Feeder Neutral Counting as a Current Carrying Conductor for derating?

[4thCuppaCoffee]

Hi All! Long time lurker, first time poster!

I'm trying to figure out if the grounded conductors in the feeder supplying my 208V/120 (3ph,4w) panel would count as a current carrying conductor. The 208V panel has ~20 branch circuits. All of them (except for 1) are 2P circuits and they each feed a 10kVA L2 EV charger. This particular charger has a 208V single phase connection that takes (2) #6 hots and an EGC. No neutral is run to the charger. The last circuit is a 120V circuit to a maintenance receptacle near the equipment.

However, it's my understanding that an EV charger can count as a nonlinear load. In this case, should I count the grounded conductor in the feeder supplying my panel (which is dedicated to EV charging loads) as current carrying? If yes, we'd have to derate the wire in the conduit.

I know that nonlinear harmonics can effect the transformer, but does it also effect the feeders upstream?

I guess the easiest solution would be to stick a CT around that conductor to see how much current is actually flowing and whether or not it contributes enough heat to require derating, but we'd have to actually install it first.

Anyone have any experience with this? Thanks!

 

[infinity]

If there is no neutral load then the feeder neutral does not count as a CCC.

 

[kwired]

I guess if you want to be really nit picky since there is only one 120 volt circuit here then technically that neutral in the feeder is not carrying the imbalance of the other conductors as there is no other neutral loads to balance it with.

I think most people would still consider it not to be a current carrying conductor for ampacity adjustment purposes though. Most would not consider these EV chargers to have enough harmonic distortion to cause neutral to count as a CCC- this however would still only apply if they happened to be using the neutral for the chargers. Straight power rectification doesn't create the distortion that high speed switching does.

Data centers are just about the only thing that has enough distortion that you may need to consider neutrals as CCC's in otherwise balanced four wire circuits on a wye system.

 

[LarryFine]

However, it's my understanding that an EV charger can count as a nonlinear load. In this case, should I count the grounded conductor in the feeder supplying my panel (which is dedicated to EV charging loads) as current carrying?

Welcome to the forum.

Definitely not. Line-to-line loads do not contribute to neutral current, regardless of the amount of harmonic content of the loads.

Your feeder's neutral should be sized to suit the greatest line-to-neutral load, or equal to the required EGC, whichever is greater.

 

[wwhitney]

I guess if you want to be really nit picky since there is only one 120 volt circuit here then technically that neutral in the feeder is not carrying the imbalance of the other conductors as there is no other neutral loads to balance it with.

Not correct. That 120V circuit will involve only one ungrounded conductor, and so that circuit causes an imbalance in the ungrounded conductors. The neutral will carry the corresponding imbalance.

Cheers, Wayne

 

[LarryFine]

Not correct.

As I read it, he wasn't disagreeing with your point, just saying it differently.

He wasn't saying that there would be no neutral current, he just wouldn't call it imbalance current because there would be no other neutral loads to balance it with, or against.

Added: Unless I'm wrong, in which case kindly disregard this post.

 

[wwhitney]

He wasn't saying that there would be no neutral current, he just wouldn't call it imbalance current because there would be no other neutral loads to balance it with, or against.

The neural is carrying the imbalance of the ungrounded conductor currents; this idea doesn't depend on their being more than one circuit connected to the neutral.

Cheers, Wayne

 

[LarryFine]

The neural is carrying the imbalance of the ungrounded conductor currents; this idea doesn't depend on their being more than one circuit connected to the neutral.

I grok. Any line-to-neutral loading, other than identically-balanced, creates an imbalance.

To quote Spock:

 

[zbang]

Unless I'm quite mistaken, the neutral conductor of a 3ph 4w feeder is almost never considered a current-carrying conductor (except in cases of high neutral harmonic loads). Pity I can't find the cite at the moment.

 

[kwired]

As I read it, he wasn't disagreeing with your point, just saying it differently.

He wasn't saying that there would be no neutral current, he just wouldn't call it imbalance current because there would be no other neutral loads to balance it with, or against.

Added: Unless I'm wrong, in which case kindly disregard this post.

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?

 

[LarryFine]

Unless I'm quite mistaken, the neutral conductor of a 3ph 4w feeder is almost never considered a current-carrying conductor (except in cases of high neutral harmonic loads).

That's high-harmonic line-to-neutral loads; line-to-line loads have no effect on neutral current.

 

[infinity]

From a practical standpoint this non-linear nonsense can just about be forgotten for good.

 

[zbang]

Exactly, the neutral isn't a CCC here.

 

[CoolWill]

The neutral is only a current-carrying conductor when it is.... carrying a current.

 

[infinity]

The neutral is only a current-carrying conductor when it is.... carrying a current.

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.

 

[winnie]

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?

Plausibly correct as the code is written. Plausibly a problem given the actual physics of the system.

The setup you described puts full current on the neutral. If the feeder were near capacity then IMHO you really do have 4 conductors worth of heating even if the code only counts 3 CCC.

Definitely not. Line-to-line loads do not contribute to neutral current, regardless of the amount of harmonic content of the loads.

Absolutely correct by the physics of the situation, but as the code is written there is no distinction for L-L nonlinear loads.

Your feeder's neutral should be sized to uit the greatest line-to-neutral load, or equal to the required EGC, whichever is greater.

This is what I'd do. The single 20A receptacle circuit is in the noise compared to what the chargers are doing.

 

[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.

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

 

[zbang]

Thanks, that's the section I wasn't finding yesterday.

 

[ggunn]

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

 

[LarryFine]

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

Agreed. You certainly may, but it's rarely required.