I want to use 8/3 wire for my 75' run and use Nema 14-50. Since mobile charger only draws 32amps max, I figure it should be fine. Has anyone does this? Thoughts? I am running the BX cable outside in soffit so would like to stay with 8/3 and use 40amp vs running 6/3 and running 50amp.
Lots of comments here:
First off, when referring to "BX" cable, I assume you mean what the NEC refers to as "AC" cable. I would probably recommend MC cable instead since it has a dedicated ground cable in it of sufficient ampacity that you don't also have to depend on the AC cable sheath for grounding.
Yes, it is fine to put a NEMA 14-50 on a 40 amp circuit breaker (with sufficient ampacity wire) if the load you intend to serve is only 40 amps (or in this case 32 amps continuous which requires a 40 amp circuit). The relevant code section here is: 2017 NEC 210.21(B)(1). A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.
Furthermore, my reading of the ampacity charts and the code on AC and MC cable *might* allow you to even use a 50a breaker and have it be a full 50a circuit (even though you only need 32). AC cable may be allowed to use the 75c insulation rating (if not run in "thermal insulation"). MC cable I see no restriction on using it's 75c rating even if in insulation. This is what would allow you to use it as a full 50a branch circuit (otherwise it is limited to 40a if only able to be used at the 60c rating like NM cable - romex).
And for the folks that will call out voltage drop: Note that it is NOT a code requirement to stay within 3% / 5%. Just a recommendation. Though a quick calculator I tried online shows that as well within the recommendations even with eight gauge wire with a full 50a load on it.
As mentioned above, if you do go with a 40a breaker you may want to label it as such on the receptacle (though I know of no requirement to do so).
I should also call out that you may need to adjust the ampacity of the wire due to the heat in the soffit you are planning to run it through? Anything above 86 degrees farenheit I think. (so it is possible this could bring you back down to only supporting a 40a circuit)
Oh, and AC / MC cable might not be rated for your outdoor application (wet, etc...). I would check up on this before committing to this kind of install. See sections 320 and 330 (you can register for free access to the NFPA online now).
A lot of folks on the forums do recommend that you always run 6 AWG wire for the NEMA 14-50. It is what the Tesla spec sheet recommends, though what really matters is code. I don't think the "you must follow manufacturers instructions" thing applies since you are just installing a receptacle, not a hardwired device. (I personally would heavily consider running 6awg as long as I was at it... That might let me install a Wall Connector later at 60a).
"Using a 50A plug and receptacle is allowed on a 40A circuit because of an exception in the Electrical Code. The exception is because 40A receptacles do not exist, or rather, are not part of the NEMA standards. (There are enough varieties of receptacle already!)"
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40 amp range into 50 amp plug
Should be ok, just have to remember to check other devices that might pull over 32 amps before plugging them in. Maybe a sticker on the outlet or something.
Yes, if just doing a 40a circuit you need to make sure devices plugged in to that receptacle don't draw more than 40a intermittent, or 32a continuous.
Note that I don't believe the "special" code sections / exceptions surrounding the use of NEMA 14-50 receptacles on a 40a circuit for use with ovens, etc... apply in this case since it is a single dedicated outlet for use with an EVSE. The only thing that matters in this case is that your receptacle is rated to at least the ampacity of the branch circuit it is connected to.
Note that Tesla also recommends a couple brands of "heavy duty" NEMA 14-50 receptacles that can withstand the stress better of frequent insertion/removals.
The label recommended above is a good idea but I don't think it is a code requirement (if you only feed it with a 40a breaker).