Planning garage sub-main project

[Liam151807]

Hi all, 2nd year apprentice.

I'm looking to take a supply out to my garage, which is detached, from the DB at the front of the house (2 bed terraced, built 1978~)

The length of run is about 30m (having to go through either loft or under floorboards).

Intend to have a 20A radial and a 6A lighting circuit so am I right in thinking I'd allow 66% diversity for lighting and 50% for the power circuit as it's over 10A? So 13.96A total demand?

Thanks

 

[James]

in practical terms, a garage supply will always (well nearly always) want a 32A supply as a minimum.
In the winter, there will inevitably be a 3Kw fan heater and the starting current of the angle grinder, chop saw, welder, hot tub, lawn mower, or any other of the common things plugged into a garage.

 

[Liam151807]

in practical terms, a garage supply will always (well nearly always) want a 32A supply as a minimum.
In the winter, there will inevitably be a 3Kw fan heater and the starting current of the angle grinder, chop saw, welder, hot tub, lawn mower, or any other of the common things plugged into a garage.

For my needs it will just be for a fridge/freezer, tumble dryer and use of garden/home equipment to hoover my car and trimming hedges.

But yeah if and when I move on one day someone else may want the extra capacity for more. Let's say I took 40A out there, what would I do for a mains switch in the dB?

 

[James]

ha, ha.

fridge/freezer 2A continuous 10A start
tumble drier 9A continuous 13A start
Vac. cleaner 8A continuous 13A start.

I rest my case.

 

[MJPD29]

For my needs it will just be for a fridge/freezer, tumble dryer and use of garden/home equipment to hoover my car and trimming hedges.

But yeah if and when I move on one day someone else may want the extra capacity for more. Let's say I took 40A out there, what would I do for a mains switch in the dB?

What do you mean mains switch in db? You could just add a 40a mcb to your existing db if using swa and fit a small board in the garage, with rcd protection of course

 

[Liam151807]

What do you mean mains switch in db? You could just add a 40a mcb to your existing db if using swa and fit a small board in the garage, with rcd protection of course

Well I was going to use RCBOs due to the built in RCD protection at the main DB in house. Then at the garage sub main dB have a main switch to isolate the two breakers on that board.

Hope that makes sense haha

 

[diditrain]

If you fall out with the miss’s and are crashing in the shed one night and your RCBO trips right in the middle of a good movie, you will have to trail back to the house in the dark to reset the RCBO. Better design required, ideally with your RCBO at the shed end.

 

[littlespark]

Detached garage, so buried cable between? Need SWA.
SWA does not need rcd protection.
The circuits from the board in the garage will.

What is in the house board at the moment? Mcb? Rcd? Photo please.


Are you working with a company? What do your colleagues suggest?

 

[pc1966]

You will always end up needing more than you planned for!

There are a few ways to approach the design and you should go with the largest of:

• What loads do I expect to use?
• What is my basic cable choice capable of?
• What cable size can I afford if looking to future usage?

You are unlikely to select less than 2.5mm SWA so buried it should be fine with a 32A MCB, but what is your route and hence the "methods" involved? They might lower the CCC if it runs through insulated regions.

Will you meed voltage drop with this choice?

Do you have extraneous conductive parts in the garage? If so do you need to size for PME bonding requirements?

What is your supply Ze and so can you meet sub-main disconnection time with the cable length and your chosen protection?

 

[MJPD29]

Well I was going to use RCBOs due to the built in RCD protection at the main DB in house. Then at the garage sub main dB have a main switch to isolate the two breakers on that board.

Hope that makes sense haha

Your swa cable does not need rcd protection. Personally I would have it on an mcb and either small board with rcbcos or rcd in garage.

 

[westward10]

SWA supply to garage may need RCD protection, we don't have enough information.

 

[James]

SWA supply to garage may need RCD protection, we don't have enough information.

Although it might not need to be 30mA

 

[mainline]

40 amp mcb, 10mm 3 core swa, garage Cu, would cover most garage supplies and allow for future upgrades.

If there really is the likelihood of say an Ev point or hot tub, then I would use an SP+N fused connection unit.

But like already said, more information is needed.

 

[Liam151807]

Cheers for the replies, much appreciated.

So the db is in the porch on the ground floor (highlighted red) and the first floor pic with red lines is my proposed route going under the floor boards or up into the loft.

 

[Liam151807]

Here is the circuits on the board

 

[Liam151807]

Here is the swa being ran to the garage

 

[Liam151807]

I will run the T&E down the wall.either from the loft or from under the floor on the first floor (half way down) into a joint box with the swa which will go to the garage

 

[Dave OCD]

The right hand side of your board with the lighting MCBs isn't RCD protected and the blank will be a spare way- so subject to a decent Ze you could simply fit an additional matching MCB for the new SWA, but it will have to be glanded properly, possibly into a small metal enclosure to ensure the armour is earthed. EDIT= Ah I've just seen your post stating T&E for the first part of the run, so the SWA can be glanded into a weatherproof enclosure with an earthing nut and lead to the CPCs.

