Derating for 4 wires in a conduit

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steve66

Senior Member
Location
Illinois
Occupation
Engineer
I have a local inspector who is now requiring all typical 4 wire feeders and services to be de-rated by 80% per 310.15A3. By a typical, I mean any 208/120V or 480/277V 3 phase 4 wire service must be de-rated for having 4 wires in a single conduit.

In the past, I have normally used the exception, 310.15 A 5 a, for a neutral conductor that only carries the unbalanced current of the 3 phases. But now the inspector seems to think this exception can never (or almost never) be used.

He says he has talked to engineers and the ICC about this and they all agree. It seems like that opinion must be coming from the word "only" in 310.15A5a:

"A neutral conductor that carries ONLY the unbalanced current from the other conductors of the same circuit shall not be required to be counted..."

Apparently, with this interpretation, anytime a conductor carries ANY harmonics, and not just a majority of harmonics, the conductor is carrying something other than an unbalanced current, and it must be counted for derating.

I don't think that was ever the intent to require counting a neutral just because it carries some harmonics, otherwise why would A5b state a "majority" of harmonics, which most people consider 51% or more.

Does anyone else agree with this interpretation, or does anyone else have anyone enforcing this?
 

MAC702

Senior Member
Location
Clark County, NV
He's wrong. Otherwise there would be no point to even discuss the neutral and whether it might or might not have enough harmonics to qualify. Typical means neutral is NOT counted.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
No



And no.


In the past, I have normally used the exception, 310.15 A 5 a, for a neutral conductor that only carries the unbalanced current of the 3 phases. But now the inspector seems to think this exception can never (or almost never) be used.

Same here too. The inspector is incorrect. More of the opposite is true you because of the over 50% requirement for non-linear loads.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Everyone has a supervisor for a reason. Time to climb the food chain.

His supervisor is an architect, so I'm not sure he will be willing to override the electrical guy on an electrical issue.

But the EI did seem reasonable, and said he welcomed any backup I could provide to support my interpretation. Although on the other hand, I was really thinking it seems pretty clear in the code. He did mention one other inspector from another area he talked to, and I might go to him and see if I can figure out where this new reasoning came from.

It wouldn't be a big deal, but it can really add some cost to a service. Especially when the service is sized somewhat conservatively to start with.



Same here too. The inspector is incorrect. More of the opposite is true you because of the over 50% requirement for non-linear loads.

That's how I've always interpreted it. I've never had a project where I thought the non-linear loads could be more than 50% except a couple of data center projects, or a couple of projects where there were a lot of VFD's.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If he's reasonable as you say then he should be able to understand that the requirement to count the neutral as a CCC doesn't apply until after you reach a majority of the load being non-linear.
 
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