounding/earthing be reliant on the switch if N-E are bonded on the main panel?

This is what I meant:

https://www.electriciansforums.net/attachments/gen-changeover-jpg.61804/

@pc1966 do you have any comments on this BEFORE or AFTER changeover switch N-E bonding?

And while you correctly point out that your two boards have their earth terminals connected together via the neutrals, I would explicitly bond them with an earth wire. Actually, I'd consider splitting the earth off the neutral at your meter (so L1, N, L2, and E from meter to each board) and avoid having the shared PEN (protective earth and neutral) internally - again it's something that's expressly prohibited in our regs. The issue is what if someone comes along and starts working on the system, on the assumption that (having pulled the supplier fuses) it's OK to disconnect the neutral. You now have an installation where part of it is supplied by a generator, and the earthing is split across two separate earth electrodes. Under fault conditions, that disconnected neutral could now carry a hazardous voltage relative to other neutral/earth conductors/terminals.
This is either-or - you don't need to run a separate earth wire from the meter location to each board AND also run a dedicated bonding wire between them. Either way, you'd have a solid single earth reference for the whole installation regardless of what's being done to the wiring. Also, this can be done just with conections to the earth bars in each board - you don't need to dig anything out to access the earth electrodes.

Understood and I think this is what's happening to my installation now because the generator does not have a neutral. So when the B1 main panel is switched over to gen power, the B1 socket connections are:

220V -> L1 and L2 from genny, E from B1 ground rods and bonded to supplier N
110V -> L1 or L2 from genny, E from B1 ground rods and N from supplier (E and N bonded)

Now the fault protection.

As you can see, the neutral from the auto-transformer is connected to the earth bar. This means that (within the ability of the genny and transformer), an L1-E or L2-E fault within the installation will trip it's respective breaker. But the genny and/or transformer may not have the "oomph" to trip anything more than a smaller breaker so you could overload one or the other for an extended time. An RCD (GFI) would trip on the imbalance and disconnect the whole board.
With the assumption that the installation is otherwise adequately protected, this RCD is only to protect the equipment - so it can be of a larger trip current and time delayed (time delayed being the most important part) to give you some discrimination between that and what you have in the distribution board. E.g., if you already have an RCD/GFI on a circuit with a fault, then the one in the board would trip and disconnect the fault, while the time delay on the supply from the transformer will mean that it won't trip as well. But if you have a fault on a non-RCD/GFI protected circuit, then the RCD/GFI in your transformer supply will disconnect the whole system - inconvenient, but better than burning out your equipment.

What kind of RCD should I be looking at and should it have a larger trip current than the changeover switch breakers? I thought RCD's should be very fast acting? With a time-delayed RCD, are those just used to protect equipments? If we're talking about RCD's in subpanels, these should be very fast in case a person touches a live wire, isn't it?

Also, how high of generator changover switch breaker should I be looking for my current genny?

You may have noticed an additional breaker on the output of the genny. This needs to be a true 2-pole (i.e. senses in both poles) breaker to protect the genny.
The genny only has a single pole breaker in one line, if there is an internal fault to earth, then a large current could flow with nothing to trip - and again there's a risk of burning out the genny or transformer. Consider fault between the genny winding on the side with the internal breaker and earth - then draw the path the fault current would take and you'll see what I mean.
Selecting this breaker could be tricky. Too low a tripping current and you'll get nuisance tripes, too high and the genny won't be able to drive enough current through it to trip. Another RCD/GFI might be more appropriate.

So is it going to be like this?

Genny outputput -> RCD -> breaker -> changover switch breaker

This part I'm confused.

 

[pc1966]

@pc1966 do you have any comments on this BEFORE or AFTER changeover switch N-E

It seems this thread has run and run!

The critical point is you should NEVER be without a ground connection no matter what the switch is doing (even if stuck half-way).

Typically here we separate N & E from the incoming supply before you do anything, so in that sense the bond is before a changeover switch, but then we don't haver the link in the DB panel as you do which would normally be after a change-over switch.

 

[pc1966]

To some extent if you are linking N-E before the panel you might as well have only L1 & L2 switched as N & E are always common, and that would also be the auto-transformer centre tap (after any protection).

