Help with AC Disconnect for 3-String Micro Inverter Based System

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The overall system consists of (3) strings of modules with rack mounted micro-inverters mounted up on the roof of the building. The result of this is 3 20 amp AC circuits coming down and back-feeding (3) 20 amp 2 pole breakers in the Main panel at the service entrance. Everything seemed to be fine until the Utility Provider stated that an AC Disconnect would be required between the Solar Generation and the Main Service. Breakers are not considered to be acceptable due to the fact that there is no visible air gap when open. So...to the question.

We intend to pull the (3) 20 amp breakers out of the Main Panel and install them into a new PV-Sub-panel on the backside of the wall come out of that panel and through a Non-fused Disconnect next to the Main Panel and back in to the Main Panel to a back-fed 60 amp Breaker. Can we continue the neutrals down and into the Main Panel or do we need to combine them as we are combining the (3) #12 L-1 and (3) #12 L-2 conductors into (1) #6 L-1 and (1) #6 L-2 conductors into the 60 amp Breaker? (Please See Picture)

Thanks, CD
 

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The code requires branch circuit neutrals to originate in the same enclosure as the overcurrent protection or an attached box that the circuits run through. So by code, no, you can't do this. It just so happens that solar inverters are about the only devices that wouldn't put current on the neutral and thus there would be no danger to my knowledge of doing it your way. But in general you should be concerned about inductive heating and not do things this way. If anyone unscrupulously added a breaker to the PV sub then there could be a danger.

I also have to say I don't understand why you would want to do it that way. Seems like an extra piece of conduit to run.
 
Thanks

Thanks

Thank you for your quick response. I was afraid there might be a catch in what we were looking to do. There are a number of reasons that we wanted to run the wire per the sketch. #1 We would only have to run #6-2 into and out of the Disconnect rather than #6-3 with the neutral just passing on through. #2 the conduit and wire are already run where we would be cutting in the Sub-panel. We would just interrupt and combine the hot legs the neutrals would be pulled out and then back in and through. Oh well ...we will do it right and make it work.
 

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Jim Jim Jim

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Location
CA
Do the microinverters require Neutral?

Do the microinverters require Neutral?

CDavis,

I guess (from your original design) they probably do, but if the microinverters don't require a neutral to operate, then your entire configuration could omit all of the neutrals.

Regards,

Jim x 3
 
Done Plan B it is

Done Plan B it is

Going with Plan-B. I also had a feeling that pulling wire through a panel enclosure may be contrary to code...not sure but don't have to worry about it anymore.
 

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Jim Jim Jim

Member
Location
CA
Neutral Required

Neutral Required

Hmm.. Jimx3 I am not sure I am following you on that no neutral comment. Standard Enphase Engage Cable L-1, L-2, Neutral, Ground. But thanks for chiming in...this forum is great.

CD:

There are inverters that operate @ 240VAC without a neutral connection. You're using an Enphase 240V split phase cable, therefore Enphase microinverters. Out of curiousity I checked - the M215 (for example) DOES require a neutral (for power line communication, I'm told).

So yes, Plan B it is.

Regards,
Jim x 3
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
There are inverters that operate @ 240VAC without a neutral connection. You're using an Enphase 240V split phase cable, therefore Enphase microinverters. Out of curiousity I checked - the M215 (for example) DOES require a neutral (for power line communication, I'm told).

Enphase inverters required the neutral for voltage measurement and phase detection even if the powerline communication feature is not used.
 

Joe Villani

Senior Member
I'm not the greatest with PV but based on plan B, isn't there an issue with the 120% rule with the 60 amp backfed breaker in a 200 amp panel.

I thought it would be limited to 40amps.

Am I missing something?

Thanks,
Joe
 

Zee

Senior Member
Location
CA
I'm not the greatest with PV but based on plan B, isn't there an issue with the 120% rule with the 60 amp backfed breaker in a 200 amp panel.

I thought it would be limited to 40amps.

Am I missing something?

Thanks,
Joe

Good point. It does look like a load side connection to me...in which case 60 A of PV is too much in a 200 A MSP.

Matter of fact to make it worse it seems to be drawn to show the 60A brkr is located right adjacent to main breaker instead of opposite it at opposite ends of loads.....
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'm not the greatest with PV but based on plan B, isn't there an issue with the 120% rule with the 60 amp backfed breaker in a 200 amp panel.

I thought it would be limited to 40amps.

Am I missing something?

Thanks,
Joe

It would be okay if the panel rating was 225 amps. The panel rating is not stated. But yes, good point.
 

