Conductor Sizing for Single Phase Inverters on 3PH System

[GoldDigger]

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Not a single one of us is designing AC combiners and the conductors between them and the point of interconnection such that all the breakers fed by inverters plus the OCPD at the interconnection are less than 120% of the combiner busbar and conductor ratings. Not a single AHJ that I know of has held any of our feet to the fire to make us do this. The crux of the biscuit as I understand it is that either the inverters can feed the panel in normal operation, or the service can in the event of a fault, but not both.

If an AHJ ever calls me on this, I will deal with it. Until then, this is the way it is done as far as I am concerned.

AFAIK, the issue comes up much more often with GTI output going to a general use subpanel than for a dedicated combiner panel. In the former case, some AHJs will apply the 120% rule to the feeder as well as the subpanel bus (even though there is currently no way to connect a load to the feeder wires in the middle.)

 

[Smart $]

So what is your point? That everyone designing PV and all the AHJ's approving their designs are wrong? Or do you just want to argue about something with which you have no experience?

Yep, I just want to argue. :thumbsdown:

And I didn't say I have no experience. I just said I don't work in the PV area... and that is because I am more versatile than that. On top of that, I am PV certified as far as the IBEW/NECA/NJATC are concerned:


My point is simply that Code makes no "ampere reduction" considerations for 1Ø inverters connected to 3Ø systems when it comes to its "sum" rules. But it seems you want to bring every other possible issue into the scope of discussion. If you ask me, it appears you want to argue more than I. :happyyes:

If AHJ's are allowing such considerations then that's great... but it doesn't change what the Code says. It actually makes it harder to get what the Code says changed...

 

[ggunn]

Yep, I just want to argue. :thumbsdown:

And I didn't say I have no experience. I just said I don't work in the PV area... and that is because I am more versatile than that. On top of that, I am PV certified as far as the IBEW/NECA/NJATC are concerned:

Whatever. I design PV systems in the real world and I know what passes inspections.

I could scan and post my BSEE diploma, my PE certificate, my engineering firm registration certificate, my NABCEP Solar Professional certificate, and my DBA if I thought it would impress anyone, but I know it wouldn't.

 

[Carultch]

Whatever. I design PV systems in the real world and I know what passes inspections.

I could scan and post my BSEE diploma, my PE certificate, my engineering firm registration certificate, my NABCEP Solar Professional certificate, and my DBA if I thought it would impress anyone, but I know it wouldn't.

From this conversation, it sounds like the NEC seriously needs a definition and passage for dedicated PV system panelboards.

 

[jaggedben]

I can't believe this is becoming another runaway thread, but here are a few points I hope will wind it down...

- Under the 2014 code, the 120% no longer applies to anything but panelboards. A feeder fed from opposite ends is not required to meet it, and a feeder tapped in any fashion in the middle is not allowed to take advantage of it. So this whole discussion of applying the 120% rule on top of a 'sum of breakers' for conductor sizing is simply not applicable under the 2014 code.

From this conversation, it sounds like the NEC seriously needs a definition and passage for dedicated PV system panelboards.

- Under the 2014 code, a PV combiner panelboard can take advantage of the rule that allows any combination of OCPDs for non-primary sources and loads as long as they don't exceed the rating of the busbar. While not explicitly defining and addressing 'AC combiner' panels, this rule effectively allows them to be sized at 125% of the inverter output (rounded up for breakers). Previous codes are admittedly much more ambiguous on this question. Unlike ggunn, I've heard of a couple AHJs requiring the 120% rule to both combiners and opposite-end-fed feeder conductors. Not for a while though.

- With regard to 3-phase combining, one could still read the 2014 rules as requiring that inverter currents or OCPDs (depending on the rule), be added simply rather than with vector math. For example, the AC combiner busbar in OPs example would be rated for 80A, instead of 63A. But this is a far cry from requiring the busbar or feeder to be rated for 119A. There's no situation under the 2014 code where the 'inappropriate' oversizing of any busbar or conductor would be by more than the ratio of 2/1.73. That's an oversizing of 15% instead of 91% (with a crazy strict AHJ). Again, previous codes are not so clear.

- Using three phase inverters avoids the problems we're talking about, and others. I hope to never have to combine single phase inverters.

 

[Smart $]

Whatever. I design PV systems in the real world and I know what passes inspections.

I could scan and post my BSEE diploma, my PE certificate, my engineering firm registration certificate, my NABCEP Solar Professional certificate, and my DBA if I thought it would impress anyone, but I know it wouldn't.

I too have additional "papers"... but I'm not the one questioning or doubting your qualifications. I don't think anyone here does. But you may need some guidance in a less technical area.

 

[ggunn]

I too have additional "papers"... but I'm not the one questioning or doubting your qualifications. I don't think anyone here does. But you may need some guidance in a less technical area.

I have no idea what you are driving at.

 

[Smart $]

...

- With regard to 3-phase combining, one could still read the 2014 rules as requiring that inverter currents or OCPDs (depending on the rule), be added simply rather than with vector math. For example, the AC combiner busbar in OPs example would be rated for 80A, instead of 63A. But this is a far cry from requiring the busbar or feeder to be rated for 119A. There's no situation under the 2014 code where the 'inappropriate' oversizing of any busbar or conductor would be by more than the ratio of 2/1.73. That's an oversizing of 15% instead of 91% (with a crazy strict AHJ). Again, previous codes are not so clear.

...

Amen.

A 2 to 1.732 isn't much when discussing a 21kVA system, but being it's a ratio the ampere difference is exacerbated on higher power systems, and will on more than one occasion force using the next greater equipment rating.

 

[Smart $]

I have no idea what you are driving at.

Exactly...

 

[ggunn]

Exactly...

Whatever. I'm not going to play mind games with you no matter how condescending you get. I've said all I'm going to say unless more tech stuff comes up. The last word is yours and you are welcome to it.