 

[MJPD29]

Skip the twin and earth in conduit and run swa straight to board. What is the purpose of the t+e? Looks like part of that board isn’t rcd protected, get an mcb to fit it

 

[Liam151807]

Skip the twin and earth in conduit and run swa straight to board. What is the purpose of the t+e? Looks like part of that board isn’t rcd protected, get an mcb to fit it

Well I was thinking twin and earth (e.g. 6mm) would be cheaper and easier to run under floor boards etc.

Tbf I could use that blank space for a 40A MCB to supply the garage couldn't I

 

[Liam151807]

The right hand side of your board with the lighting MCBs isn't RCD protected and the blank will be a spare way- so subject to a decent Ze you could simply fit an additional matching MCB for the new SWA, but it will have to be glanded properly, possibly into a small metal enclosure to ensure the armour is earthed. EDIT= Ah I've just seen your post stating T&E for the first part of the run, so the SWA can be glanded into a weatherproof enclosure with an earthing nut and lead to the CPCs.

So I could join the SWA into a IP65 rated box and attach a banjo, nut and cpc to Earth the SWA?

 

[Dave OCD]

So I could join the SWA into a IP65 rated box and attach a banjo, nut and cpc to Earth the SWA?

Yes that's right. Have a look at earthing or 'Piranha' nuts though, easier and IMO better than the old Banjo method. I'd be using 3 core SWA though.

 

[mainline]

Do we know what Ze is ?
Do we know if there is any extraneous ?

 

[Liam151807]

Do we know what Ze is ?
Do we know if there is any extraneous ?

Afraid idk the ZE at present.

Ummm garage door I suppose

 

[pc1966]

Cheers for the replies, much appreciated.

So the db is in the porch on the ground floor (highlighted red) and the first floor pic with red lines is my proposed route going under the floor boards or up into the loft.

You don't want to put the garage on the RCD side, just asking for trouble for the rest of the house being tripped by garage trouble.

If it can go on the MCB side (assuming SWA all the way, end of cable Zs acceptable, etc) then do that.

Off hand I don't know if RCBO are available for that board should you need 30mA additional protection there, but keeping it off the existing RCD would be sensible.

However, you might want to plan to replace that CU in the near future anyway so keep enough cable slack to along that.

 

[Liam151807]

You don't want to put the garage on the RCD side, just asking for trouble for the rest of the house being tripped by garage trouble.

If it can go on the MCB side (assuming SWA all the way, end of cable Zs acceptable, etc) then do that.

Off hand I don't know if RCBO are available for that board should you need 30mA additional protection there, but keeping it off the existing RCD would be sensible.

However, you might want to plan to replace that CU in the near future anyway so keep enough cable slack to along that.

You mentioned SWA all the way, is this because a join in cable with (T&E) would affect Zs readings or something?

Tbf I am leaning towards a board change at this point I won't lie.

 

[James]

Think about requirements for rcd protection of cables.
apart from the fact that where possible it is better to design a circuit with fewer or no joints in it.

 

[timhoward]

ou mentioned SWA all the way, is this because a join in cable with (T&E) would affect Zs readings or something?

The point is that depending on the run, T&E might need RCD protection for Impact Protection. SWA definitely wouldn't for that reason. That is one reason why a few people are suggesting SWA all the way.

For fault protection purposes we need to be sure that a fault will trip the protective device (breaker) in the house. Until you know the Ze, the cable length and the cable size, the resistance (R1+R2) of the circuit can't be calculated and added to the Ze and then checked against the max Zs value for the device. (An RCD may be required for fault protection if the Zs exceeds the limit for the device. )

A further consideration is whether there are any metal services like water pipe for a tap entering the garage, as if anything needs bonding the far end you need to make sure the CPC is big enough to support this.

So this job can be easy (Low Ze, nothing to bond the far end) or quite difficult (High Ze, RCD needed for fault protection, larger SWA needed to bond services the far end, then decisions about selectivity of RCDs etc. etc.).

 

[MJPD29]

You mentioned SWA all the way, is this because a join in cable with (T&E) would affect Zs readings or something?

Tbf I am leaning towards a board change at this point I won't lie.

No joints and the t+e may need rcd protection depending on the route. Swa back to the board it a better solution and will cost minimally more due to the short length of the cable

 

[Liam151807]

Ok so lots to think about haha.

Realistically I'm leaning towards fitting a new board at the house with RCBOs. Then to supply the garage DB, a 40A MCB. Am I right in thinking I'd need RCD protection for this sole MCB?