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10 AWG wire is rated for 30A peak / 24A continuous for up to ~50 ft.

For a very short run ~1 ft, can 10 AWG wire support 30A continuous? Assuming the breaker and outlet can support 30A continuous.

Clarification: wire is Romex NM in-wall.

For those wondering what this is for: it's connected to an EV charger that can draw continuous max current for 1-8 hours. The garage has an in-wall sub-panel with a NEMA 14-30 outlet directly underneath the panel. Can't use conduit unless I make a huge hole in the wall. At some point I may replace the 10AWG. 24A is good enough most of the time, I was curious whether I could draw 30A if I really really needed to top off quickly in an emergency. However I don't want to cause another emergency by doing so :D

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    How long a time is "continuous"? What kind of wire - NM cable? Individual wires? Commented Feb 9, 2022 at 3:58
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    Based on the answers so far, it seems that what you're attempting to do is find wiggle room around the NEC code. Finding the wiggle room is how buildings burn down and insurance companies deny claims.
    – FreeMan
    Commented Feb 9, 2022 at 14:44
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    For the title question, you already know the answer. 24A Any load that lasts 3 hours or more is continuous by definition and requires the 125% sizing.
    – Ecnerwal
    Commented Feb 9, 2022 at 15:07
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    No, because of 240.4(D) regardless of what 310.15(B)(16) says. For NM you need #8, since NM can't use the 75C column even when 240.4(D) does not apply. Keep in mind, for a thing like this, you should by buying your wire "By the Foot" from better stores. Those who are married to the idea that certain specific stores are always a bargain and they must never shop anywhere else, should disregard this advice. Commented Feb 9, 2022 at 22:20
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    Re: edit and "huge hole in wall" - no place like a garage to start leaning how easy drywall repair actually is.
    – Ecnerwal
    Commented Feb 10, 2022 at 23:33

3 Answers 3

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No extra safety by keeping it short

Shortening a wire will not allow you to operate a heavier load and run more current than what the wire is specified for.

The maximum recommended length of the wire is determined by the maximum voltage drop at the heaviest allowable load. Shortening the wire brings the voltage at the load closer to the line voltage, but it doesn't reduce the current or temperature of the wire; instead, shortening increases the current.

The heat generated by the current in the wire affects the temperature of the wire, which is the primary limiting safety factor.

The heating of a #10 wire under 50ft of length is only marginally affected by its length. Provided the wire is properly installed, you generally don't get extra safety margin by keeping it short.

For a given load, a short wire generates more heat per foot than a long wire. This is due to the higher voltage at the load if the wire is short, and the resulting higher current through the entire wire.

While length is an important factor in the voltage drop it is not a factor in the wire's temperature, provided the wire is installed correctly.

Long wires at maximum current generate more heat over their full length than short wires at about the same current, but if installed in runs (e.g. in joist spaces, from panel to kitchen), and not in bunches or loops, the heat generated per unit of length matters, not the total heat. This is illustrated in the example below.

Example: 30A over 10AWG

Have a look at the calculations below. They are based on a 120V supply and a 4 Ohm load to produce a nominal 30A current.

As the cabling distance (and wire length) increases, the resistance of the wire increases and reduces the current in the wire.

At the 50ft distance (100ft wire, return loop), the voltage a load has dropped to 117V (about 3% drop).

This is where the maximum length recommendations for wires (such as 50ft for 10AWG) come into the picture: the intent is to not let the voltage at a heavy load drop by too much.

As the wire takes on more resistance, so does the power generated by it over its full length. It's up to 85W over a 50ft distance.

However, the power generated "per foot of length" is more indicative of the temperature rise of the wire, and that power drops from 1.8W/ft to 1.7W/ft.

This drop is not much but it should dissuade anyone from thinking that the the maximum current of 30A for a AWG10 wire can somehow be exceeded if the wire is short. No, short wires cannot tolerate more current than long wires.

enter image description here

Power drops for very long wires

I did make an earlier statement that the power per foot as well as the total power drop as the wire is lengthened. This is partially true: the total power increases and the power per foot decreases for the "electrician's" use cases such as the one illustrated here, whereby the wire resistance is (much) smaller than the load's resistance.

But in in a more general context (not applicable to an electrician's case) the wire can have a resistance equal or more than the load. Then as the wire is lengthened more and its resistance starts to exceed that of the load the total power generated by the wire will diminish as the wire becomes longer.

