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#95327 09/18/05 07:17 AM
Joined: Sep 2003
Posts: 650
Ya know Local, I think that you make a plausible semantic argument that indicates a (small) hole in the code. But I think that the semantic argument is wrong.

If I understand the semantic argument correctly, you need to claim 1) that the neutral in a two wire circuit is carrying _unbalanced_ current, and 2) that 'other conductors' can mean _1_ or more conductors.

I would respond that in a two wire circuit the neutral is carrying the current which perfectly balances the current flowing in the corresponding 'hot'. So the neutral is carrying _balanced_ current. And that _other conductors_ means more than one, meaning that this only applies to multiwire circuits.

All current flowing in a circuit must be balanced by the current flowing in other parts of the circuit. You _cannot_ have current flowing into a point, and not have the same amount of current flowing out of the point. Electrons simply don't pile up [Linked Image]

Actually, if you take this semantic argument too far, then you have to recognize that all current flow in a multiwire circuit is balanced by current flow elsewhere, and therefore must _always_ count the neutral as current carrying [Linked Image]

But I believe that the correct reading of 310.15(B)(4)(a) is (A neutral conductors) that carries only (the unbalanced current from other conductors)..., meaning that you have a set of 'hot' conductors, and current flow is being variously balanced between those hot conductors, and the neutral is only carrying what is left over. This is the reading that happens to agree with the physics [Linked Image]


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#95328 09/18/05 08:18 AM
Joined: Oct 2000
Posts: 2,749

Please contact me via email. Proposals are being solicited by NFPA for 2008 NEC revisions. I would like to discuss this with you.

Joe Tedesco, NEC Consultant
#95329 09/18/05 10:02 AM
Joined: Jul 2004
Posts: 19
The main reason I interpreted 310.15(B)(4)(a) to mean a two wire circuit is because (b&c) specifically mention multi wire. I read (a) to be 2 wire, (b) to be three wire, (c) to be 4 wire. I notice no real mention of single phase service, guess (a) would cover that if unbalanced was changed to balanced.

(b) uses the wording:

"In a three wire circuit consisting of two phase wires and the neutral of a 4 wire 3 phase Wye connected system..."

(c) uses the wording:

"On a 4 wire 3 phase Wye circuit where the major portion of the load consists of nonlinear loads..."

The way I read this is 2 and 3 wire circuits sharing a neutral the neutral must be counted.

But now I am starting to believe unbalanced was meant to be balanced in (a)

When I first started pushing my coworkers to start derating and to be more conscience of what they stuffed into a conduit I made it a point to say "for the majority of our work the neutral will count, period" because I feel this is the safest way to determine conduit loading. Looks like I did mis-interpret (a). They are going to hate me when I bring this up on Monday.

I would like to say that I don't think many circuits are ever balanced consistently and therefore we never truly have a neutral. I also feel that the majority of "round robins" are used to feed nonlinear loads. Therefore we should always count the grounded conductor as a current carrying conductor. At least for the work our shop does this seems to ring true.

What do you think?

#95330 09/18/05 10:53 AM
Joined: Sep 2003
Posts: 650

Take a look at this other thread: Kinda of funny how the same topics can come up multiple times in different locations.

One of the points that I was trying to make in the other thread is that you don't need _exact_ balance in order for the neutral to not _count_ as a current carrying conductor for 310.15(B)(4).

Perhaps a better way of looking it is not to ask 'does the neutral carry current?', but instead to instead examine the sets of 2, 3, or 4 wires comprising a circuit and ask the following two questions: 1) Do any of the individual conductors carry excessive current? 2) How much total heat is generated by this _set_ of wires when loaded in the worst possible heating condition?

In those situations where the code permits the neutral to not _count_ as a current carrying conductor, you will find that the worst case heating for a given set of N conductors happens to be equivalent to N-1 conductors fully loaded.

In a 3 wire _single phase_ multiwire branch circuit, the worst case heating happens when 2 of the 3 wires carries full current, and the other carries no current. You can get this situation with one branch of the circuit fully loaded, and the other branch unloaded, or with both branches fully loaded. You might have one hot and one neutral fully loaded, or both hots loaded with the neutral unloaded. But in either case the total heat production is maximized. In the case where the current is _not_ fully concentrated in 2 of the conductors, the _total_ heat production is reduced, even though all three conductors are actually carrying some current.

Consider the above case where the resistance of each conductor is 0.1 ohm (roughly 50 feet of #12 wire), on 20A circuits run on 12ga wire. In the case of both circuits fully loaded, you have 20^2 * 0.1 = 40W being dissipated in hot A, and 40W in hot B. In the case of one circuit fully loaded you have 40W in hot A and 40W in the neutral. But in the case of one circuit loaded at 20A and the other circuit loaded at 10A you get 40W in hot A, but only 10^2 *0.1 = 10W each in both the neutral and hot B. So even though all three conductors are actually carrying current, the heat produced is less than if only two conductors were carrying the current.

The same analysis will hold for a 4 wire three phase circuit with nice resistive loads. The worst case heating for these for conductors is the same as for 3 conductors fully loaded.

In the case of a three wire three phase circuit, the neutral ends up carrying almost the same current as the phase conductors, so the worst case heating for these three conductors is the same as three conductors fully loaded.

For a 4 wire circuit with non-linear loads, the worst case heating is harder to figure out, and in some case is _worse_ than 4 conductors fully loaded.

For a 3 wire single phase circuit with non-linear loads, things are a bit different. Most of the non-linear loads used today produce _odd_ order harmonics....which happen to balance on a single phase circuit, and don't load the neutral. So loads which would heat the shared neutral in a three phase feed will not be a significant problem for the shared neutral in a single phase feed.


#95331 09/18/05 11:12 AM
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
A couple of thoughts here....

I consider a neutral to be a current-carrying conductor. Multi-wire branch circuits only are a means to reduce the number of current-carrying wires.

Generally, running a smaller neutral is a bad idea, and best avoided, even where the code allows. I will concede one such application, though.
Many industrial machines have nothing but three-phase loads, except for the controls and, maybe, a light for the operator. I have seen such equipment set-up with the control/ light power being taken from one leg, fused on the machine, then only a very small #12 neutral run back to the panel. It does look a little sily, that #12 coming out a pipe with a set of #2's- but was once a fairly common arrangement. (This arrangement is disappearing, a victim of the computer age).

Now I think I see where your issue originates. The code wants you to begin de-rating at four conductors....which seem kind of silly when the "power" wires are #2, and there is only that little #12 neutral. I think this might be where that exception is meant to be applied, and no de-rating is needed.

For a conventional branch circuit, there is no doubt- the neutrals carry current.
Consider the following example...two circuits, different phases, one using 10 amps, and the other 9.
As a multi-wire circuit, the three wires carry 10, 9, and 1 amp respectively.
As circuits with separate neutrals, the hots carry 10 and 9, while the neutrals also carry 10 and 9. There's a lot more amps running through that pipe- so de-rating makes sense.

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