I see a 3-pole MCB appears above, it could be for the 100% load with the transformer rated at 50% as then the tap is good to that current anyway. So a 5kVA transformer on a 10kVA generator and a 40A or 45A 3-pole MCB is a sane choice, etc.

 

[kevindd992002]

It seems this thread has run and run!

The critical point is you should NEVER be without a ground connection no matter what the switch is doing (even if stuck half-way).

Typically here we separate N & E from the incoming supply before you do anything, so in that sense the bond is before a changeover switch, but then we don't haver the link in the DB panel as you do which would normally be after a change-over switch.

I apologize if the thread has run longer as it should.

As for the critical point, that is what I thought. I guess it's just really a matter of country specification like you and simon have mentioned. With N and E bonded at the DB panel (after the changeover switch), then I satisfy the "never be without a ground connection" criterion as it is never part of the switching mechanism.

To some extent if you are linking N-E before the panel you might as well have only L1 & L2 switched as N & E are always common, and that would also be the auto-transformer centre tap (after any protection).

I see a 3-pole MCB appears above, it could be for the 100% load with the transformer rated at 50% as then the tap is good to that current anyway. So a 5kVA transformer on a 10kVA generator and a 40A or 45A 3-pole MCB is a sane choice, etc.

I see what you're saying. Now circling back to what you mentioned in the beginning of this thread, I thought that we should NEVER let the neutral of the generator touch the supply neutral? If N-E are linked before the panel, then both neutrals are linked through the ground connection. Wouldn't that have the same undesirable effect?

So 45A for the 3-pole MCB for the transformer/generator combo, ok. Can I keep the 200A MCB that I currently use for the supply coming from the meter?

How about for the RCD, what rating should I be looking at? And with N-E linked on the DB panel on my setup, I'm assuming the RCD should also be installed on the DB panel itself possibly replacing the main MCB on that same panel?

 

[Simon47]

So, I meant to get back to this earlier ?

I'd agree with pc1966 - don't switch the neutral at all, use a 2 pole changeover switch.

It's not really a problem having the genny share the neutral with the supply - the supply is after all grounded and means you are sharing the ground. The genny is fully floating (best check that actually !) and you were going to earth the centre tap anyway.

As to breaker ratings, TBH it's too far outside my comfort zone to determine what's suitable - at least, not without spending time with my head in the books. I was hoping someone else would chime in - there are people on here who deal with gennys and portable setups as part of their normal job. Nothing like practical experience of what these things actually behave like. I have a 3kVA genny, and I know that it really really can't handle overloads - even transitory ones like trying to start a motor.

The RCD can be omitted IFF you can guarantee that under fault conditions either the load breaker will trip OR the genny breaker will trip. The problem there is that small generators are notorious for having poor overload capacity - so probably just isn't capable of producing enough current to trip even a moderately sized breaker on it's fast magnetic trip. For example, you have an 8kVA genny, so that's nominally 36A full load. A curve B MCB needs between 3 and 5 times it's rated current for the magnetic trip to operate - so a (say) 20A MCB will need between 60 and 100A. The genny won't produce that, so a fault on a 20A circuit might simply overload the genny until something else trips - while possibly exposing a user to an unsafe voltage on an "earthed" item. Even a 6A MCB (commonly used for lighting circuits here) needs between 18 and 30A (so assume it needs at least 30A) - and if there are a few amps of other load, again the genny might not be able to trip it.
So I'd say the RCD is probably essential for safety of people and animals. The only downside to fitting it is the possibility of nuisance tripping of the whole supply if "everything combined" has a bit of leakage. In general it's frowned upon here to have the whole installation on one RCD for this reason - but running from a small genny is a different situation.

The MCB on the genny output is to protect the genny from certain faults. With a winding-earth fault in the genny, "significant" current could flow round the circuit but NOT through the breaker that's on the genny panel. There's potential for this to overload the genny and the transformer until the genny burns out. There's an argument that as soon as such a fault occurs, the genny is effectively scrap anyway - but you don't want the risk of a fire, nor of burning out the transformer.
Again, I suspect that the genny would be incapable of tripping a 45A MCB even on it's thermal trip (which might not trip below 90A). Given the max rating of the genny equates to 38A, I suspect that it might struggle to even trip a 32A breaker.
And unless you are particularly agrresive with electricity usage, I think 32A (or even 25A) would be more than enough to keep the house running. That's 32A (or 25A) each on both the 110V sides, or 32A (25A) on 220V, or some combination in between.

