Service alteration - electric

[Russdl]

I've just got off the phone to a very helpful chap at Scottish and Southern who was helping me through the service alteration process. When it came to the temporary building supply he said they would need "information regarding construction equipment used for the TBS - total expected kW".

 

He couldn't see my blank look, so I made it clear to him that I was indeed looking blank. I told him that I had no idea, and asked what he thought, he said they weren't permitted to make any assumptions so it looks like it's down to me to make the assumption, however, I'm still looking blank.

 

Can anyone help me with this? We're planning on a timber frame build on an raft foundation so I don't foresee any electric cement mixers. There's bound to be a kettle a radio and probably a handful of power tools either plugged in (or having their batteries recharged) and perhaps a heater. Can someone give me a sensible figure I can fire back at SSEN or guidance on how I can come up with a sensible figure myself?

 

Thanks.

 

Russ.

[jack]

Is your temporary supply going to become your permanent one when the house is finished? If so, you should spec for that.

[ProDave]

Just tell them 10KW

 

They are after a figure for the "system"  The man on the end of the phone does not know if this is a building site for 1 house, or a nuclear power station.

[joe90]

Mine was different but, we asked for a temp supply into a fibreglass box ( meter housing for outside walls) on temp poles next to where it would be built into the back of the garage, a waterproof double socket was put on one of the poles for extension leads and whatever for the build.  when the garage was built we simply swung it from the poles and inserted it into a hole left in the wall and it was connected up by my electrician. No further charges for a permenantly supply or moving it.

Edited June 15, 2018 by joe90

[Russdl]

Ah, good point Jack. That is the plan that it will become the permanent supply but I'm still none the wiser on the "total expected kW"

[Russdl]

3 minutes ago, ProDave said:

Just tell them 10KW

 

That's the kind of thing I'm after Dave. I'm as thick as pig-pooh when it comes to this elec-trikery and I've no idea if 10kW is high, low or somewhere in the middle but I'm guessing it's more than adequate for a single residence?

[ProDave]

We are actually on what is rated as a 12KW supply, and that will be plenty for our house, even with an ASHP for heating (that's not much over 1KW anyway)

 

In our case that came about because there is a 100KVA transformer serving 8 houses and I know if I had asked for a 21KVA supply there was a high probability of them wanting to upgrade that to a bigger transformer.

 

The reality is I have the same size supply cable and same 100A fuse and could draw 21KW if I wanted to. As long as we don't all do that at once.

[Jeremy Harris]

I had a chat with SSE about this ( @Russdl we're in the same DNO area) and the chap who came out on site to do the preliminary site survey, before we put in the application, was pretty helpful and said they would install an 18 kVA supply.   In practice they've fitted a 23 kVA supply, 100A company fuse at 230 VAC. 

 

Worth talking to them about local distribution network capacity, as @ProDave mentioned, if you ask for more than the local network can supply they can hit you with network upgrade costs.  I found the chap that came out to do the preliminary site check was very helpful; he told me that there was lots of spare capacity as there was only one other property connected to the 90mm² three phase cable that was already running under our plot, and that we could have three phase if we wanted it.

 

If you stick something like 12 kW to 18 kW on the form they will almost certainly just give you a 23 kVA supply, which in reality will be around 24 kW, because it's rare for the supply voltage to drop as low as the fudged nominal of 230 VAC (the tolerance is +10%, -6%, a fudge to make the EU accept that we'd harmonised at 230 VAC, when in reality we've stuck to the pre-EU 240 VAC...)

[Russdl]

3 minutes ago, JSHarris said:

the chap who came out on site to do the preliminary site survey, before we put in the application,

 

Thanks Jeremy, it seems I have to do it the other way round, i.e. tell them the total expected kW before they arrive for the survey. Sadly I wasn't bright enough to think to ask the network capacity question as mentioned by @ProDave. I'll get back in touch and see what I can find out.

