Believe it or not, code references for determining the calculation to adequately size a PV inverter breaker are longer than the calculation itself. Don’t be intimidated into making a costly mistake when designing a customer’s solar system.
The calculation is simply the maximum output current of the inverter multiplied by a 125 percent safety factor, then rounded up to the nearest breaker size.
Two standard PV breaker examples:
A maximum output current of 16A multiplied by a 125 percent safety factor equals 20A. This happens to be a standard breaker size.
A maximum output current of 22A multiplied by a 125 percent safety factor equals 27.5A. The next standard breaker size is 30A.
An odd PV breaker example:
A maximum output current of 24.1A multiplied by a 125 percent safety factor equals 30.12A. If you rounded up, the next standard ampere rating is 35A. However, if you have a decimal fraction that is smaller than 0.5 the fraction can be dropped. Therefore you would only have 30A, which happens to be a standard breaker size.
NEC Code References
NEC® 2014 690.8 (A)(3) Inverter Output Circuit Current.
NEC® 2014 690.9 (B) Overcurrent Device Ratings.
NEC® 2014 240.6 (A) Standard Ampere Rating.
NEC® 2014 220.5 (B) Fractions of an Ampere.
Ronnie Raxter (guest author)
Design Application Engineer