Interconnected Power Sources — The 120% Rule

When both the main breaker and a backfed solar breaker feed the same bus, their combined current can exceed the busbar rating. The 120% rule lets you exceed the bus rating by a limited margin when the breakers sit at opposite ends.

This is the section every solar installer lives in. Get the busbar math right, or place the interconnection as a supply-side or load-side tap that avoids the problem entirely.

When it applies

Whenever solar, a battery, or another source backfeeds a panel that the utility also feeds, you have to prove the busbar will not be overloaded. The classic 120 percent allowance lets a back-fed breaker plus the main add up to no more than 120 percent of the busbar rating, with the solar breaker at the opposite end.

Common mistakes

• Putting the back-fed PV breaker anywhere on the bus. It has to be at the opposite end from the main for the 120 percent rule to apply.
• Adding the interconnection breaker to a bus that is already near capacity and blowing past 120 percent.
• Forgetting there are other compliant paths, like a sum rule or supply-side connection, when 120 percent will not fit.

In the field

On a residential solar tie-in I ran the numbers and a 200-amp bus with a 200-amp main could only take a 40-amp PV breaker under the 120 percent path. The customer wanted more. Rather than fudge it, I moved them to a supply-side connection ahead of the main so the bus was never the limit. Always do the busbar math before you promise a system size.