Panel Upgrade - Conductors sized for Bus or Main?

[Xamacho]

First time poster - long time reader.

I have, what I feel, is a simple question. A three phase, 480/277 panel upgrade is needed to allow for a three phase backfed PV breaker. The Bus rating is 400 A, the Main OCPD is 250 A, and the PV OCPD is 200 A. Do the conductors that service this panel need an ampacity capable of handling the bus rating (400A) or the Main OCPD (250A).

Thanks so much for any help.

Michael

 

[petersonra]

I find the PV rules confusing but in this case I am going to just ask how would you ever get more than 250A of current in the service conductors?

 

[electrofelon]

First time poster - long time reader.

I have, what I feel, is a simple question. A three phase, 480/277 panel upgrade is needed to allow for a three phase backfed PV breaker. The Bus rating is 400 A, the Main OCPD is 250 A, and the PV OCPD is 200 A. Do the conductors that service this panel need an ampacity capable of handling the bus rating (400A) or the Main OCPD (250A).

Thanks so much for any help.

Michael

With an OCPD of 250 A, you would need 250 amps of conductor, or you could use the "next size up rule" if your conductors ampacity was 226-249 and the ampacity was greater than or equal to the calculated load.

P.S. I am assuming this is a service, is it?

 

[Carultch]

First time poster - long time reader.

I have, what I feel, is a simple question. A three phase, 480/277 panel upgrade is needed to allow for a three phase backfed PV breaker. The Bus rating is 400 A, the Main OCPD is 250 A, and the PV OCPD is 200 A. Do the conductors that service this panel need an ampacity capable of handling the bus rating (400A) or the Main OCPD (250A).

Thanks so much for any help.

Michael

In 2014, the 120% rule only applies to busbars fed from opposite ends. It does not apply to service or feeder conductors, as the main supply for the panelboard. Those only need as much ampacity as is needed for being protected by the main OCPD rating. In previous editions, this was omitted.

If Kirchhoff's current law were all that mattered, it could in concept be a 200% rule, because current will diminish to zero at some point in the busbar fed from opposite ends. Neither side of that point will the busbar amps ever be greater than the source on that side of the zero point. But, it isn't just busbar ampacity that matters. It is also the sum total of the heat generated by the accumulation of breakers in the unit. You can take credit up to 20% for placing the sources at opposite ends, but that is it.

 

[Smart $]

I agree with Ethan, along with the same assumption.

 

[publicgood]

Agree - busbar sizing aside and assuming your OCPD are sized to code, the feeders only need to match these OCPDs and not the bus rating.

 

[Xamacho]

With an OCPD of 250 A, you would need 250 amps of conductor, or you could use the "next size up rule" if your conductors ampacity was 226-249 and the ampacity was greater than or equal to the calculated load.

P.S. I am assuming this is a service, is it?

This is a Feeder from a Main Panel. On the existing 2000 A Main Panel, there is 250 A OCPD for a new MLO Panelboard (400A Bus, 225A OCPD for Transformer Load (existing), 250A PV OCPD) located roughly 10 feet away. This new Panel, which is MLO, is the one in question, and this new Panel will contain all stand-by loads in the event of Utility failure. As of this moment, I am waiting for more name plate data and existing panel setup to analyze each leg of the wye-wye transformer load for balance.

 

[jaggedben]

What you describe is all fine with respect to a grid tied PV install. But your mention of standby loads raises a question of how you expect the system to island. I don't see any mention of a transfer switch or other islanding device, and a typical utility interactive inverter will not work with a typical generator. It sounds like either a more complete description is needed or the mention of standby loads involves some mistake.