Converting into 12 flats, KVA, phases etc. Help!

[Confusedsoconfused]

Hello,

I'm new to this forum, we are in the process of converting into 12 flats all electric, no gas incl. heating, showers etc.

An electrician was hired to apply to the National Grid on our behalf for a three-phase connection.

He then quoted us £1,800 plus VAT for civil works i.e. to dig within our boundary the trenches, lay the ducting.

It was later discovered that he has bumped several former companies and has also charged us VAT when not VAT registered (Another issue for another time).

Anyways, we finally received the offer from the National Grid but given what we discovered we are sceptical and desperately seeking some knowledge and advice.

On the application he wrote, "the maximum load capacity for the site at 180kva"


The Connection Offer:

"1 three phase LV service (with whole current metering provided by your supplier).

"12 new connections to flats 12 way MSB".

"Your supply will have the following electrical characteristics...

Voltage - 230V
Phase - Single Phase
Agreed Capacity - 18.4 kVA
Earthing - PME


It would be great if someone could explain all this to me in layman terms...


Do we need a three-phase connection? if so why?

Why is there 1 three phase LV service but the supply characteristics are single phase and a capacity of 18.4kVA?

Is the digging of the trench on soil within our boundary that is a maximum of a few metres really justify that price?

 

[timhoward]

Your supply will have the following electrical characteristics...

Voltage - 230V
Phase - Single Phase
Agreed Capacity - 18.4 kVA
Earthing - PME

That will be for each flat. 18.4 KVA is roughly 80 amps, so it looks like they are planning 12 flats with an 80 or 100 amp service head for each flat.

Do we need a three-phase connection? if so why?

It works out a LOT more practical. To the point that the alternative is actually almost impossible.
If you need 80 amps X 12 flats you would need a 960 amp single phase supply which would require an enormous cable that costs a fortune, and would likely need it's own supply all the way back to the substation.
By employing all 3 phases of the electricity supply the requirements are roughly divided by 3 (simplified explanation)

On the application he wrote, "the maximum load capacity for the site at 180kva"

Sounds like a 400A or 450A 3 phase cut-out.

Why is there 1 three phase LV service but the supply characteristics are single phase and a capacity of 18.4kVA?

It looks as though the design is for each phase to supply 4 flats, each flat being an 80/100amp cutout.

Does that help?

 

[pc1966]

As above, almost all power distribution systems use 3-phase as for a given voltage to Earth you get 3 times the power for twice the copper compared to single phase (more if delta load so no neutral like HV side of substations)

Typically for a block of flats you have a big 3-phase supply coming in and then a Ryefield board that has 80A fuses or so to split it in the X separate single-phase feeds to each flat. Billing meters might be in the main electrics room with the board, or more likely at each flat so the tenant can read it without needing access to the switch room.

 

[Confusedsoconfused]

As above, almost all power distribution systems use 3-phase as for a given voltage to Earth you get 3 times the power for twice the copper compared to single phase (more if delta load so no neutral like HV side of substations)

Typically for a block of flats you have a big 3-phase supply coming in and then a Ryefield board that has 80A fuses or so to split it in the X separate single-phase feeds to each flat. Billing meters might be in the main electrics room with the board, or more likely at each flat so the tenant can read it without needing access to the switch room.

Hello,

I thank you all for your comments, they've been very helpful!

Today I was told by National Grid, that with an MSDB board we will require 12 separate meters in either the rooms or the communal areas.


As this will be student accommodation and all bills included, I'd rather have one meter positioned in the existing meter room.


National Grid told me that for this, only with a Cut Out will it be possible to have one meter only.



Can someone please help me out with this?



- What is the difference between a Cut Out and MSDB?


- How does the difference between the two impact the number of meters required?

 

[pc1966]

- What is the difference between a Cut Out and MSDB?

This is not really my area, but:

Basically a "cut out" has fuse(s) to protect the supply network from overload, and to allow supply isolation, to a single load as far as the supplier is concerned. Normally there is the billing meter immediately after the cutout (cutout and meter are not your property and must not be interfered with) and following the meter is the installation (your ownership) with your distribution board(s) that separate and protect each circuit (lights, sockets, etc).

With a MSDB they treat all of the fuses in there that service each flat as separate points of isolation for the flat's billing meters as separate installations. I.e. the MSDB becomes their property and they treat it as their cutouts per flat.

- How does the difference between the two impact the number of meters required?

It comes down to how they treat the boundary between the DNO side and the billed customer.

From an electrical point of view you can split up the power afterwards in many ways, but where it gets messy is if the flats are sold separately (i.e. different ownership) or if you are billing tenants for electricity used where various regulations apply on the type of meter and how electricity cost is charged, etc, etc.

