Solar 120 Rule Sub Panel

In a previous article the 120 rule explained 2011 nec 705 12 d 2 we clarified the philosophy of the 120 rule for load supply side interconnections of solar pv systems.

Solar 120 rule sub panel.

The inverter breaker in the sub panel is the breaker to use in the calculations. Examples are given to illustrate how it works in real situations. In the 2014 national electric code 705 12 d 2 was expanded to describe how to do a load side tap on feeders buses. Because the subpanel will backfeed into the main panel i assume i will have to apply the rules to the main panel too.

However if we add our pv breakers into a sub panel that is dedicated to only the pv breakers we can land the sub panel into a 40a breaker in the main panel board 10 4a 21a 31 4a x 1 25 39 25a. The solar is acting like a backdoor allowing additional current to feed the loads. Grounds and neutrals in electrical panel. The 120 rule also known as 690 64 b 2 is explained.

This is what the 120 rule is protecting against. That sub panel can have a 40a breaker added to it for the 7600w inverter as 100a 40a 140a which is under the 150a. More about wiring a sub panel. The 2011 code was clean understandable and easy to safely apply.

For example picture a 200a main breaker box feeding a 125a sub panel through a 100a breaker with a 100a main breaker in the 125a sub panel. That 50a breaker in the main panel that is supplying the power to the sub panel is located in the middle of the main panel. The bus will overload overheat and fail possibly catastrophically. If i have to apply the rules to the main panel i have to follow the 120 rule unless i upgrade that panel.