Threshold for 3-phase supply

[endecotp]

Dear Experts,

I've just had a conversation with a friend who has acquired a 12-bed guesthouse - bought at auction at a bargain price and in need of some renovation. (He has done similar projects in the past.)

He started by telling me that his electrician has told him that "new laws" mean that all hotels, however small - and even houses rented as holiday lets - now need to have a 3-phase supply. This didn't seem likely and triggered my "is this a scam?" reaction. But then he went on to say that this place has 6 electric showers (though no electric cooking or heating). So I guess that maybe the electrician has looked at it and correctly determined that it needs a supply upgrade, the showers perhaps having been DIY-installed inappropriately. It currently seems to have what looks like two domestic-type single-phase meters (apparently it was subdivided at some point, maybe one is redundant).

So a few questions for anyone with experience of this scale of installation:

- What is the typical threshold (in Amps?) at which a supply would be 3-phase, rather than single-phase?
- If the supply from the road needs to be upgraded, is laying a 3-phase cable significantly more expensive than laying a larger single-phase cable?
- Is some sort of diversity calculation appropriate for determining the required supply capacity to supply 6 electric showers?

The property does have gas, and I'm surprised that it has these electric showers rather than a large gas-heated hot water cylinder. But that's another topic...

Thanks.

 

[JohnW2]

- What is the typical threshold (in Amps?) at which a supply would be 3-phase, rather than single-phase?

I don't think a DNO will ever provide a single-phase supply greater than 100A

- If the supply from the road needs to be upgraded, is laying a 3-phase cable significantly more expensive than laying a larger single-phase cable?

As above, I doubt that "a larger single-phase cable" is likely to be an option, but common sense suggests that the nature of the cable being buried should not have much impact on work/price. What does matter is what is 'under the road' and whether the existing distribution network has capacity for what is required. If not, one could be talking 'very serious money'!

- Is some sort of diversity calculation appropriate for determining the required supply capacity to supply 6 electric showers?

Probably, yes, particularly in a single-family residential property. Other situations are probably different - e.g. in a hotel, or even HMOs etc, multiple people showering simultaneously would not be uncommon. Just two electric showers simultaneously would use most of the largest available single-phase supply, and more than two would be 'over the top'.

 

[ericmark]

The need for three-phase is to run three-phase motors, one can have a 300 amp single phase supply or 3 x 100 amp supplies and the result is nearly the same, except not need for balancing with a single phase supply.

There is also the split phase supply, with is single phase 460 volt centre tapped.

The recommendation for the ordinary person is that the supply is from a consumer unit, the maximum rating for a consumer unit is 125 amp single phase, I have never seen one over 100 amps, but this means we have two conflicting recommendations. So it depends on what you read as to single, or three-phase, basic it seems is three-phase needs some sort of plant room. Which needs a key or tool to access it. So there is no definitive answer.

Three-phase is being fitted for EV charging, but the question is who is allowed access? It needs a tool to remove a cable tie, so even a cable tie through the locking hole is likely enough.

 

[JohnW2]

The need for three-phase is to run three-phase motors, one can have a 300 amp single phase supply or 3 x 100 amp supplies and the result is nearly the same, except not need for balancing with a single phase supply.

In theory, yes - but, as I said, in practice I don't think any DNO will provide a single-phase supply greater than 100A

... so, when people talk/ask about 'needing a 3-phase supply' I think that, in practice, they are effectively asking whether they need a supply of more that 100A (single-phase).

 

[flameport]

his electrician has told him that "new laws"

There are no new laws about 3 phase or any other electrical installations.

- What is the typical threshold (in Amps?) at which a supply would be 3-phase, rather than single-phase?

100A in most cases. Single phase above 100A does exist, but is unlikely to be offered or even be available.
In the last 20+ years I have seen one, which was a single phase 200A service for a small B&B type place.
3x100A would be the usual starting point. 3x200A may be an option, 3x400A is common.

acquired a 12-bed guesthouse

3 phase required without any doubt.

, is laying a 3-phase cable significantly more expensive than laying a larger single-phase cable?

No, most of the cost is the installation which is identical.

Is some sort of diversity calculation appropriate for determining the required supply capacity to supply 6 electric showers?

