TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker and becoming a Supporting Member. For more info: Support TMC

Ok to use Nema 14-50 plug with 40amp circuit?

Discussion in 'Model 3: Battery & Charging' started by gujudog, Jul 12, 2018.

  1. gujudog

    gujudog Member

    Joined:
    Jun 28, 2018
    Messages:
    50
    Location:
    11001
    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.
     
  2. Glamisduner

    Glamisduner Active Member

    Joined:
    Aug 2, 2017
    Messages:
    1,176
    Location:
    Escondido, CA
    "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!)"

    - 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.
     
  3. eprosenx

    eprosenx Member

    Joined:
    May 30, 2018
    Messages:
    991
    Location:
    Beaverton, OR
    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).

    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).
     
    • Informative x 1
  4. ewoodrick

    ewoodrick Active Member

    Joined:
    Apr 13, 2018
    Messages:
    1,032
    Location:
    Buford, GA
    In general, when you install a plug capable of 50A, the circuit behind it should be capable of handling the entire capacity. What happens when you sell the house and someone else tries to use it? If it is 50A, it should be capable of it. If the circuit can only carry 30A, then the next lower plug should be used, then the breaker set for the plug capability.
     
  5. Glamisduner

    Glamisduner Active Member

    Joined:
    Aug 2, 2017
    Messages:
    1,176
    Location:
    Escondido, CA
    But since there's no 40 amp receptacles you are allowed to use a 50 amp. I'd just label it so I don't forget (I didn't lookup codes, but it's easy enough to do :) )

    Also, I'm not an electrician.
     
  6. T3forMe

    T3forMe Member

    Joined:
    Jul 12, 2018
    Messages:
    14
    Location:
    Los Angeles
    My question is what is the rating of the circuit breaker in your power panel. A nema 14-50 is designed for a 50 AMP. circuit breaker and the allowable current draw is recommended to be 40 Amps or lower (a 20% safety current reduction). To draw 40 Amps, I believe 6 AWG wire is recommended. For 32 amps, 8 AWG wire is okay. The "smart Nema 14-50 adapter" supplied by Tesla will limit the current draw to 40 Amps automatically. You can change the charging current to any lower value through your on screen charge icon. Hope this helps
     
  7. eprosenx

    eprosenx Member

    Joined:
    May 30, 2018
    Messages:
    991
    Location:
    Beaverton, OR
    No. This is not how the National Electrical Code is written. Go read the section I quoted.

    You are fully allowed to have a 40a breaker with a dedicated 50a receptacle as long as the intended load is not greater than 40a (or 32 amps continuous).

    I would agree with you that from a practical standpoint it is probably a good idea to have a wire of sufficient ampacity for 50a along with a 50a breaker, but this is NOT A CODE REQUIREMENT!

    The most important thing here is that the *WIRE* and *RECEPTACLE* be capable of handling the current that the breaker opens at. As long as this is the case, if a future owner plugs in a 40a car charger into a NEMA 14-50 that was only wired with a 40a breaker then they will likely just get nuisance trips when the breaker heats up after hours of operation.

    Lots of things to dissect here:

    1. A NEMA 14-50 is rated for up to 50 amps. But it is fine to run less current through it than that (it won't catch fire or anything). The receptacle itself does not care what size circuit it is on as long as the circuit is equal or lower to the receptacles rating.

    2. You are fine running 50 amps through a dedicated 50 amp receptacle as long as it is a non-continuous load. Where the 80% derate comes in is for continuous loads (which all EVSE's are continuous loads so yes, 40a is the limit there for this use case).

    3. On the 6 AWG comment, this is generally mostly correct, but wire sizing is complex and it depends on the type of wire (Romex, AC, MC, THHN, etc...), whether it is in conduit or not, whether it is run through insulation, what the ambient temperature is, what the terminal ratings on both ends are, whether it is copper or aluminum, whether it is run in conduit with other current carrying wires, etc... As an example, 8 AWG THHN in conduit with a breaker and receptacle rated for 75c is fine to be used on a 50a circuit in ambient temperatures of 86f or less.

    4. By the "smart NEMA 14-50 adapter" you speak of I assume you mean the Universal Mobile Charger from Tesla. The Gen 1 version of the device will limit current to 40 amps (its max). The Gen 2 version of the device will limit current to 32 amps (its max).

    5. Changing the current draw in the Tesla software is not recognized by the NEC and I would not trust it to not screw up and accidentally charge more. It is a great feature for a lot of reasons, but I would not rely on it to avoid burning my house down. It has been known to reset itself to defaults from time to time...
     
    • Like x 3
  8. T3forMe

    T3forMe Member

    Joined:
    Jul 12, 2018
    Messages:
    14
    Location:
    Los Angeles
    Thanks for the info. I was not aware that a Gen. 2 version was available.
     

Share This Page

  • About Us

    Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.
  • Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


    SUPPORT TMC