Yeah minimum circuits it a good way to do it sensibly. The heaters will be oil filled electric radiators, about 8 of them around the house, either 800W or 2kW each (hasn't been decided yet). Worst case scenario but my main concern is if its cold in winter and the electric heating radiators are all put on (25A or 60A), then you have a shower (44A), someone is in the kitchen cooking (17.5A) and uses the hot tap to wash up (24A).

Just simple oil-filled rads? That feels like a odd choice choice for primary space heating (both economically and EPC wise) unless the place is very well insulated (in which case you're not likely to need anything like that total output).

Worst case isn't just about how much is switched on at the same moment - there's a time element in there too - as both cables and the thermal side of protective devices take a significant length of time to warm up. Put 120A through a 100A fuse and 100A cable starting from cold and it'll probably take an hour or two even to reach normal operating temperature, let alone start to overheat (which is why we can put 19kW cookers on a 32A circuit even though the chief can easily turn everything on at the same time initially). For smaller cables the time interval is a lot shorter for the same percentage overload, so more care is needed, but as things get larger, a few tens of amps that lasts for only a few tens of minutes becomes a lot less of a worry.

Also thanks for the minimum circuit tip, do you mind saying the other method of calculating maximum demand you mentioned or what you usually use?

There are several approaches - in many commercial situations I understand people often use standard Watts per sq m of floor space (for lighting, cooling and heating) and ignore circuit layout all together. In other situations, experience and/or measurements from "similar" units elsewhere can provide reasonably reliable numbers. For space heating I like to look at the building's heat loss figures - generally you can't be chucking more power into the space heating of a building that it can loose otherwise it'll get unbearably hot. Obviously it'll take more power when trying to heat up the building from cold - but that should only be for relatively short periods - maybe longer than it takes a cable to warm up - so not something you can ignore completely - but still nothing like as onerous as having to support those kinds of power levels for infinite time (which is what most BS 7671 calculations assume, to cover the worst case). Other than possibly hot water, almost all other uses of electricity will end up as heat that will contribute to warming the building to some extent, so there may be some double-counting there that you can claw back on (e.g. what is used by lighting or portable appliances will reduce the space heating load by an equivalent amount).

   - Andy.

Yeah minimum circuits it a good way to do it sensibly. The heaters will be oil filled electric radiators, about 8 of them around the house, either 800W or 2kW each (hasn't been decided yet). Worst case scenario but my main concern is if its cold in winter and the electric heating radiators are all put on (25A or 60A), then you have a shower (44A), someone is in the kitchen cooking (17.5A) and uses the hot tap to wash up (24A).

Just simple oil-filled rads? That feels like a odd choice choice for primary space heating (both economically and EPC wise) unless the place is very well insulated (in which case you're not likely to need anything like that total output).

Worst case isn't just about how much is switched on at the same moment - there's a time element in there too - as both cables and the thermal side of protective devices take a significant length of time to warm up. Put 120A through a 100A fuse and 100A cable starting from cold and it'll probably take an hour or two even to reach normal operating temperature, let alone start to overheat (which is why we can put 19kW cookers on a 32A circuit even though the chief can easily turn everything on at the same time initially). For smaller cables the time interval is a lot shorter for the same percentage overload, so more care is needed, but as things get larger, a few tens of amps that lasts for only a few tens of minutes becomes a lot less of a worry.

Also thanks for the minimum circuit tip, do you mind saying the other method of calculating maximum demand you mentioned or what you usually use?

There are several approaches - in many commercial situations I understand people often use standard Watts per sq m of floor space (for lighting, cooling and heating) and ignore circuit layout all together. In other situations, experience and/or measurements from "similar" units elsewhere can provide reasonably reliable numbers. For space heating I like to look at the building's heat loss figures - generally you can't be chucking more power into the space heating of a building that it can loose otherwise it'll get unbearably hot. Obviously it'll take more power when trying to heat up the building from cold - but that should only be for relatively short periods - maybe longer than it takes a cable to warm up - so not something you can ignore completely - but still nothing like as onerous as having to support those kinds of power levels for infinite time (which is what most BS 7671 calculations assume, to cover the worst case). Other than possibly hot water, almost all other uses of electricity will end up as heat that will contribute to warming the building to some extent, so there may be some double-counting there that you can claw back on (e.g. what is used by lighting or portable appliances will reduce the space heating load by an equivalent amount).

   - Andy.

Thanks for the info. Yes oil-filled rads, seemed the best choice for an electric system, or ceramic. They heat up and cool slowly like normal water rads. The internal front and rear of the property, loft and maybe under ground floor will be insulated with 50mm PIR. Solid brick walls. TBH this is my own property. Maximum demand is something that I've questioned a few times on other jobs though. I saw the benefits of electric heating being efficiency, not relying on one boiler, independent thermostat and on/off on each rad, can install myself, no gas / carbon monoxide, minimal water pipes / leaks, no yearly boiler service, easy to add to, quiet, phasing-out of gas and can power from solar and batteries in the future. Only issue could be bills cost but with the insulation and independent thermostats I'm thinking it should be the same as gas or hopefully cheaper.

The question though, if this was your job, would you be upgrading to 3 phase or do you think single phase 100A will be safe with these kind of loads?

Given the cost, and its your own place..
Organize things so that a change to 3 phase in future would not be too painful - leave room for 3 boxes or a 3 phase  box, but leave it on all on one phase for now.

If it ever blows the company fuse then only after the 2nd time you need to call the DNO to change the it, book the upgrade..

My money is on it never blowing...
Mike.

Jimi said:

TBH this is my own property.

I can think of three reasons for upgrading to 3-phase:

(1) you want to run 3-phase machinery;

(2) you want a fast EVCP;

(3) get it while you can.

One further thought: if you get a 3-phase supply, it does not mean that you have to use all three of them. You could install a 3-phase switch-fuse and then take just one phase (for now) to your CU.

Thanks