Would it need a larger supply, then, probably no.

People have lived in typical houses with all electric heating and cooking for 70+ years with no detrimental effects on the incoming 60 or 80 amp fuse.

The actual loads used are nothing like the figures you have used for diversity, if lighting was using 8 amps, they'd be running a 24 hour office environment. The total wattage for lighitng in my house will be around 130 watts, less than an amp, and that use is for less then 6 hours a days for individual rooms, so if averaged out, a load of <5 watts for 24 hours.

Sockets are the same, rated at 20 or 32 amps, but actual use is very low. The biggest draw in most houses will be the kettle. Ovens are intermittent, even electric heating is intermittent use. Storage heaters draw a large (10 amps?) load at start up, but this is usually when nothing else is being used, so not a large load overall on the system.

Water heaters, where is the 45 amps from - a shower? Intermittent use. Immersions are only on for 2 hours from cold, so again, limited intermittent use.

I did a diversity calculation for my house it came to 140A, no electric heating. I then added a 9KW pool heater which ran flat out for weeks at a time, thankfully long since gone. It ran OK on  single phase 60 A fuse on the submain from the garage. When replacing the TP board in the garage I also replaced the switch fuse to the house and it was obvious the 60A fuse had been hot at some point but had not blown or melted anything. Despite ordering and paying for a 80A per phase supply I found out years later the supply was fused at 60A per phase.

Yeah, lights and sockets are realistically lower, but if its cold in winter and the electric heating radiators are all 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). That's probably the worst likely case and the true maximum demand. I'd prefer to keep it single phase but obviously it needs to be correct and safe.

Basing things on circuits when the circuits are over-sized for the actual application will naturally give over the top results - e.g 20+20+32A = 72A  could serve a floor area of around 240m² - over two normal size houses. Ditto 6A circuits for a could of hundred watts of LED lighting. If you want to do it circuit wise (there are other methods) then base it on the minimum size/number of circuits that could have been installed to serve the actual loads. We're only worried about maximum demand at this point, not minimising inconvenience or optimising cable runs.

What's the nature of the "25A or 61A heating" - is it off peak or 24h space heating? - 14kW feels rather a lot for heating even a fairly poorly insulated house if it could be on constantly.

   - 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).

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?

The current cost of electricity for 24 hours is about £2 per amp so to get anywhere near to that consumption would require a pretty prosperous householder.

It's just the maximum demand figure though in the worst case scenario. In reality things are used on and off. Each rad has it's own thermostat and can be turned on and off, unlike a gas boiler where the boiler has to work even if only 1 rad is on.

Jimi said:

unlike a gas boiler where the boiler has to work even if only 1 rad is on.

Yes, but then it will short cycle - it won't be firing continuously.

Diversity is probably more useful for single circuits rather than a whole installation.

Another thing to remember is that even when the fuses is 100 A, the DNO has probably agreed a maximum demand which is less than 23 kVA.

And of course the DNO sizes the transformer/ substation on about 4 amps per house, because so long as there are enough houses in the street that so that one is off when others are on, that total 'after diversity maximum demand'  is unlikely to be exceeded long enough to set fire to anything - though it must be noted that short term (less than an hour) 100% overload (i.e. twice design current) is not totally unknown in some 'hotspots' .
Of course if in future lots of houses have cars to charge or install heating like this, then the picture changes a lot and assumptions may need revision.
Mike