How hot does an electrical house wire normally get?

Question

From a theoretical point of view, how hot does an electrical wire in a house get in normal use?

To be more specific, in the United States, there are two usual residential circuits, 20 amp circuits that use a 12 gauge wire and 15 amp lighting circuits that use a 14 gauge wire. The usual method of cabling these wires nowadays is to use Romex, but I would be interested in the heat profile of coated wire in EMT as well. I assume it does not get as hot as the Romex because the Romex is considerably more insulated. If we assume an ambient temperature of 72-degrees Farenheit, how hot do these different wires become if they are fully loaded (meaning using 20 amps continuously in the first case and 15 amps in the second)?

If you only want to do one of these types in your answer, please do a 20 amp load on a 12 gauge wire in Romex at an ambient temperature of 72-degrees Farenheit. Assume no wall insulation, just the Romex.

Note that this question can be answered by experiment or by theoretical calculation. If you do the experiment, note that you need to measure the wire temperature, not the outer temperature of the Romex jacket.

Answer 1

I expect the “ampacity” rating should define the current at various temperature ratings is proportional to diameter and the resistance per unit length which will vary according to different thermal insulation ratings.

• for example AWG 10 is about 1 mΩ/ft has a standard ampacity of 20A, 30A & 40A at temperatures 60’C, 75’C, 90’C at an ambient of 25’C . This translates to a temperature rise of 35, 50, 75’C for a wire rated for 30A.
• In other words a 55’C rise at a rating of 30A with AWG 10 having a diameter of 0.1019” = 2.588 mm
• Since wire resistance is approx. doubled every changes of 3 AWG, AWG 13 is 2R ( AWG10) and AWG 16 is 4R greater than AWG10 .
• Yet AWG 10/16 diameter ratio is ~ 2 and the ampacity rating at 90’C (+65’C) for AWG10/16 is also ~2 so I expected AWG 16 ampacity to be ~1/2 of AWG 10 which was 18 A which is close to my estimate of 20A
• Therefore for standard insulation I expect the rated temperature rise for AWG 16 rated for 15A to be 15A/18A of +65’C = +55’C above any temperature in ‘C and that this will be close to the safety limit for the wire.
• Whereas the 10s melting current for AWG16 is 117A

This my “paper napkin analysis” of rated temperature rise for insulated wire without looking up and wire any cable standards for household wiring.

Although personally I have only experienced extension cord temperature rise on the outside jacket around 15’C rise and not the bare live wire temperature rise.

• Don’t use ‘F if you want to compute temperature rise per A per AWG or compute wire temperature rise from ampacity for any size but I expect it is dependent on wire diameter and insulation thickness.

• for EMT (thin-wall) refer to this or that with 5 x AWG 10 wires permitted, presumably at rated max safe thermal ratings of perhaps 90’C for wire temperature.

Answer 2

The NEC does not allow 20 amp continuous loads on a 20 amp branch circuit. It requires an 80% limit. See 210.19(A)(1)(a) (2020 NEC).

Any load that is continuous for 3 hours or more must be up-rated by a factor of 1.25 to create this 80% headroom.

This headroom is needed not for the wire, but because normal breakers are not listed for continuous use at their rated trip point. They are 80% breakers. So a 15 amp breaker is only rated for 12 amps continuous, and a 20 amp breaker is only rated for 16 amps continuous.

The real-world duty factor of a 20 amp branch circuit in a home is complex, but it's never 20 amps constant unless the circuit is overloaded. Consumer devices are built to these limits so hairdryers on 15/20 amp plugs don't go over 16 amps (120x16 = 1,920 watts). Bathrooms must have 20 amp circuits per NEC.

To create a constant 20A load for an extended period to generate maximum heat, one must have the perfect combination of constant load devices like light bulbs that add up to something near 20 but not enough to trip the breaker for hours.

So the heat on a normal 20 amp residential circuit won't normally ever be from a 20 amp constant load, without intentionally creating the perfect load. Any random combination of consumer devices will either go way over 20 and trip the breaker or be likely under 16A.

So real-world wires in real-world homes are more than a little hard to find at over 16 amps.

At 16 amps. and .0018 ohms per foot, we have I²R = .46 watts per foot. In Romex where we have the current going both ways in two wires, we have about 1 watt per foot of internal heat in the cable.

I do not know how to calculate the heat dissipation of Romex and the steady-state temperature from this, but I've never felt a Romex wire that felt warm.

So I ran a test using a hair drier on a 20 amp circuit where the 12 gauge Romex was exposed.

Wattage was measured at 1355 W and voltage at 122V (123V before turning on). So 1335/122 is 10.9 A.

Using an IR thermometer I measured the basement floor at 66F and the plywood sheet on the wall measured 72F.

After running for about 30 minutes, with the small gauge cord wrapped up to generate more heat, I measured the heat of the coil at a max of 90F. It felt noticeably warm.

The #12/2 Romex stapled to the plywood was registering 76F (+4F rise from 72) but this was too small for me to feel any heat by touching.

The plug of the dryer when I removed it was the hottest at 128F!

Conclusion.

So this 11 amp test on a 20 amp #12/2 Romex branch circuit stapled to plywood saw a 4°F temperature rise in 30 minutes of the outer jacket. No clue what the inner wires were running but I would assume much hotter. It didn't seem to be changing so a longer test I do not suspect would produce much difference in the numbers.