Increase supply current?

[camngnp223]

Hi everyone!

Sorry for what might be useless questions, but I'm just starting out with my electrician courses.

My mum lives in a flat and wants to add a 10.5 kW electric shower which would draw about 44 amps. She already has underfloor heating that uses a maximum of 44 amps (10,590 W) as well as all the other stuff (including sockets, lights, immersion heater and cooker) which, at worst, might draw up to a further 50 amps.

At the moment, the underfloor heating is supplied from the meter by a 10mm cable. The rest of the house is supplied by another 10mm cable.

An electrician has advised that a third 10mm cable from the meter would be needed for the electric shower. However, he has also said that she would need to get in touch with her supplier (EDF) to increase the supply current upwards from the current 100 amps.

Mum has asked me for advice but, as I'm just starting out, I really couldn't say much for sure.

My questions are:
(1) Will the supplier agree to swap out the 100 amp fuse (and any inadequate cable) to increase the supply to 150 amp or more? Will this cost a lot?
(2) After the meter, will she need to have three separate 60 amp switch fuses (one for each of the three 10mm cable feeds)?
(3) Will she need three consumer units in her flat? (1 split load unit for the general lighting and power, 1 rcd-protected unit for the underfloor heating, and 1 rcd-protected unit for the electric shower.)

My guess is that the answer should probably be "yes" to all three of the questions above but I'd be very grateful for any advice or extra information you might be able to give me.

Thanks a million!

Cam

 

[breezer]

diversity

 

[ban-all-sheds]

AFAIK the answers to (1) are "No", and "No, because they won't do it".

If the supply is 100A then I don't think the installation will trouble it - e.g. will she use the cooker and the shower at the same time?

 

[camngnp223]

Great: thanks for the answer to question 1!

If the supplier won't increase the supply current, what sort of switch fuse would be needed then? Just one 80 amp switch fuse that all three 10mm cables run out of? (The underfloor heating at 44 amp and the shower at 44 amp might well get used at the same time. Won't this be a problem?)

I can definitely tell mum not to cook while she's in the shower!

Huge thanks,

Cam

 

[ban-all-sheds]

At the moment, the underfloor heating is supplied from the meter by a 10mm cable.

From where?

The rest of the house is supplied by another 10mm cable.

From where?

An electrician has advised that a third 10mm cable from the meter would be needed for the electric shower. However, he has also said that she would need to get in touch with her supplier (EDF) to increase the supply current upwards from the current 100 amps.

10mm² is way too small to be protected by a 100A fuse, let alone an even bigger one.

Or even an 80A one.

Whoever this electrician is I advise you to stop listening to him and to listen to your lecturers instead.

(2) After the meter, will she need to have three separate 60 amp switch fuses (one for each of the three 10mm cable feeds)?

No - she will need zero switchfuses, and no lengths of 10mm².

(3) Will she need three consumer units in her flat? (1 split load unit for the general lighting and power, 1 rcd-protected unit for the underfloor heating, and 1 rcd-protected unit for the electric shower.)

No - she will need 1 CU for the lot.

If the supplier won't increase the supply current, what sort of switch fuse would be needed then? Just one 80 amp switch fuse that all three 10mm cables run out of?

You started out concerned that a 100A supply would be inadequate, and now you want to reduce it to an 80A one if you can't get more than 100A?

How does that work?

 

[Stoday]

AFAIK the answers to (1) are "No", and "No, because they won't do it".

BAN — If you read the Utilities Act 2000, 16A, (2) (c) you will see that it's up to the consumer to state what maximum power he requires.

Of course, the DNO will recover up front the full cost of increasing the connection capacity to meet this requirement.

 

[Steve]

As far as I know, the only upgrade from a 100 amp single phase supply is a 100 amp 3 phase supply. Unless your out in the sticks or something where they may give you 2 phases or some combination.

A conversion to 3phase might not be too much trouble since you're in flats where the building probably has a 3ph supply. But the DNO will charge stupid amounts for it.

It sounds like the metering equipment is remote to the flat, so it is supplied by submains (one for the UFH, one for main supply)

 

[Taylortwocities]

Lets take a step back.
Why does your mother want an instant heat shower? If there is hot and cold water already in the bathroom then why not just install a shower pump to boost the water flow.

 

[ban-all-sheds]

AFAIK the answers to (1) are "No", and "No, because they won't do it".

BAN — If you read the Utilities Act 2000, 16A, (2) (c) you will see that it's up to the consumer to state what maximum power he requires.

Of course, the DNO will recover up front the full cost of increasing the connection capacity to meet this requirement.

I didn't think that going for a standard 3-phase supply, or anything not available as standard from the existing street LV supply made any sense whatsoever.

The question "Will the supplier agree to swap out the 100 amp fuse (and any inadequate cable) to increase the supply to 150 amp or more? Will this cost a lot?" was clearly asking about sticking with the existing SP supply simply uprated to 150A+.

 

[camngnp223]

Thanks so much for all the advice! The consensus seems to be that the NDO would not be likely to increase the supply current. It's great to know this and I'd love to be able to give Mum some suggestions on how she can get around her shower problem.

