How to 'distribute' a residential supply greater than 100 Amp

[Tas-T]

Hi all,

Firstly, this is not a DIY question as such. I am renovating and will have a proper electrician do the work, but he isn't around for a while and I want to do some of the design work and get a feel for where cables can be run.

What's completely new to me on this project is that when I add up all the ovens, hobs, and heaters it's clear that a 100 A supply is not going to be enough - more like 200 A. I will be arranging for the meter to be moved by UK Power Networks  in a few months and hopefully they will make sure the supply is adequate. That part I hope isn't a problem.

What I don't understand is how the larger supply will be distribute. All the residential consumer units I've seen handle a maximum of 100 amp. I was expecting to branch out from a primary supply to some 'sub' consumer units in a couple of locations to simplify cable runs and 100 A will be ample for those but what about the primary unit? What is used to serve as a primary distribution unit ?

Many thanks,

Tas

 

[Essex1]

Welcome,

what at is the scale of the building?  How many flats?  3 phase/1 phase?

 

[Tas-T]

Thanks for the reply. It's a large-ish 5 bedroom house. Current supply looks like a regular domestic 1 phase.

 

[shanky887614]

what sort of things? it seems to me that you might not be allowing for diversity.

most houses will never pull more than 20amps, i clampled the supply at my granparents before i did a board swap and they were pulling all of 4amps......

im in a 4 bed house and i can honestly tell you that with 2 x 10kw showers (40amps a piece aparently) an induction hob on a 40/50amp supply (cant remember which)

never tripped the supply here with both showers running, everything used on the hob, oven on and tv's+ lights on in every room  on xmas day

most things dont pull there rated current constantly for example an oven cycles on/off same as your boiler and a lot of other electronics and that is why we have diversity.

if you do need a bigger supply you will probably find its cheaper to get a tpn supply, i dont know of any 200amp double pole isolators for a start.

just think of the size of cable you would need for 200amps then think again if it is a good idea

 

[Tas-T]

I understand what you are saying but won't regulations require that the supply is capable of appliances demand?

There is:

4x 16A Ovens

2x 3A Ovens

1x 45A Hob

4x 16A underfloor heating

The ovens and hob are likely to all get used at the same time every now and again. They will all do a lot of work when you turn them all on to pre-heat.

 

[Manator]

Hi Tas and welcome to the forum. It is not clear in your question as to what you are intending to do with the property. You have mentioned various ovens and hobs, I would assume from that you are planning in converting a single phase domestic property into bedsits? flats?

You assume a consumption of nearer 200amps than 100amps that you have. Amps are a good indicator and probably easier to use for calculations, but is not what electrical designers will do for their calculations.

If using amps as a calculation method you have to utilise diversity, this means in effect that whilst all your total demand may be 100amps, it would be very unlikely that you will use this all at once. Electrical designers will sometimes calculate over run for limited time periods, utilising the characteristics of protection devises. For instance, an appliance that draws 60amps may run for only 10 minutes per day, and say a 45amp devise will allow that 60amp through for 18 minutes before tripping, the cost savings of using that calculation can be huge in larger installs.

Crude example but should give an insight into how electrical designs are done. (none of the data supplied is real)

Your electrician should be able to complete the form for you, I have completed quite a few over the years. They will want to know what the property will draw in kVA or kw, and not amps. The standard assumption is that single phase 100amp supplies are 15kVa, and three phase supplies are 45kVA.

If your electrician calculates say 18kVA then they will not upgrade the supply, the percentages will allow for this over run.

(apologies to Sidewinder for the incorrect Si values) 

 

[Essex1]

Diversity is the key. Chances are you will never use all parts of all ovens at the same time. 

This is is bread and butter for any electrician so leave the design to them. The sooner you get your sparks involved the less pain you will experience later on. 

 

[NozSpark]

The number, type and rating of circuits in a domestic installation has very little relevance to the maximum current that the installation will pull.

The number and ages of the occupants is far more relevant.

Imaging a bungalow with 3 circuits

30A cooker

30A sockets

5A lights

now imagine that this property has now been rewired with circuits (same occupants)

32A cooker

32A kitchen sockets

20A living room sockets

20A bedroom sockets

6A boiler

6A lights (kitchen)

6A lights (bedroom)

Now just because it's been rewired with the above new circuits does not mean that the new installation is now going to draw 120A as opposed to 65A previously

even using the standard diversity calculations is a joke,, but far closer to reality than just adding up the circuits

 

[Tas-T]

Many thanks to everyone for their contributions. A truly awesome forum.

First just to answer a question. It's going to remain a 5-bed single family house. 

The ovens are all in one kitchen. Yes, it seems like a lot and crazy as it sounds I have a similar set up in my current house and it does all get fired up at the same time when entertaining but a lot of it is gas right now. Anyway, what I am hearing from people is that even if fired up at the same time- in practice the appliances won't run at rated power. I get that. 

I also get the impression that people feel a 100 A supply is going to be more than enough for what I'm describing. 

If this is the case then I can assume regular consumer units will be used.

 

[Essex1]

Many thanks to everyone for their contributions. A truly awesome forum.

First just to answer a question. It's going to remain a 5-bed single family house. 

The ovens are all in one kitchen. Yes, it seems like a lot and crazy as it sounds I have a similar set up in my current house and it does all get fired up at the same time when entertaining but a lot of it is gas right now. Anyway, what I am hearing from people is that even if fired up at the same time- in practice the appliances won't run at rated power. I get that. 

I also get the impression that people feel a 100 A supply is going to be more than enough for what I'm describing. 

