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Gentlemen:

Today's inspection had a main distribution panel with a 150 amp main breaker and appropriate service conductor. This breaker supplied the main portion of the house. It also supplies the subpanel that services the new addition complete with a heat pump system. This subpanel has a 100 amp breaker at the main distribution panel as well as a 100 amp service disconnect at the sub panel being feed by an appropriate size cable.

Am I just being overly cautious or is that subpanel to large for the main panel?

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Main breakers should be at least twice the size of any downstream breakers they feed. That helps to insure that an overload or short circuit current in a feeder or branch circuit will trip just the downstream breaker and not the main, otherwise the whole house goes black because of a localized problem.

Other than that, the 150 must be large enough to serve all loads. Forget the 100 amp, it's just a breaker, not a load. Feeder breakers are often oversized.

At about 2,000 SF or higher, I'd be wary of an overloaded main if its a 150 and might call for a load analysis by a sharp sparky. Sometimes I do my own rudimentary analysis. Takes 5 minutes. I just add up nameplate electrical consumptions of major appliances around the house and ignore the smaller of heat/cool wattages on each HVAC system - no one runs the AC and heat at the same time, ehh? Any sum coming even close to 80% of main...call the sparky.

Marc

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Gentlemen:

Today's inspection had a main distribution panel with a 150 amp main breaker and appropriate service conductor. This breaker supplied the main portion of the house. It also supplies the subpanel that services the new addition complete with a heat pump system. This subpanel has a 100 amp breaker at the main distribution panel as well as a 100 amp service disconnect at the sub panel being feed by an appropriate size cable.

Am I just being overly cautious or is that subpanel to large for the main panel?

The 100-amp sub panel is not a problem in itself.

The 150-amp service might or might not be fine, depending on how the house is outfitted with electrical appliances.

It is very easy to perform a load calculation. I suggest that you learn how. You don't have to tell anyone that you did it, but doing it will give you confidence to recommend or not recommend having an electrician do one.

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