That's my reasonings. I rather hope someone with first hand experience in this area can give their input on what rating devices they'd use.

 

[kevindd992002]

Ok, that's interesting. With a 2-pole changeover switch and bonding N and E before it, I think the modifications I need to make will be so much easier than originally planned. If you remember, the first diagram in your post here is my current setup. With that I mind, should I be doing all these?

• Disconnect N and E bonding both DB main panels?
• Connect ground bus bar of B1 DB main panel to ground bus bar of B2 DB main panel?
• What's the best way of bonding both Neutrals (supply and generator) to ground? Remember, my earth rods are already embedded into the ground under cement.

And yes, my generator has a floating output, that has been tested in the earlier parts of this thread here.

Also, I'm still concerned on @pc1966 's posts here and here where he specifically said:

"The autotransformer to create the neutral point must be on the generator side of your change-over switch. It must never be connected to the utility supply in that manner as that will likely destroy it."

"Actually if the local regulations don't require N to be on change over then best not to change things. Messing about with the board supply is definitely not a DIY or similar activity.

What is essential is you never put the generator or the autotransformer on to the utility supply, not at any time."

What is the reason for that? How will sharing the neutral of the supply with the generator destroy the supply?

As for the breaker ratings, I'm still trying to wrap my head around your reply I need to draw my own schematics as my head is hurting trying to understand just by reading. I'll circle back with you on this breaker rating topic.

 

[pc1966]

The point about the auto-transformer is it must not be directly on the supply L1-N-L2 at any point as any imbalance will cause a huge current as the transformer attempts to maintain it's winding ratio voltages.

When it is on the generator the source of power is only L1-L2 and the resulting transformer current is partly from that magnetising the core (and losses) but mostly it is from the load imbalance. The load in your house is not capable of such a huge current as your incoming supply, as fundamentally it is limited by the generator output.

However, as you are likely to have a transformer that is 50% or so of the generator output it really requires some protection in case of an overload that results in too much current on the transformer. Here a 3-pole MCB could be used to protect it and the generator, though as you have already realised they often just stall if seriously overloaded and don't deliver anywhere like the prospective fault current that a test would suggest (same as many UPS whose regulation suggests 2kA fault but in reality drop at 50A or so load).

So a 10kVA generator is about 42A at 240V. With a 240V-120V transformer for the centre tap at 5kVA is is the same on the 120V end (5000 / 120 = 41.67). Hence something like a 3-pole (linked operation) 45A MCB would provide reasonable protection for long-term thermal overloads, even if a short is likely to just stall the generator.

In terms of switching, in the UK the N-E link is on the supplier's side and it is prohibited to link N-E in the installation, so here we would have another N-E link in the generator and use a change-over switch that swaps neutrals as well as the line conductor(s). That way the E is always connected, but at any one time there is only on N-E link and it is always before the DB (panel).

But you are using USA style rules and there the N-E link is in the panel. In this case it really does not make sense to switch the N as you will have it linked in any case, and the less messing around with supply lines the better. That is why I later though you would be as well just switching the L1/L2 pair between the incoming supply and the 3-pole MCB, but leaving the N & E there all the time.

 

[pc1966]

This sort of idea:

 

[kevindd992002]

The point about the auto-transformer is it must not be directly on the supply L1-N-L2 at any point as any imbalance will cause a huge current as the transformer attempts to maintain it's winding ratio voltages.

When it is on the generator the source of power is only L1-L2 and the resulting transformer current is partly from that magnetising the core (and losses) but mostly it is from the load imbalance. The load in your house is not capable of such a huge current as your incoming supply, as fundamentally it is limited by the generator output.

However, as you are likely to have a transformer that is 50% or so of the generator output it really requires some protection in case of an overload that results in too much current on the transformer. Here a 3-pole MCB could be used to protect it and the generator, though as you have already realised they often just stall if seriously overloaded and don't deliver anywhere like the prospective fault current that a test would suggest (same as many UPS whose regulation suggests 2kA fault but in reality drop at 50A or so load).

