Question Concerning Service Overload

[Eric Hyde]

I had a client today who claimed to be an electrical engineer and had a problem with the main electric panel. There was a 100amp feed, no single main disconnect, and instead 4 disconnects with combined amperage exceeding 100amp by 60 or 70 amps. He assured me that the 100amp feed could overheat unless a 100amp main breaker was added. I told him he was full of crap. Kidding.

I see this set up all the time from the 70's and 80's and never heard of it being an issue. Any thoughts?

[Robert Meier]

Take a look at NEC 230.90(A)Exception #3.

[Larry D. Kage]

VII. Service Equipment — Overcurrent Protection
230.90 Where Required.
Each ungrounded service conductor shall have overload protection.

(A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit. Single-pole circuit breakers, grouped in accordance with 230.71(B), shall be considered as one protective device.

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.

[Cameron Anderson]

Would the calculated load be the all equipment used by any paticular home owner added up?

[William Blackmon]

If I understand correctly, you are describing a service entrance feeding multiple units? If that is right, then the connected load is not the sum of all the individual loads. There is demand factors applied to the calculations, in addition to the minimum required dwelling circuits. Logically, for example, it stands to reason not every occupant would be cooking, or doing laundry at the exact time. That is the pupose of the demand factors.

[Robert Meier]

Originally Posted by canderson5

Would the calculated load be the all equipment used by any particular home owner added up?

No, but William touched on it. The NEC in Article 220 has a calculation to determine the connected load for a dwelling. This takes into account demand factors which reduce the minimum service size from an ampacity created by the method (simply adding the loads together) that you've mentioned.

So if by using your method the total connected load were 150 amps and using the Article 220 method the connected load was calculated to be 100 amps then the service entrance conductor size would only need to be 100 amps.

Even if you had up to 6 service disconnects when you add their ampacities together (as in the OP) they could total more than the ampacity of the service entrance conductors and still be code compliant by the Exception #3 in 230.90(A).

[Eric Hyde]

You guys are great. Thanks a bunch!