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I understand that for feeder wire from a service entry panel to a subpanel the conductor wires need to be sized according to 125% of the continuous load plus 100% of the non-continuous load. I've also found various definitions (CEC and non-residential NEC) for continuous loads being those on for more than 1 hour in a 2 hour period.

For residential usage, I generally won't have all the continuous loads running simultaneously (ex. lights, an EV charger, HVAC, etc.), so do I still have to sum all the continuous loads or can I size for an average continuous load?

EDIT: The question also applies to non-continuous loads. Do I need to sum all of them, even if not all will be active simultaneously?

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Under the NEC there is no differentiation between residential and non-residential with regard to continuous loads. A continuous load has a very clear definition that most people interpret wrong. It's not any load that "is on" for a certain amount of time. It is a load that is expected to run at maximum current for a certain amount of time, three hours.

Continuous Load.

A load where the maximum current is expected to continue for 3 hours or more.

The key words here are "expected" and "maximum current".

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  • I actually didn't find the definition in the NEC directly, but on an industrial electrical website, thus my confusion. Also, the definition you provided was identical to the CEC definition. Under this definition, though, does anything in a residential setting even qualify as continuous?
    – Hari
    Jan 20, 2017 at 22:30
  • Some things certainly could. Outside lighting, especially floods and security lights could very well be considered a continuous load. Certain things are required to be considered as continuous loads regardless of their usage, such as fixed electric heat. Jan 20, 2017 at 22:32
  • Gotcha. Can you add which section of the NEC has the requirements for loads that are always considered continuous? I have a heat pump, which may qualify then.
    – Hari
    Jan 20, 2017 at 22:34
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    It's not that they are all listed together. Some things are listed that way. Some are just required to be rated at 125% of the load, which is the same basic thing. A heat pump is one of these things. The circuit would need to be rated at 125% of the running load current. Jan 20, 2017 at 22:36
  • Feeder load calcs can be tricky. The loads in a dwelling can be so transient it's hard to tell what to add together to get "the sum of the non-continuous loads". Jan 20, 2017 at 22:37

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