So a 10kVA generator is about 42A at 240V. With a 240V-120V transformer for the centre tap at 5kVA is is the same on the 120V end (5000 / 120 = 41.67). Hence something like a 3-pole (linked operation) 45A MCB would provide reasonable protection for long-term thermal overloads, even if a short is likely to just stall the generator.

In terms of switching, in the UK the N-E link is on the supplier's side and it is prohibited to link N-E in the installation, so here we would have another N-E link in the generator and use a change-over switch that swaps neutrals as well as the line conductor(s). That way the E is always connected, but at any one time there is only on N-E link and it is always before the DB (panel).

But you are using USA style rules and there the N-E link is in the panel. In this case it really does not make sense to switch the N as you will have it linked in any case, and the less messing around with supply lines the better. That is why I later though you would be as well just switching the L1/L2 pair between the incoming supply and the 3-pole MCB, but leaving the N & E there all the time.

Ok, again thank you yet another diagram. What you said made much more sense with that.

1. On the neutral topic:

So with your latest diagram, when on generator power, it is fine that the neutral from the auto-transformer is connected to the supply's neutral?

2. On the generator output breaker topic:

Would it make sense to use a 3-pole RCBO (MCB+RCD) instead of just an MCB to make sure that any current imbalance in any non-RCD/GFI protected circuit in the DB panel will cut the generator output and protect the generator, people, and animals?

Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?

3. On the change-over switch topic:

So my current change-over switch is composed of 2 x 2-pole 200A MCB's (the line side of one connected to the supply and the line side of the other is connected to the generator output) and have that metal strip that only allows one of those breakers to be turned on. The load side of both is connected to the DB panel main circuit breaker.

With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?

 

[kevindd992002]

The change-over switch I have is similar to this one here:

https://www.carousell.ph/p/mts-manual-transfer-switch-in-nema1-enclosure-complete-with-common-busbar-and-ge-bolt-on-breakers-1039798954/?t-id=_1606805940319&t-referrer_browse_type=search_results&t-referrer_request_id=IGinHlqHtLkdxYsS&t-referrer_search_query=manual%2Btransfer%2Bswitch&t-referrer_sort_by=popular

 

[pc1966]

1. On the neutral topic:

So with your latest diagram, when on generator power, it is fine that the neutral from the auto-transformer is connected to the supply's neutral?

Theoretically no issue. But I don't know if the USA code allows it to remain in place. It could be switched as for the L1/L2 but with the N-E link for all cases still at the panel side.

Probably @Megawatt will know what the USA would allow for generator change-over.

2. On the generator output breaker topic:

Would it make sense to use a 3-pole RCBO (MCB+RCD) instead of just an MCB to make sure that any current imbalance in any non-RCD/GFI protected circuit in the DB panel will cut the generator output and protect the generator, people, and animals?

A RCD before the N-E link won't protect anything really, other than a winding to chassis fault in the generator resulting in current outside of the senses set.

Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?

It is to protect the autotransformer against overload if a large imbalance or fault occurs in the home.

If you put any means of isolation in to the N it must also disconnect all L simultaneously for safety, so here it ensures that any 120V fault that might overload the transformer also isolates all lines so you don't get a floating N and over-voltage on one 120V set.

3. On the change-over switch topic:

So my current change-over switch is composed of 2 x 2-pole 200A MCB's (the line side of one connected to the supply and the line side of the other is connected to the generator output) and have that metal strip that only allows one of those breakers to be turned on. The load side of both is connected to the DB panel main circuit breaker.

With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?

No, keep the switch as it is. No point in modifying it as it is simpler to add a 3-pole one elsewhere.

 

[kevindd992002]

Theoretically no issue. But I don't know if the USA code allows it to remain in place. It could be switched as for the L1/L2 but with the N-E link for all cases still at the panel side.

Probably @Megawatt will know what the USA would allow for generator change-over.

@Megawatt , I'd really appreciate if you can chime in. I live in the Philippines but our electrical system follows USA standards so if I can wire up the neutral from both utility supply and generator that way, it would make the modifications I need to so much easier.

