Feeder Neutral Counting as a Current Carrying Conductor for derating?

[wwhitney]

My understanding is that as long as all the phase conductors are in a conduit the N does not count as a CCC.

kwired's example of two branch circuits, one A-B, the other C-N, is a case where at least on the basis of the physics the N should count as a CCC.

Cheers, Wayne

 

[CoolWill]

For derating purposes that's not true. For a 3 wire MWBC on a 120/240 volt system the neutral will carry current but it is not a CCC for derating purposes.

And for good reason. It will carry it's maximum current only when one of the ungrounded conductors is carrying zero current.

 

[jaggedben]

That's kind of the road I was going down.

If you put a 208 volt branch circuit and a 120 volt branch circuit in a raceway and utilize all four system conductors it counts as 4 CCC for ampacity adjustment reasons.

Now make it so these are the only loads in the panel. The feeder has same number of conductors, same load on each of the conductors but it only counts as 3 CCC's. See what I'm getting at?

You raise a good point. Although it's not clear to me that the code language draws a distinction between your two examples; it depends on how you interpret the word "circuit."

2017 NEC 310.15(B)(5)(a) says "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted . . ." Reading this fresh, I have no idea what it is trying to say. In any circuit, with a consistent sense of positive current, the sum of the currents in the circuit conductors will always be zero; otherwise charge would be accumulating in the downstream portion of the circuit. From this point of view, any single conductor always carries only the unbalanced current from all the other circuit conductors, in that if the other circuit conductors currents sum to zero, the conductor current will be zero; and if it not, the conductor current will be the negative of the sum of the other conductor currents.

But clearly 310.15(B)(5)(a) does not intend to exclude the neutral conductor of a 2 wire circuit. I was hoping that the definitions in Article 100 would let us say that a 2 wire circuit does not have a neutral conductor, but a neutral conductor is defined as a circuit conductor that is connected to the source neutral point.

Cheers, Wayne

If we want to be thoroughly legalistic about it, we could say that because the neutral in kwired's feeder doesn't carry the unbalanced current from other conductors, plural, it doesn't qualify for the exemption from being counted as a CCC. It only carries the unbalanced current from one other conductor of the circuit. This also should go some distance to clearing up Wayne's uncertainty about the meaning and clarify why a neutral in a 2-wire circuit is a CCC. When we speak of "unbalanced" current it has to be from more than one conductor that is not a neutral.

(Also, Wayne, I'd say it is not so that "any single conductor always carries only the unbalanced current from all the other circuit conductors". Only a neutral does, kind of by definition and nature. If I have two loads, L1-N and L2-N, on a 4-wire MWBC, L3 does not carry any unbalanced current. Only N does.)

Unfortunately this isn't a wholly satisfying answer, for at least a couple reasons:

1. It really give us an unreasonable answer to the OPs question, as winnie explained in post #16.
2. If we add a second 120V branch circuit to kwired's example, we still have the same problem that all conductors could carry their max current at the same time, even though the neutral now meets the language of carrying unbalanced current from 'other conductors'.

The intent is clearer than the language. Where the following is true (or close enough) ...

And for good reason. It will carry it's maximum current only when one of the ungrounded conductors is carrying zero current.

then the neutral shouldn't be counted as a CCC. But as we've seen, there are examples where this isn't strictly true for a feeder. What Will said here can only be counted upon for an MWBC that complies with 210.4(C) without using exception 2, i.e. it has line to neutral loads only.

 

[wwhitney]

(Also, Wayne, I'd say it is not so that "any single conductor always carries only the unbalanced current from all the other circuit conductors". Only a neutral does, kind of by definition and nature. If I have two loads, L1-N and L2-N, on a 4-wire MWBC, L3 does not carry any unbalanced current. Only N does.)

I will disagree. In your example, the current in L3 is 0. But the imbalance in the currents of L1, L2, and N is also zero.

The current cancellation we sometimes see in circuits with more than two wires is not dependent on there being a neutral conductor, nor is any one wire distinguished, current-wise.

