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TOPIC: Question on Service Box and Distribution Panel Ratings

Question on Service Box and Distribution Panel Ratings 3 years 1 month ago #63136

To start with I'm a new home inspector and have a question about the following:
In my Principles of Home Inspection text book it talks about the service box and distribution panel need to be big enough to accommodate the service size (service size is 200 amps) which I understand.  It indicates  the box (main panel) and panel (sub panel) can be rated for the same or more amps, but not less than the service size.  So I'm thinking this means for example if the main panel is rated for 200 amps, the sub panel in the garage would also need to be rated for 200 amps.  In this situation the box in the garage is rated for 125 amps, the power coming into the subpanel has over current protection (breaker) at the main panel rated at 60 amps.  That being said the most power that will get to garage would be 60 amps thus never exceeding the box rating of 125 amps; therefore, in my mind no problem.  In my friends mind who is a licensed electrician no problem.  In another text book I read as long as there is overcurrent i.e, 60 amp breaker on the main for power going to the garage no problem.

Question:  Due to the fact that the sub panel is not rated the same or more as the main panel would you write it as a deficiency and why?  Thank you so much for your consideration.

Jim Giacobbe

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Re: Question on Service Box and Distribution Panel Ratings 3 years 1 month ago #63137

Not a deficiency for the distribution panel (subpanel) to have a smaller rating than the service equipment (main panel). The limiting factor for the service is the capacity of the smallest component. You can have a 200 amp service feeding a 200 amp panel with the proper size feeders and the capacity will be 200 amps.

Now add a 60 amp breaker to feed a distribution panel (subpanel) with a 200 amp rating using conductors rated for 60 amps. The capacity of the distribution panel is now 60 amps, no matter what the rating is, the panel will only receive 60 amps as the breaker will limit the current and trip at an overload condition.

You may also run across a structure with feeders sized for 200 amps, panel rated at 200 amps and a 150 amp main breaker (service disconnect). The capacity of this set up is 150 amps, with the limiting factor being the main breaker. I see this quite often as the local utility companies require a minimum 200 amp service. This is a plus to me, the extra capacity is there if demand goes up at some point, change out the main breaker and add breakers for new branch circuits.

Many don't consider it, but disconnects at AC condensors, furnaces, water heaters and the like are all distribution panels.

Hope this helps a bit

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Alton Darty    AR Lic. #HI-1496
ATN Inspection Services, LLC
Home & Commercial Inspections throughout the NE Arkansas and SE Missouri regions.
www.arinspections.com
Osceola, Arkansas 72370

Re: Question on Service Box and Distribution Panel Ratings 3 years 1 month ago #63139

Alton,

Thank you for your reply, yes your answer was helpful.

Sincerely,

Jim Giacobbe

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