ECN Forum Forums General Topics of the Trade General Discussion Area Neutral Current Carrrying?

Bot whatever we call it, the ungrounded, the white wire or if you want neutral is always a current carrying condutor in a two wire circuit.

If the current does not flow on the neutral the circuit is off or has a ground fault.

The only time neutrals are sometimes not current carrying conductors are when they are a part of a multiwire circuit.

Which is a circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

iwire the question was in reference to a multiwire circuit. I understand the theory behind a grounded conductor becoming a "neutral". The reason I asked for a field example was because it seems rare to have a true neutral. I know that the loads on cancel each other out but how often do you see the loads on each ungrounded circuit exactly the same? I was just thinking that this is barely ever used in determining conductor size or am I wrong?

I got you now bot and you can call me Bob.

I run as many multiwire branch circuits as I can.

It is fairly unusually for the neutral not to actually carry some current.

It is much more likely that each phase will be carrying different currents and the neutral will be carrying the amount of imbalance.

This does not change the fact that for derating purposes many times the neutral in a multiwire circuit that carries current is still not counted as a current carrying conductor.

bot,

There are two different meaning for the term 'current carrying conductor'. One is to ask the question: Is there current flowing in that conductor? The second is to ask the question: Given a particular arrangement of conductors in a conduit, how much must I derate the current handling capacity of the conductors?

The answer to the _second_ question is not based upon the actual current flow in the wires, but instead on the current flow in the wires during the 'worst case' possible condition.

In the case of a single phase three wire circuit, the worst case condition is two wires carrying full current and the other wire carrying nothing. Call the two hot wires HotA and HotB; then you might get 20A on HotA and 20A on HotB, or 20A on HotA and 20A on the neutral, or 20A on HotB and 20A on the neutral. These are the worst case scenario for heating, and in this state there are only two wires carrying current. So for the purpose of derating, this set of conductors _counts_ as two 'current carrying conductors'.

Now what happens to all of the other possible states? You could have, for example 20A on HotA, 10A on HotB, and 10A on the neutral. Clearly all _three_ conductors actually have current flowing in them...but if you do the calculations you will find less heat produced than in any of the 20A and 20A conditions. So by treating this set of three conductors as only two 'current carrying conductors', you end up with a conservative estimate of heat production.

Similarly, in three phase service, with three hots and one neutral, the worst case is _three_ current carrying conductors, except for the case of harmonic current flows, where you can see all _four_ conductors carrying heavy current.

-Jon

There is actually an even worse case scenario.
When feeding 3 phase dimmer racks, such as in a theatre, where the loads on each phase vary, under certain conditions, the load on the neutral can exceed the load on any individual phase.
If you look in the NEC under section 520, you will find a requirement that feeders for dimmer racks be provided with a neutral twice the size of the phase conductors.

Techie actually those dimmer racks are non-linear loads that cause the harmonic currents on the neutral that Jon mentioned.

Audio amps have the same issue.

This fall I will be installing two additional 400 amp three phase 4 wire disconnects on a 'shed' stage for lighting, video or sound.

I am trying to get the customer to go with K rated transformers and double neutrals.

Bob

Techie I can not find anything in 520 that requires that feeders for dimmer racks be provided with a neutral twice the size of the phase conductors.

What I do find is requirements to treat the neutral as a current carrying conductor.

I think there is a move among the better traveling shows to want double neutrals from the venue disconnect to their distribution equipment.

I don't have a copy of the code handy, but google and mikeholt indicated that it might be 520.53(o). I think it was added in 1993.

(O) Neutral.
(1) Neutral Terminal. In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the supply neutral terminal, its associated busbar, or equivalent wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.

Greg, Techie, 520.53(O)(1) only applies to the internal construction of the portable switchboard equipment. 520.53(O)(2) is more interesting.

IV. Portable Switchboards on Stage

Quote

520.53 Construction and Feeders.
Portable switchboards and feeders for use on stages shall comply with 520.53(A) through (P).

(O) Neutral.
(1) Neutral Terminal. In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the supply neutral terminal, its associated busbar, or equivalent wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.

Exception: Where portable switchboard equipment is specifically constructed and identified to be internally converted in the field, in an approved manner, from use with a balanced 3-phase, 4-wire with ground supply to a balanced single-phase, 3-wire with ground supply, the supply neutral terminal and its associated busbar, equivalent wiring, or both, shall have an ampacity equal to at least that of the largest ungrounded single-phase supply terminal.

(2) Supply Neutral. The power supply conductors for portable switchboards shall be sized considering the neutral as a current-carrying conductor. Where single-conductor feeder cables, not installed in raceways, are used on multiphase circuits, the grounded neutral conductor shall have an ampacity of at least 130 percent of the ungrounded circuit conductors feeding the portable switchboard.

So when I run my permanent wiring to the disconnect / Cam-loc location I only have to treat the neutral as a current carrying conductor.

When the roadie plugs into the Cam-loc their neutral shall have an ampacity of at least 130 percent of the ungrounded circuit conductors and the Neutral terminal and bus bar of the portable switchboard shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.

I guess the harmonics stop at the Cam-locs.

Bob