Suppose you have a single phase transformer that has a 360V secondary with a 1/3 "tap." It creates conductors A, B, C, where you ground B (call it 0V) and then A has potential -120V, while C has potential 240V. From a 3-wire circuit you can supply 120V loads A-B and 240V loads B-C. B is not a neutral as the A-B and B-C voltages are not equal.

But for currents with a consistent choice of sign, and power factors close to 1, you will still have the constraint I_A + I_B + I_C = 0. So there are only two current-carrying conductors, physics wise. Each conductor carries the unbalance from the other two conductors, in the sense of the negative of the sum of their currents. No one of the conductors is distinguished in this regard.

Cheers, Wayne

 

[ggunn]

I am not going to count the N as a CCC when all phases are in a conduit until someone tells me I have to.

 

[kwired]

My understanding is that as long as all the phase conductors are in a conduit the N does not count as a CCC.

Which is more clear if it were only carrying unbalanced current in a multiwire circuit. But if you have a 2 wire 208 load on A and B phase and a 120 volt 2 wire load on C and N - you have 4 CCC's when it comes to determining conductor ampacity and related adjustments.

If you have 2 four wire MWBC's in a raceway then neither neutral counts as a CCC in that situation.

A raceway containg switch loop with a single "hot" and three returns from three switches counts as four CCC's even though the sum of the current in the returns will never exceed the current on the "hot". Now add a neutral to the raceway even though the occupancy sensor type switch puts a load in milliamp ranges on it, and it counts as another CCC.

 

[infinity]

But if you have a 2 wire 208 load on A and B phase and a 120 volt 2 wire load on C and N - you have 4 CCC's when it comes to determining conductor ampacity and related adjustments.

Yes it's that simple. If they were both 20 amp circuits you could have 20 amps on each of the 4 conductors meaning that there are 4 CCC's. It's really not complicated.

 

[kwired]

Yes it's that simple. If they were both 20 amp circuits you could have 20 amps on each of the 4 conductors meaning that there are 4 CCC's. It's really not complicated.

But now run this from a three pole breaker enclosure (so there is little to no chance of having any other loads on the feeder and most will say the feeder only has 3 CCC's even though the feeder conductors could be exactly the same thing as the branch circuit conductors. Or put a three pole breaker in as the branch circuit device and now it is three CCC's even though loading of the conductors has not changed a bit.

NEC has a rule here that sort of works most the time but can have some holes in it. The idea behind this is to account for heat contributed to the raceway by each conductor and this method is somewhat simple but can't account for all situations nor is it all that accurate when it comes to how much it may actually impact actual conductor operating temperatures.

If you had both 20 amp circuits (with the 208 and the 120 loads situation) you have 4 conductors each carrying up to 20 amps.

If you had a 4 conductor MWBC and load all three sub circuits to 20 amps you only have effective heating in the raceway of 3 conductors carrying 20 amps each.

The 208 plus the 120 volt loads in same raceway is not considered a MWBC by NEC. But feeder circuit for this generally does consider this same situation to consider the feeder neutral as a non CCC for ampacity calculations. Keep in mind most the time a feeder would have more line to neutral branches on it and we kind of assume the neutral does only carry unbalanced loading of the ungrounded conductors

 

[wwhitney]

The 208 plus the 120 volt loads in same raceway is not considered a MWBC by NEC.

How's that? Seems to meet the definition, depending on the meaning of the word "circuit".

Cheers, Wayne

 

[infinity]

How's that? Seems to meet the definition, depending on the meaning of the word "circuit".

Cheers, Wayne

The 208 volt load is not sharing the neutral.

 

[wwhitney]

The 208 volt load is not sharing the neutral.

I see nothing in the definition that discusses the load end of the MWBC, other than perhaps the word "circuit" itself:

Branch Circuit, Multiwire. (Multiwire Branch Circuit)
A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a neutral conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral conductor of the system.

Cheers, Wayne

 

[infinity]

I see nothing in the definition that discusses the load end of the MWBC, other than perhaps the word "circuit" itself:

It's two different circuits (1-208 volts and 1-120 volts) neither of which is a MWBC. No matter how hard you try you won't twist the definition to say otherwise.

 

[LarryFine]

The 208 plus the 120 volt loads in same raceway is not considered a MWBC by NEC.

No, that is clearly two separate 2-wire circuits.