A RCD before the N-E link won't protect anything really, other than a winding to chassis fault in the generator resulting in current outside of the senses set.

And we don't necessarily need protection against winding to chassis fault, why? Does it not work like a normal RCD that is installed on the DB panel? It should still detect current imbalances on any of the circuits since it would technically be the protection device that is most upstream in the tree, no?

It is to protect the autotransformer against overload if a large imbalance or fault occurs in the home.

If you put any means of isolation in to the N it must also disconnect all L simultaneously for safety, so here it ensures that any 120V fault that might overload the transformer also isolates all lines so you don't get a floating N and over-voltage on one 120V set.

Ok, got it. That clears that up.

No, keep the switch as it is. No point in modifying it as it is simpler to add a 3-pole one elsewhere.

Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.

 

[Simon47]

This sort of idea:
View attachment 63594

Yes, that's it. I know we've been a bit around the houses - that's partly because we are used to different wiring standards and so we (or at least, I) have tended to work from what we'd do here and adapt.

Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?

It's only really for a genny winding-frame fault. If you add in the built in breaker to your diagram, you'll see that there's one side of the output that isn't really protected. You could circulate fault currents through the genny "neutral" which is really your L2, the auto-transformer, the neutral-earth connection, and back to the genny frame. The genny breaker may or may not trip - but even if it does, it can't stop the current flow in the genny neutral.
But now I think about it a bit more, and look at your last diagram, if properly sized the 3 pole breaker after the transformer should trip and that will stop the fault current.

With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?

I wouldn't bother - it just won't trip, and so you are using it just as a switch.

The change-over switch I have is similar to this one here:

https://www.carousell.ph/p/mts-manual-transfer-switch-in-nema1-enclosure-complete-with-common-busbar-and-ge-bolt-on-breakers-1039798954/?t-id=_1606805940319&t-referrer_browse_type=search_results&t-referrer_request_id=IGinHlqHtLkdxYsS&t-referrer_search_query=manual%2Btransfer%2Bswitch&t-referrer_sort_by=popular

Hmm, I guess US standards are somewhat different to ours. I don't like that design as it's just too easy for someone to remove or bend the bit of metal that's "getting in the way" and close both switches. Also, there's no terminals for your neutral - in answer to you question of how to connect them.
The ideal would be to have a neutral block in your changeover switch housing - then you have 3 wires from meter to switch, 3 wires from transformer to switch, and 3 wires from switch to panel, and thus your wiring is made much simpler.
Over here I don't think that would be accepted as it's just too easy to connect both the genny and mains supply - which means you can back-feed the mains which over the years has led to a number of accidents due to circuits being live when they are supposed to be dead. We'd normally be using a switch where all poles are operated by one actuator - making it impossible to close one switch without the other having opened first. Rotary switches are common, but in single phase DBs, it's possible to replace the two pole isolator with a 4 pole device like this one (we use DIN rail mounted devices as standard over here) which is effectively two switches with a common operating handle, and one of the switches working "upside down" to the other.

As an aside, it's also important to run the 3 wires of each group in close proximity to each other - if you were to (for example) run the neutral wire a different route, then you'd create a single turn of a transformer, creating magnetic fields according to how much current there is in the neutral.

I've seen some interesting effects on old CRT screens when there's been magnetic fields like that ?On one occasion, I got called to a client because all the screens in one part of the office were "wobbling" at certain times of the day. I checked with the electricians, and they'd done some work in the shop below - but a neutral had come adrift in a ring final circuit*. So when the electric curtain fan heater was one, it's 13A was split both ways round the ring for the live, and only one way for the neutral - thus creating a large loop that effectively had something in the order of 6-7A flowing round it. This was enough to interfere with the magnetic scan of the CRT screens. As the screesn were using something near to our 50Hz mains, the result was a visible wobble rather than it going fuzzy as it would do if the wobble was at the full 50Hz.
The other occasion was when they were extending the offices. The steel frame was bolted, but also welded. The builders just clamped the welder earth to the steelwork, and went roound welding the joints. Thus the welding current similarly appeared in a loop. Because of the magnitude of the current, when they struck up and arc, the terminals the other side of the wall just went nuts - the display just shimmered well off both sides of the screen.

