Breaker rating for PV sub panel feed (120% rule?)

[deadshort]

Hi all,
So we designed a system with 2 x 3800W inverters (20A breakers each for a total of 40A) with the PV breakers in a 125A sub panel. The sub panel is being fed by a 60A breaker in the main panel which has 200A bussing and a 200A main breaker. I get the 120% rule allowing us the 40A of PV (which we have) but the plans examiner says that the 60A sub feed exceeds this rating. The reason we went with a 60A sub feed breaker is in case the homeowner wants to add an EV circuit to the sub panel in the future.
The way I understand it, this circuit will never be more than 40A (PV). Right?
The label in the main service panel allows for up to a 110A branch breaker. Isn't the 60A sub feed essentially a branch breaker. Even if the homeowner were to add more circuits in the sub panel they would be loads and not a source (like the PV). We used #4 AWG for the sub feed so we're good to go as far as amperage is concerned.
Can anyone give an answer or point me to a section of code regarding this? It would be much appreciated.
Thanks!

 

[GoldDigger]

In the opinion of some inspectors, backed up by their AHJs, the 120% rule has to be applied in three places:

1. In the 125A sub panel. No problem at all there, since 60 + 40 is less than 1.2 x 125.
2. On the feeder between main panel and sub panel. Whether you meet the 120% rule there depends only on what the ampacity of the feeder wire is.
Technically the ampacity would have to be at least .8 x (60+40) =80A, since the feeder is fed from both ends. In practice there could only be an overload of that feeder if somebody tapped it in the middle and connected loads at that point.
3. At the main panel: Here you have an allowance of 1.2 x 200 = 240A. What saves you here is that under NEC 2011 you count up the amps of backfeed based only on the sum of the ratings of the breakers closest to the inverters, so you would add 200+40 not 200+60.
In NEC 2014 it gets even better for you since you add up 1.25 times the rated output of each inverter instead of looking at the values of the first breakers.

Now if you are subject to [2008] you might have a problem, since I am not sure whether the "first breaker" rule exists there.

PS: IMHO the only legitimate room for judgement is in item #2 above. The rest is very clear.

 

[deadshort]

In the opinion of some inspectors, backed up by their AHJs, the 120% rule has to be applied in three places:

1. In the 125A sub panel. No problem at all there, since 60 + 40 is less than 1.2 x 125.
2. On the feeder between main panel and sub panel. Whether you meet the 120% rule there depends only on what the ampacity of the feeder wire is.
Technically the ampacity would have to be at least .8 x (60+40) =80A, since the feeder is fed from both ends. In practice there could only be an overload of that feeder if somebody tapped it in the middle and connected loads at that point.
3. At the main panel: Here you have an allowance of 1.2 x 200 = 240A. What saves you here is that under NEC 2011 you count up the amps of backfeed based only on the sum of the ratings of the breakers closest to the inverters, so you would add 200+40 not 200+60.
In NEC 2014 it gets even better for you since you add up 1.25 times the rated output of each inverter instead of looking at the values of the first breakers.

Now if you are subject to [2008] you might have a problem, since I am not sure whether the "first breaker" rule exists there.

PS: IMHO the only legitimate room for judgement is in item #2 above. The rest is very clear.

Thanks! Are you able to point me to the specific section where I can find this. I have scoured the NEC and CEC and can't seem to find the 'definitive' language I am looking for.

 

[GoldDigger]

Thanks! Are you able to point me to the specific section where I can find this. I have scoured the NEC and CEC and can't seem to find the 'definitive' language I am looking for.

Try NEC 705.12(D)(7) [2011].

...
In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.
....

 

[deadshort]

Try NEC 705.12(D)(7) [2011].

Great, thanks! I think that was the ammo we were looking for.

 

[jaggedben]

Now if you are subject to [2008] you might have a problem, since I am not sure whether the "first breaker" rule exists there.

For the record, there was no change to 705.12(D)(7) from the 2005 thru the 2011. (Which is a little surprising considering how much it changed in 2014...)