# Microinverter neutral question

Solar Expert Posts: 64 ✭✭✭✭
I have been searching the NEC for a yes or no answer to the question: with several microinverter strings going thru the same conduit to a combiner, does the neutral count as a current conductor for conduit fill calculations? My guess would be that no current flows thru the neutral except possibly due to a fault, then it would not be considered a current carrying conductor

If anyone can point me to a code article I would appreciate it.

Thanks,

Brent

• Banned Posts: 17,615 ✭✭
Re: Microinverter neutral question

Interesting question.

True it does not carry any current, but it does take up space and thus reduces the air space available for heat dissipation.

I'm inclined to say it should be included, just as any other non-continuous use wire would be. But as far as finding it in the regs goes ... Anyone?
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Microinverter neutral question
Brent wrote: »
I have been searching the NEC for a yes or no answer to the question: with several microinverter strings going thru the same conduit to a combiner, does the neutral count as a current conductor for conduit fill calculations? My guess would be that no current flows thru the neutral except possibly due to a fault, then it would not be considered a current carrying conductor

If anyone can point me to a code article I would appreciate it.

Thanks,

Brent

2011 NEC, section 310.15(C)(5).
Neutral Conductor
(a) 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(B)(3)(a).
(b) 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(B)(3)(a).
(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.
So, if there is only one neutral conductor which is used as part of the wiring harness or continuation of a microinverter system which feeds a 120-0-120 split phase circuit or a three-phase wye circuit you do not count it. But if you have a separate neutral conductor for each phase, you need to count all of them. (Seems a little strange, but it reflects the reality of cancellation of balanced current in a neutral. It also means that the voltage drop will be higher when separate neutrals are used!)
See also (6) which tells you not to count EGCs.
In fact, read all of 310.15 to see everything it discusses.

However your question has mixed two very different calculations.

The maximum conduit fill is based entirely on the area of all of the conductors (including Equipment Grounding Conductors which will only carry fault current and neutrals which will carry only unbalanced current) as a percentage of the area of the conduit or other raceway. Indeed, you even have to count both wires of a three-way switch traveler run, even though at most one can be carrying current at any moment in time.

The counting of current carrying conductors (CCC) is used for ampacity derating purposes, limiting how many amps you can run through a specific size and type wire because its heat dissipation ability will be reduced by the heat generated by other conductors in the same conduit. The section quoted above is used as a basis for ampacity derating only, not for conduit fill.
SMA SB 3000, old BP panels.
• Banned Posts: 17,615 ✭✭
Re: Microinverter neutral question

Very nice inetdog.

But just once I'd like the answer to not be "it all depends".
• Solar Expert Posts: 64 ✭✭✭✭
Re: Microinverter neutral question

Thanks for the response. I did not state my question very well as I am mainly concerned about determining conduit fill for calculating conductor ampacity. In my immediate application I have 3 strings of microinverter conductors running through a common conduit. Each microinverter string has 4 leads: L1-L2, Neutral & ground for a total of 12 conductors. From what I read here and in the code I would only count the L1 & L2 leads as current carrying so the total number of leads for ampacity calc. derating purposes would be 6 and not 9 [excluding the neutral and of course the ground lead].

Thanks,

Brent
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Microinverter neutral question
Very nice inetdog.

But just once I'd like the answer to not be "it all depends".
But that is such a handy universal answer. :-)
SMA SB 3000, old BP panels.
• Solar Expert Posts: 230 ✭✭
Re: Microinverter neutral question
Very nice inetdog.

But just once I'd like the answer to not be "it all depends".

I will disagree with inetdog and give you the answer you want.

No, you do not ever have to count the neutral conductor as a current carrying conductor on a line-to-line connected grid-tie microinverter. It does not depend.
Brent wrote: »
Thanks for the response. I did not state my question very well as I am mainly concerned about determining conduit fill for calculating conductor ampacity. In my immediate application I have 3 strings of microinverter conductors running through a common conduit. Each microinverter string has 4 leads: L1-L2, Neutral & ground for a total of 12 conductors. From what I read here and in the code I would only count the L1 & L2 leads as current carrying so the total number of leads for ampacity calc. derating purposes would be 6 and not 9 [excluding the neutral and of course the ground lead].

Thanks,

Brent

6 is correct.

You should only have to run one ground wire in any given stretch of conduit.

If you use #10 THWN-2 wire for the circuit conductors (or larger), you'll be fine on the deratings. With #10 you'll need a 1" raceway to meet fill requirements.
• Banned Posts: 17,615 ✭✭
Re: Microinverter neutral question

Jaggedben;

This would be because in a micro inverter situation the neutral line is never current-carrying because it does not ever operate in a split-phase state?
• Solar Expert Posts: 230 ✭✭
Re: Microinverter neutral question
Jaggedben;

This would be because in a micro inverter situation the neutral line is never current-carrying because it does not ever operate in a split-phase state?

In a word, yes. And that's where I'd begin with appealing to any inspector's common sense. That, and showing that you don't get a current measurement on it with your Fluke.

For the code argument, if you look at inetdog's code quotes above, there's no question about 120/240V split-phase. For 3-phase it's a little more vague, but I don't believe that the neutral in this case fits the definition of a 'common conductor'. There are no load or supply circuits that use the conductor this way. Rather, it fits under 705.95(B), a "Neutral Conductor for Instrumentation, Voltage Detection, or Phase Detection."

Note that I specified line-line inverters. There's no reason a micro-inverter can't connect at 120V, (and there's one not at all reputable brand that does). In that case the neutral would be a CCC. But all reputable brands that I know of are 240V or 208V. The SMA micro does not even have a neutral connection.

Note also the same logic applies to other inverters.
• Banned Posts: 17,615 ✭✭
Re: Microinverter neutral question

Thank you jaggedben for clearing that up.

The key element here is that the neutral line on these inverter is there for "Instrumentation, Voltage Detection, or Phase Detection" and not for current.

Although some may still have a bit of trouble with the part about "appealing to any inspector's common sense".
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Microinverter neutral question
jaggedben wrote: »
I will disagree with inetdog and give you the answer you want.

I resemble that remark!
I think that my answer was pretty clear that a single neutral would never be counted as current carrying (except, as you note in passing, in a three phase system with only two phase conductors present and line-to-neutral inverters.)
It was Coot that characterized it as "It depends", not me.
SMA SB 3000, old BP panels.
• Banned Posts: 17,615 ✭✭
Re: Microinverter neutral question
inetdog wrote: »
I resemble that remark!
I think that my answer was pretty clear that a single neutral would never be counted as current carrying (except, as you note in passing, in a three phase system with only two phase conductors present and line-to-neutral inverters.)
It was Coot that characterized it as "It depends", not me.

Which I did because the NEC section you quoted had three parts explaining application under circumstances.

Code interpretation can start more arguments than grounding. Code on grounding ... we're talking World War instigation there.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Microinverter neutral question
Which I did because the NEC section you quoted had three parts explaining application under circumstances.

Code interpretation can start more arguments than grounding. Code on grounding ... we're talking World War instigation there.
I would even go for interstellar. :-)
And Code interpretation is actually very simple most of the time. My interpretation is always right.
Some inspectors or AHJs may disagree, but what does that matter.
It is only a theoretical question after all. :-)
SMA SB 3000, old BP panels.