* Ring Final Circuit
Created during the second world war as a means of reducing the amount of copper needed, a quantity of sockets are wired in a ring that starts and ends at the distribution board. The sockets themselves are rated for 13A each, the cable we currently use is rated (depending on installation method) at a max of 27A, but it's protected by a 30A fuse or 32A MCB. The idea is that unless you apply a massive load right at one end of the ring, the currents gets split between the two routes between board and sockets, and it's not possible to overload the cable.
The downside is that without testing, it's quite possible for any of the individual wires to break, and no-one will ever know - unless it causes magnetic interference like I described above.
And the "which is better - ring or radial" (I believe US standards are for lots of radials) is one of those arguments that I think will go on for a very long time.

 

[Megawatt]

@Megawatt , I'd really appreciate if you can chime in. I live in the Philippines but our electrical system follows USA standards so if I can wire up the neutral from both utility supply and generator that way, it would make the modifications I need to so much easier.


And we don't necessarily need protection against winding to chassis fault, why? Does it not work like a normal RCD that is installed on the DB panel? It should still detect current imbalances on any of the circuits since it would technically be the protection device that is most upstream in the tree, no?


Ok, got it. That clears that up.


Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.

I just trying to figure out what has previously been talked about. Here’s what I’m understanding is yes you tie the neutrals together from the panel to the generator and don’t break them through your transfer switch. As far as your breakers on your generator do you not have a 30 amp 4 wire plug or just a

 

[Megawatt]

I just trying to figure out what has previously been talked about. Here’s what I’m understanding is yes you tie the neutrals together from the panel to the generator and don’t break them through your transfer switch. As far as your breakers on your generator do you not have a 30 amp 4 wire plug or just a

Just a 3 wire plug. What are you referring as a auto transformer. I’m just trying to catch up

 

[pc1966]

Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.

As mentioned above, you really want the L1/L2/N cables to follow the same route. However, a detour of 0.5m or so is not a big deal.

Running the generator neutral to the panel along side the supply neutral is one option.

If the change-over switch lacks a neutral terminal then you need some means to connect there then be careful as a 200A join has to be very good or it will overheat. For joining to something like that I would be inclined to look at using a line-tap. This is the sort of thing I mean:

Cable Line Taps

Basically the high-current wires goes straight through (minus a section of insulation that is carefully removed) and the tap wire is clamped next to it. Advantage is no additional joint(s) in the main high-current path.

Whatever you do, check if the cable is aluminium and if so you should be using something like 'noalox' on any joints to inhibit galvanic corrosion, especially against copper or brass.

 

[pc1966]

And the "which is better - ring or radial" (I believe US standards are for lots of radials) is one of those arguments that I think will go on for a very long time.

As above, you will find threads running to hundreds of posts on the subject of ring vs radial.

TL;DR answer is "it depends". A couple of sockets somewhere, us a radial. Many sockets on a floor of a flat/house, use a ring.

Also you have to be aware that the UK has fused plugs. That has a huge impact on the trade-offs possible.

 

[pc1966]

Just a 3 wire plug. What are you referring as a auto transformer. I’m just trying to catch up

Short version is they have a 240V generator but a split 120-0-120 home, so genny use causes one side to be over volts, other under-volts as not balanced load.

Suggestion is to use an auto-transformer to create a "virtual neutral" off the 240V genny and then supply the house, following that the issues of rules for generator-utility change over switching, protection of a partial load rated transformer, etc.

We should charge $500/day for this, and spend it on beer!

 

[Megawatt]

As mentioned above, you really want the L1/L2/N cables to follow the same route. However, a detour of 0.5m or so is not a big deal.

Running the generator neutral to the panel along side the supply neutral is one option.

If the change-over switch lacks a neutral terminal then you need some means to connect there then be careful as a 200A join has to be very good or it will overheat. For joining to something like that I would be inclined to look at using a line-tap. This is the sort of thing I mean:

Cable Line Taps

Basically the high-current wires goes straight through (minus a section of insulation that is carefully removed) and the tap wire is clamped next to it. Advantage is no additional joint(s) in the main high-current path.

