dedicated backfed panel board

[Joe Crecca]

I am installing two sma 6000 inverters with 30amp 2 pole breakers. I have a dedicated 100 amp subpanel box for the inverters. The box has a 100 amp main breaker and is line side tapped to the 200 amp service feeders after the meter. I have installed many systems this way. The DCA inspector is saying we are breaking the 120% rule. I thought that if you had a dedicated sub panel labeled for solar back fed only that it was okay to fill above the 120% rule. Please any information pertaining to this would be helpful. I have to have a cumulative srec meter and need to combine the ac output of all the inverters into one ac 240 volt source. I have always done this by a sub panel line side tapped. Thank you

 

[ggunn]

I am installing two sma 6000 inverters with 30amp 2 pole breakers. I have a dedicated 100 amp subpanel box for the inverters. The box has a 100 amp main breaker and is line side tapped to the 200 amp service feeders after the meter. I have installed many systems this way. The DCA inspector is saying we are breaking the 120% rule. I thought that if you had a dedicated sub panel labeled for solar back fed only that it was okay to fill above the 120% rule. Please any information pertaining to this would be helpful. I have to have a cumulative srec meter and need to combine the ac output of all the inverters into one ac 240 volt source. I have always done this by a sub panel line side tapped. Thank you

Unfortunately the 120% rule can be interpreted that way. There has been a lot of discussion in here about this, but it comes down to what the AHJ says. An alternative may be to use a pair of fused discos on the inverter outputs and tie them together in a tap box to feed your interconnect.

 

[Zee]

hmmm...lemme try to give it a go here.

I think you CAN do it.
Specifically, you can tie in on line side - just not exactly with that combination of brkrs and boxes.

These are the line side tie req.s:
1. min 60A rated enclosure and wiring from line side tap to first disco (in thsi case 2p100 brkr in subpanel). check.
2. metal conduit. check, i hope.
3. rated as service equipment. check, i assume.
4. 120% rule. Important thing in my mind: this applies to busbars and conductors only BETWEEN the "supplying power" to them. In my minds eye this means the main brkr rated 100A and the two @ 30A PV brkrs supply power. The busbar is the only thing between them. That busbar times 120% must be more( or equal) than the brkrs added all up.

Unfortunately, the busbar is 100A rated and is exceeded by
30A + 30A + 100A breakers > 120% @ 100A busbar
160 > 120 ---> no go

SOLUTIONS:
a. get a 200 Amp panel
b. easier. get a 60A main breaker.
Math:
30 + 30 + 60 = 120% of 100A busbar.
120= 120 ---> good!
The wires going to line side tap from main brkr are new service feeders and do not have the 120% rule to follow.

 

[GoldDigger]

hmmm...lemme try to give it a go here.

I think you CAN do it.
Specifically, you can tie in on line side - just not exactly with that combination of brkrs and boxes.
.....
...

Or you can hope that the 100A box actually has a 150A bus in it. In which case no changes would be necessary.

The argument is that since the 120% rule was intended to address the situation where loads are connected to the bus in addition to the main feed and the back feed, it does not apply in a backfeed-only box. But there is no exception in the language for the situation where there are no load breakers in the box.

 

[Zee]

Yes, i was just composing the following Golddigger )

"solar back feed only" labels are good ideas - but the sad, sad fact is they do not meet the LETTER of the Code if attempting to supply a busbar or conductor with excess of 120%.
(They should read "ADD NO LOADS" , IMHO........possibly also including the wording above)
I use them myself, but some AHJ's catch me on exceeding the 120% rule........

Such labeling was a gentle suggestion by Wiles for future Code cycles to deal with way-too-conservatively applied 120% rule situations in pv combining panels (AC).
Applying the 120% rule in these cases
1. of dedicated solar panels
2. especially when all full (quads and twins) of brkr.s
3. and when brkr.s are located opposite feed
4. AND also labeled "DO NOT RELOCATE!"
seems excessive to even Code experts.

But an AHJ may disagree. And they have (the letter of the) Code on their side.