But feeder circuit for this generally does consider this same situation to consider the feeder neutral as a non CCC for ampacity calculations. Keep in mind most the time a feeder would have more line to neutral branches on it and we kind of assume the neutral does only carry unbalanced loading of the ungrounded conductors

Agreed. A feeder tends to average out imbalances.

 

[wwhitney]

It's two different circuits (1-208 volts and 1-120 volts) neither of which is a MWBC. No matter how hard you try you won't twist the definition to say otherwise.

Agreed that is plausible to say it is two different circuits. Yet we don't have a definition of circuit, so that is a judgement call.

A side effect of your point of view is that if you see a panelboard, and you see a 3 pole breaker, and you see 4 wires leaving in a cable or conduit, one neutral and 3 ungrounded conductors from the 3 pole breaker, you can not conclude from that information that you have an MWBC. You would need to further investigate the nature of the load.

And I would say it is also defensible to define circuit more broadly, so that you can determine whether a collection of conductors is an MWBC without having to look at the details of what the loading is. A different judgement call.

Cheers, Wayne

 

[wwhitney]

If you put a 208 volt branch circuit and a 120 volt branch circuit in a raceway and utilize all four system conductors it counts as 4 CCC for ampacity adjustment reasons.

Now make it so these are the only loads in the panel. The feeder has same number of conductors, same load on each of the conductors but it only counts as 3 CCC's. See what I'm getting at?

I'm going to disagree. If the panel only supplies the 2 branch circuits, one L1-L2 208V, the other L3-N 120V, then if putting both of those in a single conduit means you have 4 CCCs, then on the feeder to the panel you have 4 CCCs.

Cheers, Wayne

 

[CoolWill]

I'm going to disagree. If the panel only supplies the 2 branch circuits, one L1-L2 208V, the other L3-N 120V, then if putting both of those in a single conduit means you have 4 CCCs, then on the feeder to the panel you have 4 CCCs.

Cheers, Wayne

You do, in fact, have 4 CCCs, but is it enforceable from an NEC stand point, especially if the ungrounded conductors are all simultaneously disconnected by the same breaker?

 

[wwhitney]

You do, in fact, have 4 CCCs, but is it enforceable from an NEC stand point

I have no idea, as I don't know what the language "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit" means.

especially if the ungrounded conductors are all simultaneously disconnected by the same breaker?

I'm not seeing how that fact would have any significance for any reasonable interpretation of the above phrase. If we accept infinity's point of view, the 208V 2-wire plus 120V 2-wire example will be two circuits, even if supplied by a single 3 pole breaker.

My point in the previous post is that I also don't see how the presence or absence of a panelboard in the middle of an otherwise unchanged circuit diagram would have any bearing on any reasonable interpretation of the above phrase.

Cheers, Wayne

 

[CoolWill]

I have no idea, as I don't know what the language "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit" means.


I'm not seeing how that fact would have any significance for any reasonable interpretation of the above phrase. If we accept infinity's point of view, the 208V 2-wire plus 120V 2-wire example will be two circuits, even if supplied by a single 3 pole breaker.

My point in the previous post is that I also don't see how the presence or absence of a panelboard in the middle of an otherwise unchanged circuit diagram would have any bearing on any reasonable interpretation of the above phrase.

Cheers, Wayne

I would imagine that from a code stand point, being on the same breaker is the difference between a MWBC and just two different circuits supplied from a 2 pole breaker and a single pole breaker. From a physics standpoint, there is no difference, but from a code point of view, one is a MWBC where the neutral doesn't count as a CCC, and the other is two different circuits where the neutral is a CCC. I could also envision a situation where different loads on that MWBC could be switched in such a way as to alternately cause of eliminate the scenario we're talking about here. How can the NEC account for such a setup? The only way would be to just make all neutrals CCCs.

 

[jaggedben]

I agree with Wayne on this discussion of what is an MWBC. And clearly someone on a code making panel did too, because we have 210.4(C). That section would be unnecessary and nonsensical if kwired's circuit with the three pole breaker couldn't be an MWBC.

 

[winnie]

IMHO the physics is perfectly clear.

The wording we humans make up to describe and regulate the situation...not so much.