Whatever you do, check if the cable is aluminium and if so you should be using something like 'noalox' on any joints to inhibit galvanic corrosion, especially against copper or brass.

What about installing a Henley block as y’all call them. If you break the neutrals then it is consider an separately derived system which has another article with that so I would not break the neutrals

 

[Simon47]

Hmm, with that switch the OP is using - doesn't the neutral have to go through the box along with the L1 and L2 ? And each set of 3 through the same hole ?
Thinking eddy currents here.

 

[kevindd992002]

Ok, So I guess we're all on the same page regarding the final design. I checked locally and there's either a 40A or a 50A 3-pole MCB available for the genny output. Would it be better to go with the lower 40A for the 10.8kVA genny /5.4kVA transformer combo? And does it have to be a B curve or a C curve?

The only problem now is the concern about running L1/L2/N together from the gen to the changeover switch. The L1/L2 from the gen are already routed underground from the area around the B2 building to the changeover switch on the B1 building and adding another neutral wire would be hard. I thought abother neutral wire using a different route will be just fine. Can't I get away with that since the genny power is just "temporary" anyway?

@pc1966

When you said "running the generator neutral to the panel along side the supply neutral is one option.", does that mean I can directly go from the autotransformer to the panel neutral bus bar even if it's a different route from the genny L1/L2?

I'd definitely treat you guys for a beer for real if we were close to each other But seriously, thank you.

 

[Simon47]

To save running an extra neutral through a different route, is there room to put the transformer near your distribution board ? If not, then you don't have much option really. It won't be carrying the full load current unless you manage to load only one of your 110V sides.
10.8kVA @ 220V is 49A. I'd personally go with the 40A breaker - you'll be hard pressed to trip it anyway unless you have electric showers and/or do a lot of cooking with electric. Normally I'd expect some amount of restraint when running off the genny. Go with B curve - the only difference is the magnetic trip setting (B curve is more sensitive), slow thermal trip is the same. I doubt the genny will be capable of tripping the magnetic trip on even the B curve.

As to the difficulty of running extra wires, now you know why people often recommend ducting so you can pull in the cable you need if requirements change.

BTW, for you changeover switch and distribution board - if they have ferrous metal (i.e. steel) housings. All three wires from the set (L1,N,L2) should enter through the same hole. So all 3 from your mains supply through one hole, the three from the genny/transformer through one hole, the three out to the DB through one hole, and the three into the db through one hole. If you use two holes, cut a very thin slot between the holes with a thin hacksaw blade.
The reason for this is that each individual wire carrying a current generates a magnetic field, and this will cause currents to flow in a sheet of metal if passed through a hole in it. But the fields from the three cables will total out to nothing - because the currents in the three wires must sum to zero (taking into account phase angle as well as magnitude). So if all the wires go through one hole, then the magnetic fields cancel and no eddy currents are created. If they go through different holes, then you get eddy currents which wastes power and heats up the box.
Cutting a slot where cables go through different holes breaks the circuit for eddy currents, so the whole thing acts like the wires go through one larger hole.

 

[pc1966]

The only problem now is the concern about running L1/L2/N together from the gen to the changeover switch. The L1/L2 from the gen are already routed underground from the area around the B2 building to the changeover switch on the B1 building and adding another neutral wire would be hard. I thought abother neutral wire using a different route will be just fine. Can't I get away with that since the genny power is just "temporary" anyway?

You only need the L1/L2/N together from the autotransformer, so if the generator is already wired in you can leave it, just plan to route the transformer ones more or less together (often simpler just to use 4 core cable of adequate rating, earth is there as well).

 

[kevindd992002]

To save running an extra neutral through a different route, is there room to put the transformer near your distribution board ? If not, then you don't have much option really. It won't be carrying the full load current unless you manage to load only one of your 110V sides.

It depends on the size of the autotransformer as I still don't have a clue how big a 5kVA variant is. The changeover switch and DB for B1 is actually just separated by a wall back-to-back so they are very near each other. If its size is reasonable enough, I probably can do something to make it fit in the vicinity of the changeover switch or the DB. But shouldn't it be easier if it's near the changeover switch side and not the DB? So the new MCB + autotransformer combo near the changeover switch would mean that the cables from the autotransformer to the MCB are shorter and together and the L1/L2 wires from the MCB to the changeover switch is also shorted. But the neutral from the MCB should be tapped to the supply neutral (as shown in @pc1966 's latest diagram).