If this is really more of a hotel-like environment rather than individual flats with legal boundaries then you might well be better with a single billing meter after the DNO cutout, and for each personal room, etc, you might want to put in some sort of check-meter on the sockets per room to check for anyone taking the pi55 (e.g. bit-coin mining on some heavy PC) even if those meters are not used for billing as such.

In that case (basically halls of personal rooms, few communal areas, kitchen, etc) an MSDB is not a good approach, you might as well have a three-phase distribution board (or maybe two like top & bottom depending on the floor layout, number of floors, etc) that feeds power to the various sets of sockets, lights, showers, etc.

 

[pc1966]

You must decide at the very start if this will be a single installation (i.e. hotel-like) or if it will be multiple flats that have ownership/tenant rights.

If it will be divided as flats with a view to future sale, etc, then they must have separate electrical installations and separate meters.

If it will be a halls/hotel like arrangement then you are much better with treating it as one installation with a single billing meter but I would advise that you have some sub-metering just for your own management of how power is used. It is not my area but I think this might be something the building regulations are now pushing for?

If the sub-meters are not for legal billing then they don't need to be MID certified, so various "smart meters" can be used which are cheaper and more convenient to gather the usage data in one place as often they support wi-fi or Bluetooth connections, etc.

Hopefully your electrician can advise you on this, but you do have to make your plans clear to them!

 

[pc1966]

Just to add that each "personal room" should have an independent socket circuit.

Not just for any check meter, but also if someone has brought a dodgy bit of equipment that trips the protection - you don't want that to impact on other students by tripping out their power as well, and you also want to locate the fault quickly to one person's room and thus equipment to be checked for faults.

 

[Rockingit]

Also to add that AFDD protection devices will be required by regulations - they are expensive, so be prepared!

 

[Confusedsoconfused]

This is not really my area, but:

Basically a "cut out" has fuse(s) to protect the supply network from overload, and to allow supply isolation, to a single load as far as the supplier is concerned. Normally there is the billing meter immediately after the cutout (cutout and meter are not your property and must not be interfered with) and following the meter is the installation (your ownership) with your distribution board(s) that separate and protect each circuit (lights, sockets, etc).

With a MSDB they treat all of the fuses in there that service each flat as separate points of isolation for the flat's billing meters as separate installations. I.e. the MSDB becomes their property and they treat it as their cutouts per flat.


It comes down to how they treat the boundary between the DNO side and the billed customer.

From an electrical point of view you can split up the power afterwards in many ways, but where it gets messy is if the flats are sold separately (i.e. different ownership) or if you are billing tenants for electricity used where various regulations apply on the type of meter and how electricity cost is charged, etc, etc.

If this is really more of a hotel-like environment rather than individual flats with legal boundaries then you might well be better with a single billing meter after the DNO cutout, and for each personal room, etc, you might want to put in some sort of check-meter on the sockets per room to check for anyone taking the pi55 (e.g. bit-coin mining on some heavy PC) even if those meters are not used for billing as such.

In that case (basically halls of personal rooms, few communal areas, kitchen, etc) an MSDB is not a good approach, you might as well have a three-phase distribution board (or maybe two like top & bottom depending on the floor layout, number of floors, etc) that feeds power to the various sets of sockets, lights, showers, etc.

Thank you for explaining this and everyone else's subsequent comments...

I've now been told by the National Grid that if I am to now choose a large cut out then I will require a CT Panel and half hour meter billing.

This is complete news to me.

 

[Rockingit]

I think you could do with some expert assistance on this before you end up in a muddle, and from someone that you know you can trust. There are plenty of very experienced members on this forum that operate in your area, maybe post an advert in the 'trusted electricians' section of here?

 

[pc1966]

I've now been told by the National Grid that if I am to now choose a large cut out then I will require a CT Panel and half hour meter billing.

There CT stands for current transformer.

Basically for meters up to around 100A they are usually directly connected, i.e. the current passes through the meter to be measured, and that along with the measured voltage gives power, and then power integrated over time gives you energy (i.e. kWh).

For example, 10A at 230V = 10 * 230 = 2300W = 2.3kW, after 24 hours of constant use that is 24 * 2.3 = 55.2 kWh

However, above 100A it becomes impractical to route big cables via the meter so normally what is done is a current transformer passes a known fraction of the current to the meter for measurement, and the meter then knows to multiply by 100 or whatever the CT ratio is in order to get the real power.

This is normal for every big installation, and I guess these days the half-hour billing meter is also a standard requirement from the DNO. The catch is you might have time-of-day costs applied!

However, aside for the little details of how the metering is done, I have to agree with @Rockingit that you should engage someone you trust who has experience in this sort of thing to help from very start.

I would also say that if you are treating this as one building / system you really should look in to the heating and provision of hot water for showers, etc, as the use of heat-pumps and lower-rate heating of stored water could make a BIG difference to your electricity bill, and over a few years more than pay off the costs at the start!