Feeder Neutral as Current Carrying Conductor For Adjustment Factor - 310.15(B)(3) & (B)(5)

[Richard S]

Probably getting a lot of sighs and "not again" with this topic line. I've read many posts over the years concerning Neutral (grounded conductor) as a current carrying conductor for adjustment factor purposes, but have an installation that I would like to run past the forum.
We have a 400A 120/208 3Ø4W feeder to RPP (remote power panel) in data center. This RPP will mainly feed server racks filled with servers with switch mode power supplies that are mainly 1Ø208V (line-line). Because these loads are connected line-line, these loads cannot contribute to the neutral current. For the most part (as much as practical) these loads should be balanced equally A-B, B-C and C-A. There will be some minor loads that are 120V line-neutral for ancillary equipment, but these are very minor loads. I would say maybe 40Amps mostly balanced one each phases A-N, B-N and C-N. This equipment will have switch mode power supplies so I would say that it will have harmonic content. So I have 40A of line-neutral load with harmonic content on a 400A circuit (10%). Should I count the neutral as a current carrying conductor for more that (3) CCC's in a raceway (4 = 80%) per 310.15(B)(3)?
(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.
There will definitely be harmonic content, but most of it will be on line-line so in my option 310.15(B)(5)(c) wouldn't apply here. There will be some line-neutral loads with harmonic content (40A), but this is only 10% of the circuit size and I would not think this would NOT be considered a "major portion" and again 310.15(B)(5)(c) wouldn't apply. I look forward to reading your take on this matter.

 

[infinity]

A major portion is more than 50% which is highly improbable in a feeder in just about any scenario.

 

[LarryFine]

I'd say it's not a concern with the minimal L-N loads.

 

[synchro]

There will be some minor loads that are 120V line-neutral for ancillary equipment, but these are very minor loads. I would say maybe 40Amps mostly balanced one each phases A-N, B-N and C-N. This equipment will have switch mode power supplies so I would say that it will have harmonic content.

Do these switching supplies that are powered L-N have active power factor correction that allows them to meet IEC 61000-3-2 or 61000-3-12? If so, then their input current should have relatively low harmonics and be much closer to sinusoidal than the waveform you showed. If so then the loads should not be considered nonlinear for the purposes of 310.15(B)(5)(c).

Let's assume that the neutral conductor of the 400A feeder has the same ampacity as the line conductors. Then even if there were single-phase L-N rectifier loads that have no overlap of the current pulses on different phases, the worst case power dissipation in the neutral conductor would be a factor of (40/400)² x 3 = 0.03 or 3% of that in each line conductor. And so it would definitely not represent a "major portion" of the load on the feeder that contributes to conductor heating.

If you down sized the neutral then it might need to be considered as a current carrying conductor depending on the details. But otherwise I think it's safe to say that it's not a "current-carrying conductor".

 

[Richard S]

A major portion is more than 50% which is highly improbable in a feeder in just about any scenario.

Thanks - Because the code states "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;" I read it as the major portion of the load they are referring to is loads on neutral conductor because of the subsection/paragraph it is in. In this case the major portion of the L-L loads are nonlinear and if I read that paragraph enough times I can confuse myself and wonder if it implies that it is applicable.

 

[Richard S]

Do these switching supplies that are powered L-N have active power factor correction that allows them to meet IEC 61000-3-2 or 61000-3-12? If so, then their input current should have relatively low harmonics and be much closer to sinusoidal than the waveform you showed. If so then the loads should not be considered nonlinear for the purposes of 310.15(B)(5)(c).

I'm not sure about this. Typically they server power supplies would be sophisticated and have power factor correction, but not so sure about the ancillary equipment. There are typically little "wall warts" type power supplies and seem cheaply made.

 

[zbang]

Even junk wall-warts aren't going to give you any significant harmonics. Unless you have a ton of L-N loads that generate a double-ton of harmonics, you don't have much to worry about.