10.8kVA @ 220V is 49A. I'd personally go with the 40A breaker - you'll be hard pressed to trip it anyway unless you have electric showers and/or do a lot of cooking with electric. Normally I'd expect some amount of restraint when running off the genny. Go with B curve - the only difference is the magnetic trip setting (B curve is more sensitive), slow thermal trip is the same. I doubt the genny will be capable of tripping the magnetic trip on even the B curve.

Ok, I'll settle with a 40A B-curve MCB. And yes, before we turn on the genny we manually turn off the breakers for shower heaters, airconditioners, etc. anyway.

As to the difficulty of running extra wires, now you know why people often recommend ducting so you can pull in the cable you need if requirements change.

Actually, the L1/L2 that currently runs from the generator to the changeover switch is underground but with a pvc pipe to protect them. Technically, they can be pulled but I'm not a 100% sure if it would be easy as it's a pretty tight fit and even if it's possible it would probably destroy the existing wires. A duct would've been easier, I agree.

BTW, for you changeover switch and distribution board - if they have ferrous metal (i.e. steel) housings. All three wires from the set (L1,N,L2) should enter through the same hole. So all 3 from your mains supply through one hole, the three from the genny/transformer through one hole, the three out to the DB through one hole, and the three into the db through one hole. If you use two holes, cut a very thin slot between the holes with a thin hacksaw blade.
The reason for this is that each individual wire carrying a current generates a magnetic field, and this will cause currents to flow in a sheet of metal if passed through a hole in it. But the fields from the three cables will total out to nothing - because the currents in the three wires must sum to zero (taking into account phase angle as well as magnitude). So if all the wires go through one hole, then the magnetic fields cancel and no eddy currents are created. If they go through different holes, then you get eddy currents which wastes power and heats up the box.
Cutting a slot where cables go through different holes breaks the circuit for eddy currents, so the whole thing acts like the wires go through one larger hole.

Ok, this makes sense. I'll have to check if they were done this way. As for the genny neutral-to-supply neutral bonding, with this recommendation where all wire sets go through one hole, where does the neutral-to-neutral bonding "ideally" take place?

You only need the L1/L2/N together from the autotransformer, so if the generator is already wired in you can leave it, just plan to route the transformer ones more or less together (often simpler just to use 4 core cable of adequate rating, earth is there as well).

I thought L1/L2 from either the generator or the autotransformer are one and the same? I mean, the autotransformer is in parallel with the L1/L2 output of the generator so I was assuming that L1/L2 can either be from the generator output itself (which is how it is wired from the generator to the changeover switch right now) OR get L1/L2 and N from the autotransformer output like what you're suggesting. Or are you referring to what Simon is suggesting above? Put the transformer near the transfer switch where the end of the L1/L2 generator output cables are, install the autotransformer there, and run another fresh set of L1/L2/N/E wires from the autotransformer to the changeover switch?

 

[Simon47]

Ok, this makes sense. I'll have to check if they were done this way. As for the genny neutral-to-supply neutral bonding, with this recommendation where all wire sets go through one hole, where does the neutral-to-neutral bonding "ideally" take place?

Ideally I would expect the neutrals to be linked inside the enclosure for the changeover switch.
So three wires from mains, three wires from genny/transformer combo, three wires to DB.

I thought L1/L2 from either the generator or the autotransformer are one and the same? I mean, the autotransformer is in parallel with the L1/L2 output of the generator so I was assuming that L1/L2 can either be from the generator output itself (which is how it is wired from the generator to the changeover switch right now) OR get L1/L2 and N from the autotransformer output like what you're suggesting.

Yes, the L1 and L2 are the same wherever you put the joints. So work out what's going to be easiest to do.
One way is to take L1,L2 to the transformer, then L1,N,L2 from transformer to breaker.
Or you take L1,L2 to breaker, tee off the L1,L2 there and take a 3 core (+earth) cable to the transformer.
Or put a junction box in.
It really depends on what's going to give the "best" result in terms of neatness and ease of install.