If this is the case then I can assume regular consumer units will be used.

Reguler consumer unit as opposed to????

 

[shanky887614]

The number, type and rating of circuits in a domestic installation has very little relevance to the maximum current that the installation will pull.

The number and ages of the occupants is far more relevant.

Imaging a bungalow with 3 circuits

30A cooker

30A sockets

5A lights

now imagine that this property has now been rewired with circuits (same occupants)

32A cooker

32A kitchen sockets

20A living room sockets

20A bedroom sockets

6A boiler

6A lights (kitchen)

6A lights (bedroom)

Now just because it's been rewired with the above new circuits does not mean that the new installation is now going to draw 120A as opposed to 65A previously

even using the standard diversity calculations is a joke,, but far closer to reality than just adding up the circuits

like nozspark mentioned.

the 4 bed im in is the same size as the 5 beds they are selling on the same estate the rooms in this one are just bigger.....

anyway in this property there are 4 lighting circuits all on b6 mcbs.

so that adds upto 24amps but i know for a fact it will only pull a max on 4amps over all the circuits with all the lights on.

most socket circuits pull practically nothing except the kitchen that has a kettle connected.

most modern tv's will only pull half an amp at most game consoles pull practically nothing as well for example a ps4 apparently pulls 150 watts the same as two incandesent lightbulbs.

ovens only pull current when they are first turned on to get upto temperature then they hardly pull anything.

ive just pulled up a random oven on curries its advertised as 13amp supply needed when you break it down it has a 2kw grill (8.6amps) 0.7kw oven (3 amps) now both wont be running at once.

do you see now why i said an oven wont pull much, ovens and hobs usually add all the seperate parts max draw together, you wont have the oven and grill on at the same time

 

[Essex1]

I do not think the op is actually thinking a 32 amp circuit is pulling 32 amp and a 6 amp circuit is pulling 6 amps etc etc. 

I think he has looked at the maximum demand the ovens can ask for and is coming from that point of view. 

 

[Tas-T]

Reguler consumer unit as opposed to????

I meant that it sounds like we will use the type of units I'm familiar with and see on sale in B&Q, as opposed to something special which I though I might need.

 

[Blue Duck]

Was working on a job today with a contactor for the shower so that it doesn't melt the 63a RCCB covering the shower, sockets and the cooker cct.

Like thats gonna happen....

This was 2 showers and a cooker on a 60a supply, the showers weren't on long enough to blow the fuse but over time but melted the undersized supply cable.

 

[SPECIAL LOCATION]

Most of the current consumer units on sale in B&Q do not comply with current wiring regulations.

Is this property just going to be used by a single family doing average cooking at normal mealtimes?

Or, as I have seen previously, someone basically running a small cooking business from their home..

e.g. cakes / meals and have the kitchen running solid nearly 8 hours a day 5 days a week.

opcorn  

 

[Essex1]

I meant that it sounds like we will use the type of units I'm familiar with and see on sale in B&Q, as opposed to something special which I though I might need.

This is a very basic design. A first year apprentice could design this. Engage with your electrician and he will be able to do this in 30 minutes and won't cost you a penny if you are using him anyway. 

 

[Tas-T]

ive just pulled up a random oven on curries its advertised as 13amp supply needed when you break it down it has a 2kw grill (8.6amps) 0.7kw oven (3 amps) now both wont be running at once.

do you see now why i said an oven wont pull much, ovens and hobs usually add all the seperate parts max draw together, you wont have the oven and grill on at the same time

I think so. So is the power rating for an oven generally accepted by electrician to be OTT? I know that some manufacturers do use the grill element to speed up pre-heat. Meile seem to think their ovens draw a lot of current. Does this get factored in or is everything just so over-rated that it's rarely an issue?

I understand what people are saying about lighting and socket circuits. But UF Heating, Ovens and powerful induction hobs - I thought I should be considering them as more constant. Sounds like that isn't the case.

 

[Manator]

Tas, the only time I have been asked to design an electrical distribution like this before is for a Jewish gentleman, well actually a well respected Rabbi.

He had two kitchens, two wash rooms, two laundry rooms and separate food preparation rooms. Everything was kept separate and nothing was allowed to come into contact with any other parts.

His total demand did not suddenly rise because of the installed appliances. Make sure you use a reputable electrician, they will know what you need, what you require and give you a suitable solution that should not cost you an arm and a leg.

 

[shanky887614]

I think so. So is the power rating for an oven generally accepted by electrician to be OTT? I know that some manufacturers do use the grill element to speed up pre-heat. Meile seem to think their ovens draw a lot of current. Does this get factored in or is everything just so over-rated that it's rarely an issue?

I understand what people are saying about lighting and socket circuits. But UF Heating, Ovens and powerful induction hobs - I thought I should be considering them as more constant. Sounds like that isn't the case.

ok for example we regually have 2 x 40 amp showers on, 40amp induction hob, oven and loads of electronics on a 100amp supply with no problems.

besides the dno fuse wont magically blow at 100amps a lot of older properties still have a 60amp fuse with induction hobs and electric showers etc

 

[Essex1]

ok for example we regually have 2 x 40 amp showers on, 40amp induction hob, oven and loads of electronics on a 100amp supply with no problems.

besides the dno fuse wont magically blow at 100amps a lot of older properties still have a 60amp fuse with induction hobs and electric showers etc

Shanks. Are you saying that 'regularly' you have both showers on full heat, the hob using all rings, oven, grill at the same time Plus all your electronics on full all at the same time?