690.64(B)(2) ex.

[elohr46]

I have a 200a residental service with a 200a main CB which @ 120% will limit me to 40a main CB for a PV system. I would like to install a 50a PV system, would it be permissable to replace the 200a main with a 175a main to make room for the 50a PV CB?

I looked in the 2005 NEC and did not find anything to the contrary but I wanted to check with you guys first before doing this in case I missed something.

 

[mcclary's electrical]

I have a 200a residental service with a 200a main CB which @ 120% will limit me to 40a main CB for a PV system. I would like to install a 50a PV system, would it be permissable to replace the 200a main with a 175a main to make room for the 50a PV CB?

I looked in the 2005 NEC and did not find anything to the contrary but I wanted to check with you guys first before doing this in case I missed something.

IMO You are misinterpreting this. The article is written to keep someone from feeding a 200 amp bus (for instance) with a breaker too large. If you feed a 200 amp panel, with a 50 amp breaker, the sum of current could flow on the bus now becomes 250 amps, which the bus is not rated for. So the article in my opinion is to protect a buss. You are looking at this backwards, if you ask me. Going to 175 would make your limit of backfeed even lower.

 

[iwire]

I have a 200a residential service with a 200a main CB which @ 120% will limit me to 40a main CB for a PV system. I would like to install a 50a PV system, would it be permissable to replace the 200a main with a 175a main to make room for the 50a PV CB?

I looked in the 2005 NEC and did not find anything to the contrary but I wanted to check with you guys first before doing this in case I missed something.

Yes you could do that and I have personally done that as well.

However you would have to do an NEC load calculation to prove the 175 amp is still high enough to cover the entire load without the PV system active.


mcclary's electrical, the OP is looking at that section correctly.

 

[mcclary's electrical]

Yes you could do that and I have personally done that as well.

However you would have to do an NEC load calculation to prove the 175 amp is still high enough to cover the entire load without the PV system active.


mcclary's electrical, the OP is looking at that section correctly.

I apologize, I got out my book and read it again. I was the one thinking backwards. I was thinking that the backfeed could only be a "percentage" of the main breaker, but it actully reads "the sum of amp ratings",,,,,that's where I was thinking wrong. Lowering the main to 175 would keep you inside the 120%,,,,,I was shooting from the hip and I should have re-read the article.

 

[iwire]

I was shooting from the hip and I should have re-read the article.

No problem, I have the advantage of having done a few PV systems. At this moment I am not more than 20' from two 5000 watt inverters and a 200 amp panel that I had to install to replace the existing 100 amp panel to get around this very code section.

 

[elohr46]

Yes you could do that and I have personally done that as well.

However you would have to do an NEC load calculation to prove the 175 amp is still high enough to cover the entire load without the PV system active.


mcclary's electrical, the OP is looking at that section correctly.

Thanks for the quick reply, I'll do a load calc and see if the PV system fits.