Perhaps, but the real answer is that 6 electric showers is entirely the wrong choice for a guest house.
Stored hot water is the only real solution, and surely there would be 12 showers for a 12 room emporium?

no electric cooking

There will be some. Microwaves, salamanders, water boiler, toasters, etc.

 

[SUNRAY]

As John says single phase over 100A does exist but the DNO would probably decline to fit it, in favour of 3ph. I've known it once, for a small farm 50+years ago when I was an apprentice and it had been supplied via one of our BT poles below the cross arms (I take it everyone knows what cross arms were on telephone poles) and we were unable to access the open wires to test/make a repair.The land owner told is in detail how the original feed from the other direction fizzed and sparked when it failed.

 

[JohnW2]

.... (I take it everyone knows what cross arms were on telephone poles) ...

I do - but we used to call them telegraph poles, not telephone poles, didn't we?

 

[endecotp]

Thanks everyone for your replies. My fear is that the previous owner has left the property in a very dubious condition.

Regarding single vs. 3–phase supplies, the tenement in which I live has 16 flats all on the same phase. So I am a little surprised that a single property is not generally allowed to have more than 100A (or so) on a single phase supply.

I think that my friend’s choice will be between upgrading the electrics to suit multiple electric showers, or upgrading the gas hot water to supply 6 (or more) mixer showers.

I suspect that he will choose whatever is the cheapest option!

 

[Collectors]

Easy answer is get the local DNO to give a quote. Guarantee it wont be cheap. I can remember some nice guys that worked for the lecy company's that moved & upgraded many single phase mains for us at a 1/4 of the price of the DNO. They charged us £400 20+ years ago. Hate to think what it would be now days. Oh! buy the way, they got court, but not sacked. But no more private jobs.

 

[plugwash]

- If the supply from the road needs to be upgraded, is laying a 3-phase cable significantly more expensive than laying a larger single-phase cable?

In general, a 3 phase cable will be cheaper than a single phase cable of the same power capacity

Also domestic/small commercial intake and metering and distribution gear tends to be rated at a max of 125A, and often less. If you need more than that you are getting into the world of CT metering and large commercial/small industrial switchgear which is substantially more expensive.

There may be exceptional situations where installing a larger single-phase or split phase supply makes economic sense, but I'd expect them to be very much the exception.

 

[Murdochcat]

OP - the only way for your "friend" to get the correct information is to talk to the DNO - not random people on the internet.

FWIW I think they will need a 3 phase supply

 

[ericmark]

OP - the only way for your "friend" to get the correct information is to talk to the DNO - not random people on the internet.

FWIW I think they will need a 3 phase supply

Spot on, there is no real other option. Sometimes we do get odd supplies, at work (heritage railway) we have a building with 4 x 3 phase supplies, only used as a store, but it would cost so much for them to be removed, they have been left, and they are all TN-C-S and the building connects to others with steel air lines, so to connect all the earths together is not what should really happen, but all the railway buildings likely connect together there is 8 mile of railway line. So loss of PEN would go unnoticed.

But if the DNO have a single phase supply, that's what will be offered, if they have a three-phase supply that will be offered.

With 12 bedrooms, you can't afford loss of power, so you want showers powered from stored water, once the load from showers is removed, then the load for each guest is not really that high, not as if they all have their own cookers, washing machines, tumble dryers, and dishwashers. So, likely one could work on a centre tapped 460 volt 100 amp single phase supply without a problem, which is the supply given to many farms.

But since the supply type will change how wired, and if instant water heating is used, that needs to be sorted early in the renovation.

 

[JohnW2]

My fear is that the previous owner has left the property in a very dubious condition.

Very probably, but isn't that what your friend expected, given ...

..... I've just had a conversation with a friend who has acquired a 12-bed guesthouse - bought at auction at a bargain price......

... it must surely have been obvious that there was a reason for the "bargain price", but it sounds as if your friend may not have done enough (or anything!) in terms of looking into details of the deficiencies (and the likely cost of remedying them)>

 

[SUNRAY]

.deleted double post

 

[SUNRAY]

I do - but we used to call them telegraph poles, not telephone poles, didn't we?

OK I'll go with that but as BT staff it was 'telephone pole' or just 'pole'