Cable colours for 110v fixed wiring

[Andy Hall]

It was in the original post. The circuit is to distribute 110v from a site transformer to fixed outlets (EN60309 yellow type) to run power tools in the workshop. This is because it is a PITA lugging the transformer around and more convenient just to plug them into a wall outlet.

The transformer is an isolating type and has a centre tapped secondary giving 55-0-55v with the centre tap connected to ground.

Thus there are, in effect, two phase connections of 55v each (they are balanced about earth) and the earth. Neither of the 55v connections can be described as a neutral because it is 55v away from earth. Equally, no current will flow through the centre tapped wire unless there is a fault because it will be connected to the tool case or not connected.

It's isolated and balanced.......

.andy

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[Dave Plowman (News)]

I mean the circuit - as in Andy's case, is fed from an isolating transformer, so there is no relation to earth - unless the centre tap is earthed, etc.

I know what you mean about those bl***y awful AC/DC valve amps, though - death traps.

The better valve ones did then too.

[Andy Wade]

The correct answer to your original question though is undoubtably (IMHO) to use two reds (old colours) or browns (new colours). The conductors concerned are phase conductors of a single-phase AC circuit. Neither is a neutral because of the earthed centre tap.

If you use yellow then to comply with BS 7671 all terminations need to be marked with red (brown if using new colours) sleeving. I agree with you that the distributed centre tap is a circuit protective conductor, not a neutral, and therfore should be grn/ylw and not black or blue.

That's logical but not necessary. But I'd do the same :-)

IMO a better approach, if you're concerned about identifying individual conductors or circuits in an installation where there's a large number of wires in one trunking system, is to use the correct BS 7671 colours in conjunction with wire number markings - for which systems are available from RS, etc. (as you probably already know) - and to keep relevant documentation.

[Andy Hall]

This was one of my debates with myself because considering that it is a centre tapped supply, there are, in effect, two phases albeit not with the conventional 3 phase 120 degree shift. On the other hand, the centre tap is not a current carrying conductor, so after thinking some more I came to the same conclusion as you.

In fact what I've done is to use yellows but to put red markers at each termination.

I've used numbers as well, but I had been using those as well because there are becoming quite a large number of wires with all the radial circuits for outlets and machines being run.

Yes, it's all going onto a spreadsheet.

.andy

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[Andy Wade]

See sci.engr.electical.compliance /passim/ for interminable threads on the subject of "is it 1-phase 3-wire or is it 2-phase?". I prefer the former, reserving 2-phase to refer to two phases and neutral from a conventional

3-ph system, or two phases in quadrature.

Yes, in this case it's clearly a 2-wire single-phase circuit since there is no third live [1] conductor. Since neither wire is a neutral (see definition) they must both count as phase conductors (but are not separate phases of a polyphase system). QED.

Fairynuff, that complies.

A useful approach. I record things like the terminal numbers on neutral and earth bars for each circuit from a dis-board in the same way.

[1] That's "live" in the BS 7671 sense of "load current carrying" - nothing to do with the voltage above earth. C.f. "protective conductor.

[Andy Hall]

OK. This does make sense when presented that way, although I can't think of many scenarios where you would have two out of three phases used, apart perhaps, from wiring a large building with two phases only on different floors, each single phase, and not needing the third.

Make sense.

Right. I've used the numbering from the bus bars and breakers, keeping them consistent, and prefixing with a 1 or 2 to mark which of the two stacked CUs.

.andy

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[Lurch]

On Mon, 28 Jun 2004 11:05:36 +0100, Andy Hall strung together this:

Although not very common there are 2-phase appliances like welders and other medium sized industrial machines.

[Terry]

Same looseness of terminology used here in Canada. Unfortunately. If the conductors are NOT actually the one, two or three phase wires of a three phase system, why refer to them as 'phase wires' at all? Why not use the term 'legs'. Two examples come to mind; On UK industrial sites AIUI transformers are used to step down 240 to 120 volts to power electric tools. A centre tap of the secondary of the transformer is earthed. Hence we have two conductors at around 60 volts to earth, for safety. Neither of those conductors are 'phase wires', they are two conductors of the same single phase. (Although not correct for AC one can almost think of them as plus 60 volts and minus 60 volts!). Also. In North America atypical is a SINGLE phase 13,000 volt feed to the local distribution transformer where it steps down to 230 volts centre tapped. Three wires come from each distribution transformer to several houses. The centre tap is considered zero potential and is earthed; the maximum voltage to earth is therefore 115 volts (RMS). The three wires are all on the same single phase. In fact the nearest place, in our case, that three phase is available is some distance away. Three phase is rarely, if ever supplied to a domestic installation.. After the three wire service enters the house one of the legs is generally black and the other red. The 115 volt loads are distributed, hopefully evenly between the two legs and neutral, using single pole breakers. Heavier loads such as water heaters, clothes dryers, electric cooking etc. are connected between the two legs providing 230 volts using double pole breakers. But it's all single phase! 200 amp service at 230 volts (46Kva) is normal these days for a typical house. Although I have seen one very large house that had two such 200 amp services; they must have extra load such as a indoor pool and/or some kind of elevator. (Growing marijuana also requires much light btw!).

[Andy Wade]

Yes, but if they're just connected across 2 of the 3 phases then they're really 400V single phase. (In principle you could run such an item off a

230V single phase supply using a step-up transformer.) Only if a neutral connection is needed to provide a 230V supply for control circuits, or whatever, would it be correct to describe it as 2-phase.