Earl IMO To determine the ampacity of a service requires more than adding up the combined rating of the service disconnects.

If you do that you stand a good chance of overloading the service conductors.

230.90(A)Ex. 3
Quote
Exception No. 3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.

Here is a common example of using Ex.3 This is a service a buddy of mine did.

[Linked Image]

Notice the size of the riser, it has a capacity of about 200 amps. If you add up the six main breakers inside you would get 600 amps.

This is not a 600 amp service, this is an approximately 200 amp service with six 100 amp service disconnects.



[This message has been edited by iwire (edited 01-01-2007).]

Bob Badger
Construction & Maintenance Electrician
Massachusetts