Residental mains wiring questions (USA)

Wire ampacity is not derated due to conduit fill. The derating for operating temperature is usually minor. (pretty much for small conductors #6 AWG and smaller)

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If i am fully informed about what is existing and what is contemplated i can give "official" advice. Use the local electric company and inspectors as much as you can first, though.

The NEC has cut that back pretty sharply recently. A single neutral for a single multipole breaker is allowed and very common. It may be allowed in some other cases, provided none of the load current is presented to the neutral conductor.

Load current is *always* presented to the neutral conductor (as well as the "hot"). That's the nature of any 2-wire system. No?

The number of wires in the pipe determine what percentage of their rated current they can carry. You are referring to the percentage of cross sectional area being used in the pipe. That, does NOT have any bearing on the case.

I should have been more careful in my wording.

mike

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Uhhhh....What?

Last I heard, a multipole breaker was only required if the hots went to the same device. Where two or three hots, on alternate phases, feed their own individual loads, a single neutral wire was allowed for that group of breakers.

If this has been changed, when did it happen? I haven't opened a Code book in a couple of years.

Look at office receptacle circuits. You can have three receptacles in a row, on phases a, b and c. A single white goes back to the panel. If only one receptacle is being used, the white is certainly 'presented' with the load current.

If TWO of the receptacles are being used, the neutral is STILL carrying load current. Only when all three hots are carrying an equal current is the neutral current free.

mike

All neutrals are white, but not all whites are neutrals. You have to have 2 (or 3) hots, from different phases, sharing the white in order for it to be a neutral.

In two wire circuits, the white is usually (mistakenly) called a neutral because it has a ground potential. It always carries the same current as the hot.

A real neutral carries only the *difference* in amperage between the hots. **

The right term for a white in a two wire circuit is 'an identified, grounded conductor'.

Note I said 'grounDED' conductor, not 'grounDING'. There's a difference.

mike

** could be called sum. It depends how you mark the vectors in the diagram. (+120 v at 0 degrees) plus (+120 v at 120 degrees) plus (+120 at 240 degrees) equals zero.

Multiwire circuits are now required to have a common trip (multi-pole) breaker feeding all circuits that share a neutral. In your example of 3 (20 amp) outlet circuits in an office, yes you can run 3 hots and one neutral back. But now, instead of 3 1-pole 20 amp breakers you are required to install one 3-pole 20 amp breaker. The reasoning behind this is if you turn off one of the 1-pole 20 amp breakers to work on the circuit. Yes that one hot wire is dead, but if you were to break the neutral splice, the other two circuits sharing the neutral can backfeed the white wire and kill you.

BTW, your load carrying neutral example is incorrect. The neutral carries current anytime there exists an imbalanced load between any two hot wires sharing a neutral. If you have two hots and one neutral with one hot carrying 10 amp and the other carrying 13 amp, then the neutral carries the difference of 3 amps. Assuming the 2 hots are correctly install to not be on the same phases. If they were on the same phases, then the neutral carries the combined load of 10 and 13 amps, or 23 amps. It's basically the same with a 3-phase setup except you throw the third hot into the mix and the unbalance load calculations are a bit more complex. If all hot carried the same current load, then the neutral is carrying nothing.

250.4-B requires "simultaneously disconnect all ungrounded conductors". That does not necessarily require a multipole breaker - it can be multiple breakers and a listed handle tie.

-- bud--

OK. I see that they have actually changed it.

Yes, the difference between the loads. But only in a single phase, three wire circuit. Much like the old Edison three wire circuit.

I agree. The have to be on alternate sides of the single phase, or on separate phases in a 3 phase system.

Three phase is a bit weirder. *Two* distinct hot phase wires with a shared neutral will see the neutral carry the SAME line current even when the two phases are evenly loaded.

That's why in, say, a residential apartment building, with 208/120 feeds to the units (two hots and a neutral), you can not derate the feeder neutral.

In 3 phase, You have to have all three hots feeding a load equally before the neutral balances out.

mike

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Ah, it's beginning to make some sense to me. Thanks!

Your example is for 3-phase, right? For residential split-phase, the vectors would be 180 degrees apart, right?

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Thanks, i was about to try expressing that clearly. You already did it well.

IF the loads are sine waves.

John

Well, in a single phase service, the two sides of the phase seem to be

180 out if you use the common tap as reference (black lead) and MOVE the lead (red lead) you are measuring with.

In polar notation that would be (120 V at 0 degrees) plus (120 at 180 degrees) equals zero volts. The rectangular equivalent is:

(+120 +j0) + (-120 + j0) = 0 (swapped meter leads)

In reality, it's the same current flowing through the entire secondary winding. There is NO phase angle difference between the two sides. No amount of centre taps will add a phase.

Think of two car batteries in series. Measuring from between the two batteries to the free terminals on either side will give you the same numbers. (+12v) +(-12v) = 0v. Measure between the two outermost terminals and you get 24 volts. (12v +12v) = 24v. The currents are sure not out by 180 degrees in the batteries. In a single phase, centre tapped transformer it's identical.

One reason they ground that centre point in your panel is that it limits the maximum available fault voltage to ground to 1/2 of line to line voltage. That is the absolute maximum fault voltage to ground you can get. Start moving that ground around and fault voltage rises.

The other reason the ground is in the middle is that it clamps the voltage of the phase sides to 120. If it weren't there, the two sides of the transformer secondary would act like a series circuit, with each side having a different voltage, depending on what was plugged into each. The two voltages would still add up to 240, but one side may be 80 volts and the other 160.

That situation is called a 'floating neutral'.

These guys explain it a lot better:

mike

I'm guessing that if all the waves were the same shape, it wouldn't make a difference.

Start putting a different shaped wave or one of a different period on each phase and then it's a different ballgame.

mike

I'd like put that thought on hold for a bit. I just had a horrible vision of being attacked by mutant, square shaped things. They were mocking me.

mike

The classic set-the-office-partition-on-fire situation is to have a y-connected 3-phase system, with a common neutral, and run a bunch of PCs off each phase. The PCs pull current at the peak of the AC line, at six different times each cycle, so even if the three load currents are equal and have the same waveshape, they don't cancel in the neutral.

John