Carultch

Senior Member
Location
Massachusetts
I'm not the greatest with PV but based on plan B, isn't there an issue with the 120% rule with the 60 amp backfed breaker in a 200 amp panel.

I thought it would be limited to 40amps.

Am I missing something?

Thanks,
Joe


The 120% rule only applies when you have a mixed-use panelboard, that serves both the general load circuits and inverter interconnection circuits. The NEC2014 has modified this rule, so that rounding errors aren't a show stopper.

If you have a panelboard that is dedicated to the PV system and doesn't contain any loads, then the 120% rule shouldn't need to be applied. And you need to mark it with a label stating that it is dedicated to the PV system, and not to add loads.

"No Loads"? How rigid is this? I'd like to clarify this in the 2017 NEC. It is common practice to connect auxiliary loads that are associated with the PV system, and of negligible current to this same panelboard. And it shouldn't be any safety issue to do this. Loads such as production metering, rapid shutdown/AFCI power supplies, surge arresters, etc.

Alternatively, you can use the rule that the sum of the breakers (excluding the main supply), shall not exceed busbar rating.
 

Zee

Senior Member
Location
CA
The 120% rule only applies when you have a mixed-use panelboard, that serves both the general load circuits and inverter interconnection circuits.

True.
However, the OP does show a drawing of a "PV SUBPANEL" and also a "MAIN PANEL" rated 200A .....where the 60A of PV goes.

Also not clear if 200A main breaker or 200A rated panel???
It 's a bit funky drawing with the circuits going around in a circle - IMHO .:)

He also shows the PV breaker adjacent to the main breaker - rather than opposite the main breaker from the loads!

In this case the 200 A rating is in fact the "main panel", a mixed use panelboard...it seems.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The 120% rule only applies when you have a mixed-use panelboard, that serves both the general load circuits and inverter interconnection circuits. ...

If you have a panelboard that is dedicated to the PV system and doesn't contain any loads, then the 120% rule shouldn't need to be applied. And you need to mark it with a label stating that it is dedicated to the PV system, and not to add loads.

These would be great proposals, but as of yet none of this is in the code. Anyone assuming an AHJ will agree with these comments is pressing their luck.
 

Carultch

Senior Member
Location
Massachusetts
These would be great proposals, but as of yet none of this is in the code. Anyone assuming an AHJ will agree with these comments is pressing their luck.

I think the official rule for dedicated PV system panelboards, is that you are supposed to fill all the spaces in it. In practice, it isn't easy to do this, because it is only by luck that you get a number of inverter breakers that completely fills the panelboard. How often do you get exactly 10 inverters for a 30-space panel?

I've had no trouble passing inspections with filler blanks and a "DO NOT ADD LOADS" label.
 

Carultch

Senior Member
Location
Massachusetts
True.
However, the OP does show a drawing of a "PV SUBPANEL" and also a "MAIN PANEL" rated 200A .....where the 60A of PV goes.

Also not clear if 200A main breaker or 200A rated panel???
It 's a bit funky drawing with the circuits going around in a circle - IMHO .:)

He also shows the PV breaker adjacent to the main breaker - rather than opposite the main breaker from the loads!

In this case the 200 A rating is in fact the "main panel", a mixed use panelboard...it seems.


I'd agree. He should show it opposite, and build it opposite as well. And label it opposite on the drawings with a code reference.

The 120% rule applies to opposite end of the bus interconnections. If it isn't at the opposite end, it is more of a 100% rule. And how often do you find a panelboard with an MCB that is significantly smaller than its bus?

200A bus with 200A breaker limits you to 40A of PV.
225A bus with 200A breaker limits you to 70A of PV. You may need to locate the main breaker externally, if you can't find a 225A with a 200A main.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I think the official rule for dedicated PV system panelboards, is that you are supposed to fill all the spaces in it.
What do you mean "official"? I don't think you will find that in the code anywhere.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I think the official rule for dedicated PV system panelboards, is that you are supposed to fill all the spaces in it. In practice, it isn't easy to do this, because it is only by luck that you get a number of inverter breakers that completely fills the panelboard. How often do you get exactly 10 inverters for a 30-space panel?

I've had no trouble passing inspections with filler blanks and a "DO NOT ADD LOADS" label.
I don't think there's anything official about it. Yes, a lot of AHJs will let you do that kind of thing and that's very reasonable of them. But there's no support for it in the NEC. In fact iirc there have been proposals for 'do not add loads' rules that were rejected by the code making panel.
 
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