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Size of SECs for multiple service disconnects


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What is the rule for sizing a service drop and SECs serving multiple service disconnects?

For example if three service disconnects, each with 100A OCPDs, are powered from a single service drop, does the drop and/or the conductors to meters require the capacity to supply 300A, or is there a rule that establishes a lower requirement based on the assumption that a 300A total draw is unlikely?

(And , yeah, I know what they ought to do with this entire service, but I want to understand the concept... )


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There are requirements in the National Electrical Code to do a calculation for that service (NEC 220). There is a multiple family example in annex D4(a). Unfortunately there is no easy rule of thumb that I am aware of. The wire (and other components) need to be sized properly to handle the load of the property - not the breaker sizes. In other words it is very likely that a load calculation for a triplex would come up at 150 amps, even thought the main breakers for each unit are rated at 100 amps.

If you think it is too small then I would recommend that you recommend that the owner or buyer hire a qualified electrical contractor to inspect it, and provide a proper load calculation for the property.

By the way I agree - time for a service upgrade!

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