Derating Main Panel Breaker When Installing PV System

[Yams]

I'm a student and I want to know that If I apply the 120% for ex. In a 200 busbar with 200 mdp with 60A fuse disconnect in a line side connection.
I have to derating to 150 A. The MDP but what about the busbar do I have to downsize the busbar also?

 

[jaggedben]

No, the whole point of downsizing the breaker is to have the breaker rating smaller than the busbar rating. That's so that the utility contribution to the allowable 120% is less, and the solar can be more. If you also downsize the busbar, you end up with less amps allowed all around. Plus, if you were going to do to the considerable work of replacing the panel busbar you'd probably upsize the whole thing. Changing out a breaker avoids all that work.

A few other other comments:
- if the 120% rule applies then it is not a line side connection. If it is a line side connection then the 120% rule does not apply.
- If using the 120% rule, then on a 200A busbar with a 60A PV backfeed you can downsize the breaker to 175A. 175+60= 235. 235<200x1.2
- final nitpick: 'downsize' is not the same as 'derate'. In fact it is sort of the opposite. You'd be changing out the breaker for a lower rating, instead of calculating that the larger one can only handle a lower amount.

 

[GoldDigger]

The story told by the Code Making Panel is that physics suggests a 200% rule for opposite end fed PV, but since there had not been any experimental verifications by manufacturers or third party labs to confirm that there is no problem in practice, they fudged it down to 120% to feel more comfortable.
"In theory there is no difference between theory and practice. In practice, however...."