Thread: Maximum Breaker Size for Branch Panel

If a 100A main panel has only two disconnects (50A-240V for the AC, 100A-240V for the branch panel), can the branch panel disconnect amperage equal the main service size as it does here? Thanks.



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Originally Posted by Marc Morin

If a 100A main panel has only two disconnects (50A-240V for the AC, 100A-240V for the branch panel), can the branch panel disconnect amperage equal the main service size as it does here? Thanks.

I believe so. I cao think of no reason that it cannot. Well, unless there is some requirement from the manufacturer.

Some main breaker panels have feed-through lugs to feed a sub-panel plus the space for breakers. This is same as what you are referring to I believe.

I didn't think it's an issue but the 80% rule for single power cord fixture draw off it's branch circuit breaker rating was making me think there was also a similar issue with branch panel demand from the main. Thanks for the quick response.

Originally Posted by Marc Morin

can the branch panel disconnect amperage equal the main service size as it does here?

That is a function of the rating of the breaker tabs in the panel.

Some are rated for 70 amp maximum, some for 100 amps, some for even more. You would need to read the label on the panel to know.

IF the ratings allow for it, there is nothing wrong with having a 50 amp protecting a panel with 2 100 amp breakers feeding other panels or equipment ... other than being a downright dumb idea ... as the 50 amp main is going to limit the current through IT to 50 amps, meaning that no combination of load on the panels could be more than 50 amps without tripping that 50 main.

You've made the common mistake of putting the cart before the horse .....

A breaker isn't intended to 'feed' other breakers; it's intended to protect wire. Therefore, we size breakers solely based upon the wires they're feeding.

The next step is to ask whether the power supplied is appropriate for the load. If the 'load' is a panel, the only issue is whether the panel can handle what the breaker can give it.

As for the branch circuits served by the panel, there is absolutely no relationship between the total ratings of all the breakers in it and the total load the panel can serve. Instead, the panel is sized according to load calculations. In homes, the load calculations are generally dominated by the size of the area served, rather than actual electrical loads.

Ironically, the proper application of code rules might even allow for a situation where a single branch circuit breaker is larger than the main breaker. For example, a large air conditioner served by a small panel. (70 amp max breaker fed from a 60 amp panel). I said it was legal; I didn't say it was good design!

BTW, it's that last situation that is at the center of all the discussion about "selective co-ordination,' but that's another topic.