Circuit Sizing, Receptacles, Loads, and Time

  • Sun 29 November 2020
  • misc

I've had a couple of friends recently check in with me for guidance on specifying some non-standard circuits and receptacles in kitchens and garages as part of a remodel.

Both of these friends were confused as to how much power you can pull from a circuit vs. its nameplate rating, and how OCPDs (over current protective devices), wire ampacity, and receptacles aligned with each other as well as what needed to be derated and when.

I'm neither an electrician nor an NEC code wonk, but with that disclaimer out of the way, here's the simple version:

The ampacity of the wire will vary based on type of wire insulation (THHN in conduit can have a different ampacity than NM stapled to the studs). NM gets the 60 degree C column. But a perusal of NEC Article 240 is in order.

The rule for OCPDs (fuses and breakers) is that you match to the ampacity of the wire. There are some round-up rules, but they don't generally apply to the small circuits that homeowners install (14-10 AWG wire).

What does apply in homeowner land though? Sections 210.19(A), 215.2 and 230.42(A) require the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load, where "continuous" is defined as "3 hours or longer".

So what's a continuous load? A hair dryer is not, unless you are somehow related to Chewbacca. A heater probably is, even though it cycles in normal use. A fleet of crock pots very much is, as is a car charger.

It is "safe" to upsize a gauge (i.e., use 10 gauge wire where 12 gauge is called for). The only downsides are cost and pain-in-the-ass factor (10 AWG is a bit of a bear to fight into place). The upside is lower resistive losses in the wire, which means a longer run can be accommodated without much in the way of voltage drop when you load the circuit up.