Have a look at the additional cases below. This shows what happens when the wire resistance starts to exceed that of the load. The numbers here are a bit ridiculous, but that's because I just kept using 10AWG. Nevertheless, these kinds of effects do occur in an electronic signalling systems with much lower gauge wires.

enter image description here

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  • Thanks, that's very interesting
    – pmont
    Commented Feb 9, 2022 at 12:51
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    The statement that a short wire generates more heat in total than a long wire does not seem correct. It seems to me that if the total resistance of the wire is small compared to the load, then the power dissipated in a length of wire L would be proportional to L. Commented Feb 9, 2022 at 13:18
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    @JimStewart - the load sees more voltage. Most loads (certainly resistive ones) draw more current with more voltage applied to them. More current in 1 foot of wire is more power in the resistance of 1 foot of wire so it runs hotter than less current (from less voltage applied to the load) in 50 feet of wire, so even though the 50 feet of wire generates more total heat, it generates less heat per foot. Probably not more in total, but more heat per foot and more temperature rise affecting the insulation.
    – Ecnerwal
    Commented Feb 9, 2022 at 14:29
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    Though I am Canadian, the number is a reference to a different tribe and my profile will tell you I live in NJ.
    – jay613
    Commented Feb 11, 2022 at 18:57
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    Nice edit. Deleting my previous comments pre-edit, and later this one.
    – jay613
    Commented Feb 11, 2022 at 18:59
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If the breaker can support 30A continuous it's a 40A breaker (125% rule for continuous loads.)

If the breaker is 40A, you can't connect 10GA wire to it.

Done.

Given "a very short run, about 1 foot" even if you oversize the wire - it won't cost much. While if you undersize the wire it will be hazarous and cost you the insurance coverage you won't have when the fire investigation reveals the cause of the fire to be your undersized wire. If you live through the fire.

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    Actually, get 2 feet. Need several inches on each end (both code and practical) to work with. Commented Feb 9, 2022 at 14:57
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    Where "several inches" = 6" sticking out of the wall. I'd suggest 3 feet of wire, just to be on the safe side. The extra $1 or two will be much cheaper than buying it all over again at a longer length when you get home and realize you skimped just too much.
    – FreeMan
    Commented Feb 9, 2022 at 15:09
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    I'd consider the relationship to the breaker box - a cable that short implies a box right next to the panel, at which point using a conduit offset nipple rather than cable in the wall might be more efficient. In any case, measure the actual distance and allow for the 6" out the face of the box and whatever run inside the main panel is required. Getting to the neutral, if neutral is needed can add a foot or two inside some panels.
    – Ecnerwal
    Commented Feb 9, 2022 at 15:13
  • Thanks for the tips. Any suggestions on how to stuff that extra 6" of 8AWG into the box? 10AWG is hard enough to cram in, I worry 8AWG will be a nightmare.
    – pmont
    Commented Feb 10, 2022 at 23:33
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    Use Stranded Wire; and a large enough box. 8AWG is 3 cubic inches per "count" so if you have hot, hot, neutral and ground coming in you count the ground once, and the others you have 3 for the wires and 2 more for the device - so 18 cubic inches minimum, if it's just a run to a box with an outlet. You can lose the ground wire if you use metal conduit and a metal box - the conduit becomes the grounding path, while not taking up any room. You can choose to use a larger than minimum box, which makes life easier.
    – Ecnerwal
    Commented Feb 10, 2022 at 23:50
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NEMA 14-30 outlet

That's the answer right there. You aren't trying to "make wire good for 24A continuous/30 Amp intermittent" handle "30A continuous". You are trying to make an entire circuit designed for 24A/30A handle 30A continuous. You can't do that.

But the good news: You are not trying to run a thicker cable through a barely big enough for 10 AWG hole in a brick wall. (Which wouldn't usually be a big challenge anyway with a hammer drill and a big masonry bit.) You simply need to upgrade.

As long as your subpanel has a large enough feed:

  • Replace the receptacle with a larger one. You might want to swap the box for a larger one too. If the existing box is barely big enough for a receptacle with 10 AWG wires, spend the $3 or so for a nice big box and cover.
  • Replace the 1 foot of cable (2 feet by the time you are done) with larger cable or conduit/wires.
  • Replace the breaker with a larger breaker.

As long as everything matches, this is easy. Drywall work for functional purposes is trivial. Drywall is only a (relatively) big deal if you want perfectly smooth with a perfect paint job. This is a garage, not your dining room. Cut a big rectangle. Run your cable or conduit. Patch - possibly even using the same rectangle if you cut it out carefully. A little spackle in the joints. Don't bother to sand and paint if you don't feel like it.

If your subpanel and your overall electric service can handle it, now you've got 48A of charging on a 60A circuit.

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  • Thanks. I figured I'd need to upgrade the breaker and outlet at the least. I wanted to check with the experts here whether the wire also needs to be upgraded. Threading thick gauge wire and cramming it into the box is no fun, but that's what's needed.
    – pmont
    Commented Feb 11, 2022 at 16:54

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