There can be the odd exception for both houses and for commercial use, But in the vast majority of cases, single phase means a maximum of 100 amps.

Some DNOs will under some conditions allow 125 amps single phase, but this is the exception and not the norm.

If 100 amps or exceptionally 125 amps is not enough, then 3 phase is the norm. If three phase is not available, then single phase 3 wire, often called split phase is a possibility.

In extreme circumstances the only options  may be, move, use a generator, or switch the larger loads to solid fuel, oil, or gas. That is rare.

Why would a commercial user not want 3-phase?

broadgage said:

then single phase 3 wire, often called split phase is a possibility.

Careful how you handle this, though, as U = 460 V not U = 400 V (Usually OK for cables, but products or assemblies containing or connecting to circuits from both phases can be interesting).

One reason to avoid three phase is for premises open to the public, and subject to local authority inspection.

Some local authorities have an irrational fear of three phase. I recall a case in which a busy pub installed an outdoor, 3 phase, 16 amp ceeform socket outlet for an ice cream machine.

The local authority were horrified that a "red for danger" socket outlet that used "high voltage" had been installed without consultation, in an area used by families. It was removed rather than enter into an argument.

I have also heard the objection made that the mating plugs "do not contain safety fuses"

broadgage said:

The local authority were horrified that a "red for danger" socket outlet that used "high voltage" had been installed without consultation, in an area used by families.

Well, I can see this argument ... but it would also apply to 16 A single-phase BS EN 60309-2 socket-outlets (among others) ... i.e. not shuttered.

Cue discussion on whether an outlet with an interlocked rotary "isolator" would do ... yes, but only if the isolator was actually an "isolator" - and all the BS EN 60309-4 products I've come across so far are not marked with the "isolator" symbol and therefore only contain interlocked switches, not interlocked isolators.

Thanks for the answer is basically that if possible the DNO will give a 125 amp supply that's more than enough for the load  in question 

I recently observed the following Kelly:

A ground floor shop with a three phase supply at floor level. A red link supplying a vertical earthed concentric cable running upwards to the flat above. In the flat is a single phase  100 Amp. cut out and metering. The flat has six off peak storage heaters, no gas. The on peak consumer unit supplies a shower, ring final circuit, immersion heater and cooker plus lighting.

I can imagine that on occasions the main fuse supplying more than 100 Amps. If just the six storage heaters turn on at once the total load could be about 80 Amps. The immersion heater will also come on at off peak times (including afternoon boost times) and its load will be added to the storage heater load. . The main fuse has not blown, and all the cables are in good condition with no signs of overheating or damage. The main tails are 35mm2 to a D.P. Henley block from which they reduce down in size.

Z.

Indeed, a 100A fuse is a fuse for a 100 amp load 24/7, much as a 13A fuse is a fuse for a 3kW load. Given that it must never melt at this current, it is quite capable of carrying considerably more for short periods. And house cut-out fuses are deliberately made 'slow blow'.

This set of curves is for BS1361 fuses, as seen in some cut-outs, although  it does not include a line for 100 devices. (time in seconds vertical, current in amps horizontal).

Note that even the 60A one will pass 100A for more  something like  20 minutes and 200A for a minute (the top of the graph is 1000 seconds, between 15 and 20 mins. )

Now this is fine, because the main function of the cut-out fuses is to protect the cables in the street, and perhaps the meter tails, and these too heat up over a long timescale, tens of minutes to hours.

It may not be great design, but it is certainly possible to have loads that draw rather more than 100A on a 100A supply, so long as they only do so until the oven thermostat trips off, or for a ten minute shower. ;-)

However, loads that really are on all day - like an electric car charging - are a totally different proposition and  eat into this margin and are a far more serious consideration both for the house wiring, and that in the street.

After all, the typical 500KVA substation feeding up to 70 houses per phase, fused  with perhaps a one 800A or pair of 400 A fuses per phase depending if it is in  the middle of  a street or at one end  is also sized based on having plenty of  time to cool off between short duration overloads.

But in short, the larger supply fuse is not always needed, but sometimes a better understanding of the type of load.

Mike

Thankyou all for your replys. I think il advise my customer to contact the DNO to ask about an upgrade to a split phase supply just to be absolutely sure all well either that or sell him some candles!!

Split phase is more of a rural thing, usually because the 11 kv supply is only two wire and three phase is not available.

In most urban areas, the alternative to single phase is almost certain to be three phase.  There are a handful of exceptions even in urban areas, parts of West London still have side streets with split phase only. 

Anyone in the area needing three phase will have to pay for a new cable from the substation, install converting plant, use a generator, or move !