Thread: Electricity supply question - 3 phase vs 1 phase

Originally Posted by Avery

I thought a single phase connection had 2 wires (phase and neutral) and a 3 phase connection had 4 wires.

Although most 3 phase wiring uses a neutral it is not always needed for three phase operation.
Remember all voltages have to be relative to a reference.
With single phase, a neutral is needed as a reference for the single phase.
With three phases (ABC) the voltages are relative to each other AB, BC and AC

It sounds like you have 3 phase to the meter but they decided to just wire up one phase.
If there were no 3 phase devices used in a house and the meter needed upgrading the to save a few$ sometimes they just wire up the whole house on one of the 3 phases.

Originally Posted by BobL

Although some 3 phase wiring does use a neutral it is not needed for three phase operation.
Remember all voltages have to be relative to a reference.
With single phase, a neutral is needed as a reference for the single phase.
With three phases (ABC) the voltages are relative to each other AB, BC and AC

It sounds like you have 3 phase to the meter but they decided to just wire up one phase.
If there were no 3 phase devices used in a house and the meter needed upgrading the to save a few$ sometimes they just wire up the whole house on one of the 3 phases.

Thanks you for your quick reply.

So if the whole house is connected to one phase and the other two are simply left dangling, does that mean that all the neutrals are just tied to ground at the meter box? To connect 3 phases I would need to upgrade the meter?

Originally Posted by Avery

So if the whole house is connected to one phase and the other two are simply left dangling, does that mean that all the neutrals are just tied to ground at the meter box? To connect 3 phases I would need to upgrade the meter?

Seeing that you're getting a 15amp p/point installed, raise the question with him, when you call, a sparky is really the only person who'll be able to answer the question, as he will be able to see what has been done.

FWIW I have 3 phase into the shed and it's definitely 4 wires. 3 wires have fuses on the entry and one doesn't (neutral)

Lights and power run from a single phase and there's a single 4 wire 3phase socket. 4 wire means 3 phases and earth, no neutral.

Originally Posted by Avery

Thanks you for your quick reply.

So if the whole house is connected to one phase and the other two are simply left dangling, does that mean that all the neutrals are just tied to ground at the meter box? To connect 3 phases I would need to upgrade the meter?

Essentially yes, have a read of this. Ground and neutral - Wikipedia, the free encyclopedia

It is very important not to let DIYers think that they can do this themselves - wiring up neutrals and earthing can be a tricky business that can kill and best left to qualified people.

Originally Posted by DJ's Timber

Seeing that you're getting a 15amp p/point installed, raise the question with him, when you call, a sparky is really the only person who'll be able to answer the question, as he will be able to see what has been done.

That is probably what will happen. If three phase is available I will want to use it. I have a couple of 3phase machines that I was goiing to run off single phase with electronic controllers.
I reallyjust like to know what is going on before i call the professionals in - just so I know what questions to ask.

Originally Posted by snowyskiesau

FWIW I have 3 phase into the shed and it's definitely 4 wires. 3 wires have fuses on the entry and one doesn't (neutral)

Lights and power run from a single phase and there's a single 4 wire 3phase socket. 4 wire means 3 phases and earth, no neutral.

Looks just like mine except I don't have the 4th wire.

I have 3 wires coming onto my property and it's what it known as split phase - 2 x 240 (180 deg out of phase) and neutral. The phase to phase voltage is 480V, and not 415V like proper 3 phase.

It was quite common back I the 60's and 70's in NSW to bring 2 phases into houses. 1 phase was used general supply to the house and the second phase was a dedicated off peak supply for hot water. The three wires are probably 2 active and 1 neutral. If you can post a picture of the switchboard it should be fairly easy to tell.

Andyd

Originally Posted by andyd

It was quite common back I the 60's and 70's in NSW to bring 2 phases into houses. 1 phase was used general supply to the house and the second phase was a dedicated off peak supply for hot water. The three wires are probably 2 active and 1 neutral. If you can post a picture of the switchboard it should be fairly easy to tell.

Andyd

I see a number of domestic electric ovens are designed to run with two phases.

Originally Posted by andyd

It was quite common back I the 60's and 70's in NSW to bring 2 phases into houses. 1 phase was used general supply to the house and the second phase was a dedicated off peak supply for hot water. The three wires are probably 2 active and 1 neutral. If you can post a picture of the switchboard it should be fairly easy to tell.

Andyd

What Andy and BobL said. Vernon, I think is in the country on a SWER or similar.

In SA in the 60s it was commonish to run the house on single phase and oven across two phases. Effectively a 240V house and a "single phase" 415V oven.
With three wires and an old house I would bet that this is what you have. A clue, but not definite, to look for is are they two covered wires and one bare (two phases and neutral respectively) however, they could be all covered or all bare.

After the original stove or HWS died and was replaced with a new single phase one the other active would have been left unconnected to anything (except maybe the meter, the meter would be a two phase one and you should be able to tell if it wasn't changed subsequently).

Also the 3 wires you can see (and the rating of the service fuse) would be quite small compared to a modern service, probably only good for 50A or so (maybe need to get them checked)
.

Regards
SWK

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You have a single phase connection, the additional live wire is old-school off peak supply, and will only be functional when 'off peak' is on. The newer systems use a single live wire and send a signal pulse down the line so the meter knows that it's billing 'off peak'. Those old 'off peak' supplies are somewhat dangerous, as they check out as dead, but can become live at any time. Stupid sparkys get zapped off them all the time.

Your sparky will have no trouble running a 15A line to wherever you want it to be. You don't 'need' to be able to supply the full supply current to every single point, so adding in a circuit shouldn't be a problem at all. You might get some trouble if your supply coming in isn't really enough to run everything you might want to run all at the same time though, so be aware of that.

But, get a sparky on site to sort it out.


Stu.

(Licensed Electrical Mechanic.)

Stu.
Avery said the three wires to his house came from across the road. So unless there was an common LV mains switchwire for off peak (like an old street light system) or the time clock was out in the street, I don't think it is an off peak arrangement. Something like that could have been used in NSW (I don't know) but arrangements like those were never used in SA.

I still think the circumstances indicates a two phase and neutral system of some sort. If so, the wires will be small (7/18g is what was used in SA, that's equivalent to 8mm2 in metric, about 1/2 the size of a normal service these days)

Avery,
since the house is old and things like the stove have been changed/upgraded and other circuits added over time, get your electrician to do a "maximum demand calculation" (use those words) before he adds anything to the house. I can't see it to know for sure but I am suspicious that the old service is too small for modern day use.
It used to be in SA that a service upgrade would be provided by the distribution company at no (or minimal) cost if the load on the house had increased to justify it. You may find that if the single (or two) phase service has to be upgraded there might be a discounted price to go one more step and get a three phase one if you really wanted one (it used to be like that in SA). Of course there would need to be some mods done on your switchboard and a new 3 phase run out to the shed, which will add to the cost if you do go to 3 phase. But if you haven't bought your inverters already you can weigh the cost differences of the two options.

As the moderators noted and Stu pointed out, get someone to look at it. _If_ it is a two phase system there is a chance of giving yourself a 415V shock, obviously even more dangerous than the usual 240V of a normal house. _If_ it has a timed system as Stu believes parts of it could liven up unexpectedly when you thought you were safe. Neither Stu nor I nor anyone else on this board can tell for sure what you have without looking at it. All comments given are intended so you can deal more intelligently with an appropriately qualified person, which I understood is your intention from your original post.

Regards
SWK
(SA "A" Class)