 

9 minutes ago, JSHarris said:

If you stick something like 12 kW to 18 kW on the form they will almost certainly just give you a 23 kVA supply, which in reality will be around 24 kW, because it's rare for the supply voltage to drop as low as the fudged nominal of 230 VAC (the tolerance is +10%, -6%, a fudge to make the EU accept that we'd harmonised at 230 VAC, when in reality we've stuck to the pre-EU 240 VAC...)

 

I suspect the above paragraph is not complicated, however it starts to loose me when the kW start getting mixed up with the kVA and then VAC turns up ?

 

@JSHarris Can I buy you a pint or several whilst you talk me through these (and other) problems!! 

[Jeremy Harris]

The DNO really work in terms of kVA, kilovolt amps.  The reason is to do with power factor and cable sizing, in that they need to size the cables for the maximum current, not the maximum power.  If the power factor is unity (1) then kVA and kW are the same thing, but if the power factor is less than 1 (in either sense) then the current can be greater than the power would indicate, so either the DNO need to increase the capacity available or limit the demand from the consumer.

 

The core problem is that for a power factor of less than unity (in either sense) the voltage and current aren't in phase with each other, and that causes problems with the network.  In reality, domestic loads will rarely have a unity power factor, the assumption the DNO will make is that the power factor will be around 0.8.  For that reason they will usually define a domestic supply as being about 18 kW, even though they are providing a 23 kVA supply (23 kVA x 0.8 power factor = 18.4 kW).

 

VAC is just an abbreviation for Volts Alternating Current, as distinct from VDC, which is Volts Direct Current.  Household supplies are 230 VAC nominal in the UK (in reality they are close to 240 VAC, because of the tolerance fudge) and something like a car electrical system may usually be 12 VDC.  It's not essential to use the "AC" and "DC" suffix after "V", but it does avoid any doubt, especially where both AC and DC supplies may be close to each other in the same system (for example, aircraft often have a 28 VDC bus and a 400 VAC bus in close proximity to each other).  I'm in the habit of using "VAC" from years of having to be explicit at work.

[Russdl]

Thanks for that @JSHarris that certainly clears things up a bit (I'll get the beer). I should have realised the VAC VDC thing, especially as I fly an aircraft with is primarily VDC but uses VAC for windscreen heating.

[Russdl]

Ah, the wonderful enlightenment of Buildhub.

 

We your advice on-board I phoned the DNO again, the existing supply can provide up to 23kVa so as Dave suggested

 

1 hour ago, ProDave said:

Just tell them 10KW

 

I've told them I'll need 10kW for the TBS which is just a figure they needed to put in a box before they can move forward.

 

That hurdle seems to be jumped. Thanks again all for your help as ever.

 

Russ.

[ProDave]

Have you considered doing what several of us have done. Don't get a temporary building supply (with a further cost to move it later) but instead make a permanent meter housing on a boundary and have the meter moved there, for good.

[Russdl]

@ProDave Indeed, that's exactly want I plan to do after seeing various BH members doing it.

 

I hope I've not stitched myself up by calling it a 'temporary building supply' when I was talking to the DNO, but I made it very clear that the temporary supply will be in a permanent position just inside the plot and the new house will be connected to the temporary supply at a later date.

[Jeremy Harris]

You should be OK, as the DNO don't care about it as far as their part of the supply is concerned, it's the supplier that comes to fit the meter that will distinguish between the TBS and the domestic supply. 

 

Be aware that even if you do as some of us have done and have a permanent meter box mounted remotely from the house, when it's connected as a TBS it will have to be wired with an earth rod using a TT earthing system.  They won't allow you to run the TBS as TN-C-S, using the combined earth and neutral on the incomer, but are OK with you switching the earth to the connection on the head and removing the earth rod as soon as the house electrical system has been signed off.

[Russdl]

@JSHarris Thanks for the re-assurance.

 

I'll make a note of all you've said especially the TT earthing system (when I've found out what that is) and make sure I don't run it as a TN-C-S (whatever that is!) ?

 

It's sometimes embarrassing to be this thick but I'm starting to get used to it ?

