Question about 310.15(B)(5)(c)

[winnie]

The intent of 310.15(B)(5)(c) is to deal with the situation of nonlinear loads which draw harmonic currents which result in current flowing on the neutral conductor even when phase conductor currents are balanced. Clearly this issue only presents if the non-linear loads are connected line to neutral.

However the text of the code is:

(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor

Question: Consider a 480/277V 4-wire feeder to a panel. The panel supplies 300A worth of VFDs and 50A worth of L-N 277V resistive loads. By the wording of the code is the neutral of that feeder counted as a current carrying conductor?

As I understand the physics of the situation, there will be no harmonic generating loads connected to the neutral and no issue of harmonics on the neutral. So it shouldn't count as a CCC, and IMHO common sense would say not to worry about the issue.

But the majority of the load on the feeder is from the nonlinear input rectifiers of the VFDs. So by the wording of the code I think the neutral must be counted as a CCC. I don't see anything in the wording of the code that specifies nonlinear L-N loads only.

Thanks
Jonathan

 

[kwired]

(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor

Maybe not worded quite right to be clear but I think the intent was when the major portion of the load is placing these harmonics on the neutral?

They are additive in the neutral and with a three phase wye they add to even more than on a split phase single phase supply.

A 4 wire delta is essentially the same as the split phase single phase as the high leg wouldn't normally be loaded with these loads.

Seems data centers are just about the only thing that is pretty common for this to be much of a concern with.

 

[infinity]

Question: Consider a 480/277V 4-wire feeder to a panel. The panel supplies 300A worth of VFDs and 50A worth of L-N 277V resistive loads. By the wording of the code is the neutral of that feeder counted as a current carrying conductor?

IMO since 50 amps is not a major portion of 300 amps it doesn't count as a CCC.

 

[JoeStillman]

Question: Consider a 480/277V 4-wire feeder to a panel. The panel supplies 300A worth of VFDs and 50A worth of L-N 277V resistive loads. By the wording of the code is the neutral of that feeder counted as a current carrying conductor?

As I understand the physics of the situation, there will be no harmonic generating loads connected to the neutral and no issue of harmonics on the neutral. So it shouldn't count as a CCC, and IMHO common sense would say not to worry about the issue.

But the majority of the load on the feeder is from the nonlinear input rectifiers of the VFDs. So by the wording of the code I think the neutral must be counted as a CCC. I don't see anything in the wording of the code that specifies nonlinear L-N loads only.

Thanks
Jonathan

The VFD's will have non-linear currents, but they don't flow in the neutral conductor to the panel. They flow out of the neutral point of the transformer, through a phase leg, and back in on another phase leg. Since your L-N loads on the panel are resistive, there will be no non-linear current on the feeder neutral and you can leave it out of your derating calculation.

 

[LarryFine]

Since your L-N loads on the panel are resistive, there will be no non-linear current on the feeder neutral and you can leave it out of your derating calculation.

I believe he's aware of that, but is pointing out that the letter of he law doesn't agree with your logic.

 

[JoeStillman]

I believe he's aware of that, but is pointing out that the letter of he law doesn't agree with your logic.

Good point, but I think we could apply the law at (a) before we get to (c), unless I'm not understanding the behavior of neutral currents.

Is there a way, somehow, that non-linear L-L currents can get back onto the neutral of the feeder?

 

[winnie]

@LarryFine correctly understands my question. I _know_ that in the scenario I described there are no harmonics adding on the neutral. My question is about the wording of the code.

I described a feeder supplying 300A of nonlinear loads and 50A of resistive loads. I didn't see anything in the code differentiating between balanced L-L 3 phase nonlinear loads (which don't put harmonics on the neutral) and L-N nonlinear loads (which may put harmonics on the neutral).

@kwired comes closest to answering this, pointing out the clause 'harmonic currents are present in the neutral conductor'.

As I read the paragraph as written, the code is saying 'if you have a majority of nonlinear loads on a circuit, you will have harmonic currents in the neutral, and the neutral counts as a CCC'. But IMHO it would be better if the code actually said 'if you have a majority of nonlinear loads, and if those loads are arranges so that harmonic can add on the neutral, then the neutral counts as a CCC'.

 

[wwhitney]

As I read the paragraph as written, the code is saying 'if you have a majority of nonlinear loads on a circuit, you will have harmonic currents in the neutral, and the neutral counts as a CCC'. But IMHO it would be better if the code actually said 'if you have a majority of nonlinear loads, and if those loads are arranges so that harmonic can add on the neutral, then the neutral counts as a CCC'.

Since the first version you mention is factually false, I think we are free to interpret the code text as meaning the second version. But I agree it would be clearer if the text matched the second version.

Cheers, Wayne

 

[JoeStillman]

The 2020 edition moved things around a bit and added the following pre-amble. 310.15(E) now says:

Neutral conductors shall be considered current carrying in accordance with any of the following:

(1) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(C)(1).​

(2) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(C)(1).​

(3) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.​

 

[JoeStillman]

The VFD's are not 4-wire circuits, so (3) doesn't apply.

 

[wwhitney]

The VFD's are not 4-wire circuits, so (3) doesn't apply.

Except the OP's critique still applies: (3) doesn't specify that the non-linear loads are supplied L-N, so you could have a feeder that supplies 3-wire 3 phase non-linear loads, and 2-wire single phase linear loads.

(3) should read "On a 4-wire, 3-phase wye circuit where the major portion of the line-to-neutral load consists of nonlinear loads . . ."

Cheers, Wayne

 

[kwired]

Good point, but I think we could apply the law at (a) before we get to (c), unless I'm not understanding the behavior of neutral currents.

Is there a way, somehow, that non-linear L-L currents can get back onto the neutral of the feeder?

Feeder doesn't even require a neutral if there is no loads that utilize the neutral. Seems to kind of answer the question IMO.

 

[JoeStillman]

Except the OP's critique still applies: (3) doesn't specify that the non-linear loads are supplied L-N, so you could have a feeder that supplies 3-wire 3 phase non-linear loads, and 2-wire single phase linear loads.

(3) should read "On a 4-wire, 3-phase wye circuit where the major portion of the line-to-neutral load consists of nonlinear loads . . ."

Cheers, Wayne

Agreed. We should propose a revision.

 

[don_resqcapt19]

Agreed. We should propose a revision.

You have plenty of time to get one ready. Public Inputs for changes for the 2029 code will be accepted starting in September of 2025,