I have a customer that would like an approximately 10kW AC inverter interconnected at their home. The main breaker is 200A rated and the bus/panel is also 200A. Will this work?


Assuming that this is a 240V split phase interconnection you have up to 40A of backfeed current allowed from a PV system. That would not allow a 10kW system to be interconnected. The largest system would be about 40/1.25*240 = 7.68kW. You would need a supply side interconnection or a smaller PV system. 

We just had almost exactly the same scenario come up recently and the first choice was to do a connection on the supply side of the service panel - just like Marvin Hamon suggests. Normally this works perfectly, but the problem this time was that there was no place to make that connection without some serious physical molestation of the meter/service panel internals.

We then decided to just put in a smaller main breaker - the breaker feeding the main buss bar. By swapping out the 200A breaker for a 175A version, we would instantly gain an extra 25A, thus allowing us to use the 10kW inverter (a 60A breaker). 

Apparently, the local inspector does not take kindly to folks just assuming they can derate established equipment like that. She demanded that we first analyze the loads to ensure that the smaller main breaker would not trip off under normal use and leave the homeowners in the dark...

We were stopped there, not sure if we needed to run metering equipment for a year to see what maximum loads were. However, we then realized we could download 2 years of 15 minute interval data from PG&E, the local utility.

In 5 minutes we had the homeowner download the data, (thank you Green Button), imported it into a spreadsheet, and sorted for maximum load - which turned out to be 15kW.

Following the guidelines of NEC 220.87, our system designer determined that the minimum allowable breaker was actually 90A. 175A was no problem.

In some cases, the homeowner might not be savvy enough to download interval data - in that case try Utility API. They charge $15 per report and work with many utilities across the US.

Thanks, Stuart - good solution, nicely expained!

6 years 10 months ago
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