NEC 120% Rule

[SKSolar]

Experts,

I am little bit confuse on this and I hope you, the experts can enlighten me on this.
I have a 200A Main Service Panel by Murray/Siemens. The Bus Bar is 200A and has more than 20 breaker slots for breakers. The Main Breaker is a 4-pole 200A.
By 120% rule, if I have an AC feed by a solar system, I can only use a 40A backfeed breaker.

However, I read from somewhere, the author said the total amps of all the breakers can not exceed that 120% rule. I don't think that is correct. The fact is I have 16 branch circuit breakers, a 40A breaker for AC and 70A breaker to a sub panel in my garage that is already over 300A.

What I really think is, if my total usage at any time is more than 200A, the main breaker would simply trip, correct?

In my case, I am not using a lot of power at a time, I believe I can downsize my 200A Main Breaker to 150A, and that will enable me to use a 80A solar backfeed breaker to the Main Service Panel, correct?

Thanks

 

[ron]

There is no such rule about adding breakers as related to the main breaker rating.

Many people that want to add a lot of solar will just reduce the main breaker as you have suggested, as long as the load is below that rating.

 

[Dennis Alwon]

Experts,

I am little bit confuse on this and I hope you, the experts can enlighten me on this.
I have a 200A Main Service Panel by Murray/Siemens. The Bus Bar is 200A and has more than 20 breaker slots for breakers. The Main Breaker is a 4-pole 200A.
By 120% rule, if I have an AC feed by a solar system, I can only use a 40A backfeed breaker.

However, I read from somewhere, the author said the total amps of all the breakers can not exceed that 120% rule. I don't think that is correct. The fact is I have 16 branch circuit breakers, a 40A breaker for AC and 70A breaker to a sub panel in my garage that is already over 300A.

What I really think is, if my total usage at any time is more than 200A, the main breaker would simply trip, correct?

In my case, I am not using a lot of power at a time, I believe I can downsize my 200A Main Breaker to 150A, and that will enable me to use a 80A solar backfeed breaker to the Main Service Panel, correct?

Thanks

This is correct. The best you can do is 40 amps unless your load can withstand a 150 amp main. Yep, change the main to 150 amps then you can have 90 amps solar feed into the panel.

 

[jaggedben]

...

However, I read from somewhere, the author said the total amps of all the breakers can not exceed that 120% rule. I don't think that is correct. The fact is I have 16 branch circuit breakers, a 40A breaker for AC and 70A breaker to a sub panel in my garage that is already over 300A.
...

You are correct that the first part is incorrect, and the branch breakers you describe are not a problem.

It's a stretch, but possibly somewhere along the way there was confusion with one of the alternative rules, which allows the solar to be sized higher than the 120% rule if the total of all breakers other than MCB does not exceed the bus rating. This is an alternative rule that is used less often and does not involve 120% of anything. It typically doesn't work if the panel is full of small branch breakers but sometimes works if all those branch breakers are in a different subpanel.