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I know how to use the wire sizing tables, but I'm unclear on how to size wire before a breaker, and when the panel has no main breaker.

For example, If I have a temp power service panel that is rated for 125 amps, and does not have a master breaker, and has 6 breaker slots, and I plan to install one 20 amp and one 30 amp breaker, do I size the service entrance wire (from weather head to meter to panel) based on:

a. 50 Amps, the sum of the intended number of breakers
b. 125 Amps, because that is what the panel is rated for
c. Some other calculation?

I've been reading through the NEC on service entrances, and the sizing recommendations are for things like schools, hospitals, houses, farms, etc, but doesn't (at least that I can find) clearly state the service wire sizing rules for a temp pole. Normally you would base this off of the main breaker size, but since temp poles don't have main breakers is it the sum of all breakers, or the max rating of the bus?

Does anyone know what code section covers this?

  • I do not know US codes, but assuming there is no upstream protective device, you would size it to the sum of the breakers (50A). You could of course use larger. – Someone Somewhere Mar 22 '18 at 7:26
  • What size fuse is the utility using to protect the panel? That is the size of your conductors since it isn't essentially a sub-panel at this point. – ArchonOSX Mar 22 '18 at 16:15
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According to the National Electrical Code 2017, the service-entrance conductors should be sized based on the loads being served.

National Electrical Code 2017

Chapter 2 Wiring and Protection

Article 230 Services

230.42 Minimum Size and Rating

(A) General. Service-entrance conductors shall have an ampacity of not less than the maximum load to be served. Conductors shall be sized to carry not less than the largest of 230.42(A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.

(1) Where the service-entrance conductors supply continuous loads or any combination of noncontinuous and continuous loads, the minimum service-entrance conductors size shall have an allowable ampacity not less than the sum of the noncontinuous loads plus 125 percent of the continuous loads.

(2) The minimum service-entrance conductor size shall have an ampacity not less than the minimum load to be served after the application of any adjustment or correction factors.

  • The loads will be a well pump and any power tools that need to run. I don't think any of them would be considered continuous loads. The sentence, "the minimum load to be served after the application of any adjustment or correction factors." is not very clear. If the breakers are 20 and 30, and therefore we know that we won't use more than 50, can the service conductors be sized for 50 amps? – Nick Mar 25 '18 at 5:40

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