120% Rule

120 Rule

Have you heard of the 120% rule?

The 120% rule something that gets discussed all the time in the solar industry. It refers to the rule that states the sum of the main panel breaker and the solar back feed breaker cannot exceed 120% of the main bus capacity.

I will break it down for you:

-200 amp main breaker

-40 amp solar back feed breaker

-120% of 200 is 240

-200 + 40 = 240 so this setup works. If the total is 240 or less you are good to go if it’s 250 or above you need to include an MPU in the project. A derate might be possible but it is better to go back with a credit then ask for more money after the sale.

Knowing when an MPU is needed is half the battle. The good news is we at ABP Electrical specialize in MPU’s and have performed over 10,000 installations over the past seven years. Many solar EPC’s consider us invaluable partners to their success. Let us show you how easy your MPU installations can be.