Just to clarify the details of the system, the consumer unit in the flat is fed by two submains (each made up of twin and earth 10mm) which originate in the communal meter cabinet. One of these 10mm submains runs from the meter to a 60amp switchfuse located in the communal cabinet before running into the flat. The second 10mm submain goes directly to the flat without first passing through a switchfuse located in the communal cabinet. (Incidentally, because the length of this second submain is more than 3 m, I suspect that the absence of a switchfuse near the meter is a violation of code.) The first 10mm submain runs to a 12-way consumer unit in the flat that powers most of the household. The second submain runs to a smaller consumer unit that powers only the underfloor heating.

The situation I just described is the one that Steve correctly suspected: I'm sorry that I didn't give more info up front. (Sorry especially to ban-all-sheds!) It also seems like most of you think that a third 10mm submain dedicated to the electric shower might not be the right/necessary solution.

The electric shower is definitely going to be installed: the flat is fairly big and the (already large) immersion heater can't cope with the demand for hot water. Mum is willing to have the whole set-up reworked so that she can have everything (electric shower, underfloor heating, and main supply) running off a single consumer unit. I hope you won't mind that I have a couple of follow-on questions:

(a) At the moment, each of the 10mm submains (sheathed twin and earth p.v.c.-insulated cables) is rated for a maximum of 63 amps each. Both of these two submains wouldn't fit into the main switch of a single consumer unit. Should these submains be replaced with a single 16mm cable which is rated for a maximum of 85 amps? Or isn't this necessary?

(b) If she keeps both submains, should she have her electrician install another 60 amp switchfuse at the meter cabinet for the submain that is currently unprotected by one? Also, is there any way to fit both 10mm submain feeds into a single consumer unit in the flat?

(c) Alternatively, if you advise to upgrade to a single 16mm submain, should she also upgrade to an 80 amp switchfuse in the meter cabinet?

Huge thanks for your patience and your very, very helpful advice!

Cam

 

[JohnD]

How big is the hot-water cylinder? Does it have one immersion heater, or two? How is it insulated? How many people have baths or showers within a couple of hours of each other? Is the cylinder heated by immersion heater only, or does a boiler also heat it?

If you put a bucket under the kitchen cold tap, and fill it against time, how many litres per minute does it deliver?

Is there a gas supply to the house?

 

[camngnp223]

Thanks for your response re gas/alternative immersion heaters. For now, I'd really like to focus on how a fully-trained electrician might go about wiring in the added electric shower at the consumer unit and via the submains from the meter cabinet. (I'm sure that there must be a solution and I'd kill for your good advice!)

Cam

 

[skenk]

I know diversity is subjective but the OSG states 100% of full load of the largest shower in an installation and no diversity allowable for under-floor heating.

The 2nd 10mm cable should be fused down it should not be connected direct to the supply

Better, i'd have thought, to have one switchfuse at 100A fed with 25mm tails and feeding a single CU, again with 25mm tails.

Have a search for '2 showers'

<edited for clarification>

 

[Steve]

I would:

Keep the 10mm² feed for the UFH, fed via a 63A switchfuse.

Take out the 10mm² feed for the flat, and replace with 16mm² or 25mm² cable of some variety, feeding the main CU. Take the shower feed from this CU.

16mm is the largest available in T+E, but I would be more inclined to go with 25mm² on 100A switchfuse. This would either be tails, or SWA (preferred, but much harder to work with)

 

[flameport]

(a) At the moment, each of the 10mm submains (sheathed twin and earth p.v.c.-insulated cables) is rated for a maximum of 63 amps each. Both of these two submains wouldn't fit into the main switch of a single consumer unit. Should these submains be replaced with a single 16mm cable which is rated for a maximum of 85 amps? Or isn't this necessary?

The installation should have a single point of isolation. For flats with remote meters, this is usually acheived with a switchfuse after the meter, and a single cable to the consumer unit (or isolator) in the flat.
The present arrangement with one of the 10mm T+E direct into the meter is unacceptable, since it is then only protected by the suppliers fuse.
Even the 10mm on the 60A fuse is at the limit, assuming it is surface clipped - any other method of installation is unacceptable.

(b) If she keeps both submains, should she have her electrician install another 60 amp switchfuse at the meter cabinet for the submain that is currently unprotected by one? Also, is there any way to fit both 10mm submain feeds into a single consumer unit in the flat?

No and no. The best solution will be to dispose of both the 10mm submains and replace with a single 25mm SWA, supplied from a switchfuse next to the meter.

Twin and earth is not suitable for use in this way. Parallel submains are best avoided, and using larger sizes of T+E as submains is a poor solution as the earth is likely to be undersized. There is also the issue of where this T+E is routed - is it on the surface or concealed in walls/floors? Does it pass through any insulation? What length is it?

(c) Alternatively, if you advise to upgrade to a single 16mm submain, should she also upgrade to an 80 amp switchfuse in the meter cabinet?

25mm and 100A would be more like it.

If the total loading is an issue (and it could well be), some arrangement to prevent the underfloor heating and shower being used at the same time is one solution. The short time the shower is in use will not make any noticable difference to the heating.