[jack]

I may be misremembering, but aren't there DNO limitations on how far the supply is allowed to be from the house without further protection (3m rings a bell)? Putting the temporary supply on the boundary doesn't sound like it'll achieve that (depending, of course, on how far the house is from the boundary). I have no idea what cost/complexity implications are associated with this.

[Russdl]

@jack Oh Lord! Just when I thought I was emerging from the woods. ?

 

Thanks for the 'heads-up' Jack, I'll check it out.

[Jeremy Harris]

30 minutes ago, jack said:

I may be misremembering, but aren't there DNO limitations on how far the supply is allowed to be from the house without further protection (3m rings a bell)? Putting the temporary supply on the boundary doesn't sound like it'll achieve that (depending, of course, on how far the house is from the boundary). I have no idea what cost/complexity implications are associated with this.

 

 

Yes, but it's easy to get around this, by fitting a DP fused switch (like this: https://www.tlc-direct.co.uk/Products/CGFS100.html) in the meter box.  You can then run 25mm² SWA from the fused switch to the house.  I fitted an 80 A fuse to ours, so that in the event of a major fault that will blow before the 100 A company fuse.

 

The advantage of fitting the isolator is that the meter tails only have to connect to it, leaving you free to be able to make wiring changes downstream as required later on, without having to break the seal and remove the (live) company fuse, although some makes of meter do now have a built in isolator, operated via a screwdriver slot on the front.

 

You need the fuse in the isolator switch to provide protection for the long supply cable to the house consumer unit.  For the TBS you can fit a weatherproof consumer unit connected to a weatherproof socket, close to the meter box.  I used a caravan supply weatherproof consumer unit, fed from a small garage consumer unit tucked in at the right hand side bottom of the meter box (to provide cable overload protection for the short weatherproof consumer unit cable).

[ProDave]

As Jeremy says DP switch fuse and you can run as long as you like subject to things like volt drop, disconnection time etc in BS7671

 

Don't forget also a static caravan should be connected to a TT earth as well.

[Jeremy Harris]

37 minutes ago, Russdl said:

@JSHarris Thanks for the re-assurance.

 

I'll make a note of all you've said especially the TT earthing system (when I've found out what that is) and make sure I don't run it as a TN-C-S (whatever that is!) ?

 

 

TN-C-S is pretty much what used to be called PME (the two aren't exactly the same) and is the most common earth system for new builds, except in very rural areas, perhaps.  In essence, the DNO provide a combined earth and neutral (CEN) at their termination, so that there no need to have a separate earth rod etc. 

 

TT is an earth system that doesn't have a CEN available from the DNO, just a line and neutral conductor.  The protective earth is provided by a local earth rod or plate. 

 

In general, few new installations use TT, usually only places that are at the end of a long supply cable, where the earth impedance of the DNO cable is too high.   TT is still used for the TBS, though, so you need to make sure there's an earth rod properly installed and terminated to provide the protective earth for the TBS.

[Russdl]

@JSHarris  Today is definitely a school day, I'm learning loads! 

 

30 minutes ago, JSHarris said:

Yes, but it's easy to get around this, by fitting a DP fused switch

 

I recall seeing pictures of your set up in your blog and I certainly plan to do something similar.

 

25 minutes ago, ProDave said:

you can run as long as you like subject to things like volt drop, disconnection time etc in BS7671

 

Now I'd better get on and find out more about that BS7671

[ProDave]

11 minutes ago, Russdl said:

 

Now I'd better get on and find out more about that BS7671

That is the UK wiring regs,

 

There are two things that will limit the cable length, one is allowable voltage drop, and the other is the maximum circuit impedance allowed for a particular size of circuit breaker which will affect the "disconnection time"  Put simply if the cable is too long and the impedance too high, then in the event of a short circuit, the circuit breaker or fuse might take too long to trip or might never trip.

 

Both are overcome by fitting a larger cable than the current alone may suggest is needed.

[Russdl]

@ProDave Ok. I'll obviously be getting an electrician on board (it would seem sooner rather than later would be the order of the day). My preliminary plan would have the new supply about 12m from plant room where the supply would emerge in the new build.

[ProDave]

No problem at all. I am about 25 metres from my supply box to the house.