3-phase mess

[Jon Connell]

I've just bought a new house (well, it's an old house) and the property has 3-phase coming into it, but when it was renovated in the 90s, a single-phase meter was installed and they didn't bother to get the supply fuses upgraded. So I basically have a 60A supply feeding two consumer units off a single meter. All horribly insufficient for the new kitchen that includes a 30A induction hob, and for the 30A hottub in the garden. Add in a couple of ovens and an immersion heater or two and I can foresee the mains tripping regularly.

Anyway, I am trying to untangle the mess that is the ownership of the various bits of copper between my consumer units and the power network and whilst Scottish Power are gamely trying to help, it's clear that the combination of their little knowledge and my little knowledge is not leading to a good conclusion.

What I want to achieve is retaining the two consumer units for the two "halves" of the house, but have them upgraded to something a bit more realistic (>= 120A) so that the house doesn't go dark if the oven/hottub/hob are all going at full tilt. So I *believe* that I can reinstate the 3-phase with a 3-phase meter, and use 2 of the 3 phases with each of these phases running to their own consumer unit.

No doubt uk.d-i-y has people who understand this far more than me, so advice is greatly appreciated, but so is a pointer to somebody who can simply come and take a look and give me some decent options. My electrician is helping, but a second opinion would be useful.

Jon

[Tim Watts]

On Wednesday 17 April 2013 12:53 Jon Connell wrote in uk.d-i-y:

You can allow for diversity. You don't need the sum of all the max loads of your devices as a supply.

Your oven and hob will only peak for short periods then settle back to much lower levels even during use. Your hottube might be a big draw?

Electric showers tend to run for 10-15 mins at most even though the loads are large.

Do you have electric house heating?

The cutout fuse willl take about 30 minutes to blow if you pull 120A though it. 90A through it will run for a *lot* longer.

So you might even be OK as is - but an 80 or 100A upgrade will probably give you enough comfort margin.

[Dave Liquorice]

Firstly look up "diversity" particulary in reference to the hob/ovens. They might be rated at 30A but they are highly unlikely to draw that for any length of time. What do you cook that needs all four rings flat out? They'll be ticking along at a much lower mean load and probably not all four. Same with the ovens once up to temperature the mean power use is not high.

Not sure about the hot tub but a 60A supply might be starting to struggle but for the main cut out to go you'd have to be pulling a fair overload for quite a while.

The incomer and cut out(s) are the responsibilty of the company that operates and maintains the "grid" in your area. The meter(s) are the responsibilty of who ever you pay for electricity. The consumer units are your responsibility. Not so sure about the bits of wire that join 'em together, it would make sense for the cut out to meter to be the meter operators and meter to CU yours. ie the demarcation point is the output terminals of each device in the chain.

I'd keep with a single phase if possible, it's the KISS and safer option. I think a single phase domestic supply can be 100A - 23 kW.

[Dave Plowman (News)]

The obvious way would be to put the hot tub in the garden on its own phase. No safety implications like having two phases in the same house.

[Jon Connell]

Ah, I see.

The hottub is rated 30A and is on a 25A MCB at the moment, and has been without excitement for a few years.

Just a couple of immersion in the water tank, but we're unlikely to be showering whilst cooking, so maybe we're good.

Although a bit more expensive to upgrade (perversely) that does sound like it might be a decent first step before we switch the meter.

Jon

[Jon Connell]

Yes, I see now. Some amusingly disparaging remarks about dweebs like me in electricians forums :-)

Sounds like 2 votes for that so far from here.

Jon

[Dave Liquorice]

Until you take a portable appliance out there on an extension lead from the house... Though you could have sockets outside fed from the huttub phase.

[Tim Watts]

On Wednesday 17 April 2013 14:14 Jon Connell wrote in uk.d-i-y:

Those sorts of highly specialist forums can be very snotty of you are not a clan member.

We have qualified electricians, gas fitters and lots of other specialists here :)

[Tim Watts]

On Wednesday 17 April 2013 14:12 Jon Connell wrote in uk.d-i-y:

You are probably OK.

I'm fixing a house (slowly). I have 4 oil filled electric heaters. In the first 2 months, we ran everything off 1 temporary ring while I was wiring the others. Sometimes we tripped the 32A RCBO when SWMBO had the dishwasher and dryer on and the day was cold. That's not desireable of course, but it shows how much you can "overload" a circuit before getting problems.

I don't think your 60A circuit will be much of a problem, but you'd be perfectly justified to ask for an 80 or 100A upgrade.

[John Rumm]

Probably not as dramatic as you expect... but due to the nature of how these loads present themselves, chances are you will not have a major problem.

Diversity has already been mentioned:

So even your 30A hob will in all likelihood pull and average of less than half that.

Also have a look at the response curve for a 60A incomer fuse:

You will see there is a fair amount of latitude to pull much higher currents for even relatively long periods when required.

Well the normal arrangement is that "they" own everything up to and including the meter. The "tails" and the consumer units etc are yours.

You can feed a CU with its own phase (or for that matter fit a commercial style 3Ph CU into which you take all three)

It would be a good exercise to look at what circuits you actually anticipate needing and list them. You will probably find that a single phase 100A supply is more than adequate.

[Dave Plowman (News)]

Most sensible people would keep portable mains appliances well away from a bath - regardless of phase.

[ARW]

Which might not be free if the supply cable is not up to the job:-(

[harry]

Three phase is very handy if you ever installed a wind turbine. :-) Other than that just stick with single phase and possibly ask for a

100a fuse.

[John Rumm]

Yup, you could have a big fan to keep it spinning...

[robgraham]

========================= ================\

========================= ================/

My father bought an old house (1750's build) in Scotland and was over- joyed when he found it had 3 phase. It actually wasn't that big a house (4 bedrooms) but was on 3 floors with the ground floor (open basement as it was built on a bank) as his workshop and office. This was in the 1950's and the fullscale renovation he did included re- wiring - I remember as a youngster endlessly cutting and threading conduit followed by fish-wiring the cables through.

Apart from the workshop heavy machines, he wired it as a phase per floor.

It would be just too much of a coincidence if this is the same house - you're not in Mid Calder are you, Jon ?

Rob

[Andy Wade]

It is quite so.

[Jon Connell]

Cambridgeshire as it happens. I'm wondering whether it had 3 phase because it was an previously a schoolhouse and chapel. These sorts of things get lost in the mists of time, I think. The other day somebody dropped some photos through the letterbox of how the chapel looked in the 1980s. No comment on who they are from - they just dropped them by!

Jon

[Dave Plowman (News)]

Could have been needed for the organ blower motor?

[jgharston]

My lecture notes from 1993/4/5 state that the incoming equipment up to the supply head belongs to the grid operator, the incoming and outgoing tails to/from the meter belong to the meter owner, everything after the outgoing meter tails belong to the consumer. So, if you have an isolating switch between the meter and the CU, the isolator switch and cables after it belong to the consumer. If you don't have an isolating switch between the meter and the CU, everything from the CU onwards belongs to the consumer.

JGH

[Andy Wade]

If the isolator has been provided by the supplier (& installed by the meter operator on their behalf) then it and the tails between the meter output terminals and its input will surely be the property of the meter operator.

If, OTOH, the consumer has provided the isolator, then it and the tails both sides of it should remain theirs. Of course in this case the isolator shouldn't be on the meter board - but we all know it sometimes happens and blind eyes get turned...

The meter tails (meter o/p to CU) are also the consumer's property in this case.