# 25KW system

#### gporter85

##### Member
Hello,

I am designing a rather large small commercial single phase system. The 125A Main panel is on a pole roughly 100' away from the building we will be installing the panels on. There is a 125A sub panel in the building. Here is what i am proposing. Upgrade the main and sub panel both to 125A panels with 225A bussing. Then backfeed (2) 60 amp Breakers on to the sub panel. This meets the 120% rule as 125+(2*60)<(225*1.20). I have just never back fed a system this big... Any other concerns I should have?

#### electrofelon

##### Senior Member
You could go with 200A bussing FYI. The calculation uses the inverter output circuit amps not the OCPD size.

I would be concerned with voltage drop/rise on such a small system, assuming the conductors are generally 125 A rated. Might want to look at that a bit.

#### gporter85

##### Member
You could go with 200A bussing FYI. The calculation uses the inverter output circuit amps not the OCPD size.

Are you 100% sure on that? I always thought you needed the continous duty factor.

I would be concerned with voltage drop/rise on such a small system, assuming the conductors are generally 125 A rated. Might want to look at that a bit.

From the Main to the sub is around 100' so it is really close to 2% with 1/0. Are you concerned about the utility drop? Becuase it is overhead.

#### electrofelon

##### Senior Member
are you 100% sure on that? I always thought you needed the continous duty factor.
Ok oops yes you are correct that you need to use 125% of the inverter output circuit current per 2014 705.12(D)(2)(3)(b), however that isnt necessarily the OCPD size. Some common sizes of course they design around the 120% rule, like the 7.6 KW's, so 125% = the OCPD, but that isnt always the case. What is the inveters max output current?

2% @240 is almost 5 volts. IT could very well be fine, but the last job I did had POCO voltage that had very poor regulation and fluctuated between 240 and 252 without PV. I had some problems with tripping out due to overvoltage. OBviously you have to draw the line someone and decide if cutting off a few volts is worth the money.

#### PWDickerson

##### Senior Member
You wouldn't have to upgrade the sub-panel if you interconnected at the feeder. You would have to install a new 125A AC combiner panel, but that might be cheaper than upgrading the sub-panel.