Neutral for circuits micro inverters

mryan

Member
All the conductors for the micro inverters go back to the panel. But the foreman wants us to install a busbar in the pull box and land the neutrals and pull a larger conductor to the busbar from the panel to avoid pulling all the neutrals to the panel. Never seen this done before and haven't found the code that says you can't. Please advise.
 

jaggedben

Senior Member
300.3(B)(4) permits neutrals to originate outside a panelboard in certain situations, implying that it is not generally allowed. And the feeder section also implies it's not generally allowed. And of course conductors of the same circuit must be run in the same raceway, but one could argue that a common neutral doesn't violate that.

But I cannot find a code section that explicitly requires a dedicated grounded conductor to originate in the same location as the ungrounded conductors.
 

Smart $

Esteemed Member
Location
Ohio
... And of course conductors of the same circuit must be run in the same raceway, but one could argue that a common neutral doesn't violate that.
...
Can we argue the common neutral is a mismatched feeder conductor, i.e. no ungrounded feeder circuit conductor run with it. Then there's the individual two-wire branch circuit conductors from panel to pull box with only one of the two branch circuit wires.
Ohh! another section to support the preceding, but it requires a little forethought. :p
240.8 Fuses or Circuit Breakers in Parallel. Fuses and
circuit breakers shall be permitted to be connected in parallel
where they are factory assembled in parallel and listed
as a unit. Individual fuses, circuit breakers, or combinations
thereof shall not otherwise be connected in parallel.

Regardless of allowed or disallowed, I don't see a problem with it as long as the common neutral is sized appropriately.
 
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jaggedben

Senior Member
...
Regardless of allowed or disallowed, I don't see a problem with it as long as the common neutral is sized appropriately.
From a function or safety point of view, me neither. (Probably it doesn't even matter how the neutral is sized, see 705.95.(B)).
 

Smart $

Esteemed Member
Location
Ohio
From a function or safety point of view, me neither. (Probably it doesn't even matter how the neutral is sized, see 705.95.(B)).
It does matter.

Say you have (6) 20A micro strings connected to a 120/240 1Ø 3W system, with the neutrals combined at the pull box. What is the minimum ampacity that can be used for the common neutral between pull box and aggregation panel? Would it not be the same as the aggregation panel feeder neutral?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
But would you be willing to undersize the feeder neutral in that case on the assumption that the generation will always be balanced?
I would not.
I believe you can size the neutral the same as the EGC, since the neutral is only used for a voltage reference in 240V single phase systems. It's different in 480/277V systems where the Enphase units are connected phase to neutral.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I believe you can size the neutral the same as the EGC, since the neutral is only used for a voltage reference in 240V single phase systems. It's different in 480/277V systems where the Enphase units are connected phase to neutral.
705.95(B), and when the EGC is shared among circuits which are run together it need only be sized to the largest OCPD, not the sum of all of them. Someone else can look that one up. :D
 

Smart $

Esteemed Member
Location
Ohio
But would you be willing to undersize the feeder neutral in that case on the assumption that the generation will always be balanced?
I would not.
If we agree the common neutral must be sized as if it is a feeder neutral, we have to calculate the required minimum ampacity per 220.61... which would be the maximum unbalanced load (at 125%). For 6 balanced 20A strings on 120/240, that would be approaching 3x20A (depends on output rating and actual number of micro's).

If we size it per 705.95(A) it would be maximum load ***plus*** the inverter output rating. I am a bit uncertain how we should calculate that because it sounds a lot like the ampacity must be 200% of the maximum sum of the microinverters' output rating connected to any one line. ???
 

Smart $

Esteemed Member
Location
Ohio
I believe you can size the neutral the same as the EGC, since the neutral is only used for a voltage reference in 240V single phase systems. It's different in 480/277V systems where the Enphase units are connected phase to neutral.
705.95(B), and when the EGC is shared among circuits which are run together it need only be sized to the largest OCPD, not the sum of all of them. Someone else can look that one up. :D
Line-to-neutral microinverter systems fall under 705.95(A).
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yes... and that is why I specifically said "Line-to-neutral microinverter systems..."
Sooo... since you can combine the microinverter trunks in a panel and combine neutrals there, I don't see why someone couldn't combine only the neutral and keep the hots separate. And since the neutral is only for voltage sensing, it seems to me that the neutral from the service to whatever is combining the neutral need only be a big as the EGC.
 

Smart $

Esteemed Member
Location
Ohio
Sooo... since you can combine the microinverter trunks in a panel and combine neutrals there, I don't see why someone couldn't combine only the neutral and keep the hots separate. And since the neutral is only for voltage sensing, it seems to me that the neutral from the service to whatever is combining the neutral need only be a big as the EGC.
The neutral of line-to-neutral microinverters is not just for voltage sensing. It is a full-current conductor, at least in the 2-wire portion of a circuit. A common neutral should be sized same as a feeder neutral to a 120/240 panel... and that is to the calculated maximum unbalanced neutral load.

Anyone know how to calculate ampacity per 705.95(A)? If yes, please provide an example.
 

jaggedben

Senior Member
It does matter.

Say you have (6) 20A micro strings connected to a 120/240 1Ø 3W system, with the neutrals combined at the pull box. What is the minimum ampacity that can be used for the common neutral between pull box and aggregation panel? Would it not be the same as the aggregation panel feeder neutral?
All you had to do was look up the code section I already quoted, which ggunn was kind enough to repeat. Now if the neutral doesn't qualify then yes, it does matter. But when I said 'probably' I meant exactly that. The vast majority of microinverter systems are connected line-line and no current flows on the neutral.
 

shortcircuit2

Senior Member
Location
South of Bawstin
All the conductors for the micro inverters go back to the panel. But the foreman wants us to install a busbar in the pull box and land the neutrals and pull a larger conductor to the busbar from the panel to avoid pulling all the neutrals to the panel. Never seen this done before and haven't found the code that says you can't. Please advise.
408.41 requires each grounded conductor to terminate within the panelboard.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
All the conductors for the micro inverters go back to the panel. But the foreman wants us to install a busbar in the pull box and land the neutrals and pull a larger conductor to the busbar from the panel to avoid pulling all the neutrals to the panel. Never seen this done before and haven't found the code that says you can't. Please advise.
Who is the designer of the system? How is the neutral drawn in the plans? Who is responsible for making sure the system is code compatible?

If you are the responsible party and you are not sure that what the foreman is pushing for is legit (I'm not), tell him to shut up and build the system as drawn. Diplomatically, of course. :D
 

Smart $

Esteemed Member
Location
Ohio
All you had to do was look up the code section I already quoted, which ggunn was kind enough to repeat. Now if the neutral doesn't qualify then yes, it does matter. But when I said 'probably' I meant exactly that. The vast majority of microinverter systems are connected line-line and no current flows on the neutral.
And if you would read the posts for what they truly say, and participate rather than try to steer the discussion in the direction you want it to go, we might actually get somewhere. That said, if you read my post, you'd realize the code section you already quoted* does not apply in the case of line-to-neutral microinverter system. Then again, perhaps you will not realize it. We shall see.


*You didn't actually quote it; you cited a reference to it,
 
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