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40 Amp Circuit for UMC Gen 2

Discussion in 'Model 3: Battery & Charging' started by Dynastar, Jun 3, 2018.

  1. Dynastar

    Dynastar Member

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    After doing some searching, it looks like the electrical code allows a 50 amp receptical on a 40 amp circuit. Apperantly this is because there is no standard 40 amp receptical. Since the new UMC only does 32 amps a 40 amp circuit should be fine.

    However all the discussion online about doing this is concerning fixed appliances, mostly electric ranges. Are there any electricians on here that know if it's ok for something like the UMC?
     
  2. eladts

    eladts Member

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    I don't know if it is legal or not, but it isn't a good idea. What would happen if you try to connect something that can pull the full 40A? I would say that if you are adding an 14-50 socket, use the proper 50A breaker and wiring. That would be safer and gives you the possibility to charge at 40A in the future.
     
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  3. TT97

    TT97 Member

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    I agree it is not a good idea. The Gen 1 UMC will pull 40 amps and who knows if a future Gen UMC may do the same (or another connector). Any device connected to it will assume it is a 50 amp circuit and will pull the max it can.
     
  4. animorph

    animorph Member

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    As far as I've seen it is OK to use a 40A circuit for 14-50 socket. You could wire it for 50A and use a 40A breaker if you can't fit a 50A circuit into your panel. Then you're ready for a 50A breaker if that becomes an option later, and the wiring will be perfectly safe. A 32A charging draw is appropriate for a 40A circuit, an 80% rating for the continuous load. Do you have a load calculation to see what fits?
     
  5. davewill

    davewill Member

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    Technically, it's fine. Since the UMC is a plug in unit, there shouldn't be any issue with asking to have a 40a circuit installed. I agree with the others that 50a would be preferable, but if you don't have the capacity for 50a, or the install price difference is too great, go ahead.
     
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  6. Rocky_H

    Rocky_H Active Member

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    I am not an electrician, but this comes up pretty often in the last 5 years or so on Tesla forums. I have read the sections of the NEC about this issue, and you are exactly right, that this 40/50 exception case specifically refers to fixed cooking appliances--ovens, basically. But I've seen a lot of discussion on sites about electrical inspection and permitting and code that says that even though the wording isn't more broad, in practice it is usually accepted by electricians and inspectors for other pluggable appliances where you do know it's a 40A device that will be going there.
     
  7. Dynastar

    Dynastar Member

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    Thanks all. You're right, 50 amp would be the best way to go but I already have the needed 8 guage wire and I'll only be using the UMC for the foreseeable future. I would label the receptical clearly, I was mostly worried about getting it part inspection.
     
  8. Rocky_H

    Rocky_H Active Member

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    That is a good idea. I have heard of labeling the outlet like that also being helpful or recommended in getting the big happy thumbs up from an inspector.
     
  9. davewill

    davewill Member

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    It's exactly how Blink installed the EVSE in my garage, except they used a 6-50. Nor did I have a problem doing it again when I moved into a new house (I brought the Blink with me). I think it's pretty common for non-Tesla installs that are almost always 30a EVSEs.

    I will admit that I kinda wished I had done a 50a circuit after I bought the Tesla-based RAV4EV that can charge at 40a, but I haven't really missed it.
     
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  10. Rocky_H

    Rocky_H Active Member

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    I forgot about that. Many other brands of J1772 stations are specified in their manuals to have a 40A circuit, but they have a 6-50 or 14-50 plug.
     
  11. EVnut

    EVnut Darell, the EVnut

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    This is a non-issue. Worst thing that happens if you plug something into a 40A circuit that draws more than 40A? The breaker trips. Here in CA I've been having 14-50R on 40 A circuits passing inspection for 20 years. Every one of my (many!) chargers and EVSEs has been standardized on 14-50, and every 40A circuit sports a 14-50R.

    That the UMC comes with that 14-50 connector is a great indication that this is your best choice.

    The great news? It should be no extra expense to install a 50A circuit over a 40A circuit.
     
  12. Rocky_H

    Rocky_H Active Member

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    Since when is false news great? The 50A circuit will need 6 gauge, rather than 8 gauge wire, so that's "some" extra expense, rather than "no" extra expense.
     
  13. EVnut

    EVnut Darell, the EVnut

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    Making false news great again!

    Before I can answer I will need a definition for “need” as it pertains to 6 gauge.

    I have two inspected and approved 50 amp circuits using eight gauge wire. Of course the run is one meter.

    (I offer significant credit for using “false” over “fake” BTW)
     
  14. eprosenx

    eprosenx Member

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    My reading of the NEC (I am *not* an electrician) is that 8 gauge copper wire is fine for a 50a circuit on a 50a breaker as long as the wire and breaker and device (or receptacle) terminals that you are connecting it to are rated to 75c.

    I should also note that NM (non-metallic) wire (Romex) is ONLY rated to 60c and so you do need 6 gauge copper wire if using NM per the chart below:

    Ampacity Charts

    I should also call out that I *think* using a 40 amp breaker with a NEMA receptacle 14-50 is allowed. The key is that your overcurrent protective device must NOT be a higher limit than the wire is capable of handling. So if you have NM cable of eight gauge copper then due to the 60c insulation rating for NM cable, the max size breaker you could protect that with is a 40a breaker.

    I believe the relevant part of the NEC code to look at is:
    210.21(B)(1)
    "Single Receptacle on an Individual Branch Circuit
    A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit."

    Furthermore, if you *do* put more than one NEMA 14-50 on a single circuit then you can only use 40a or 50a rated receptacles and I think this is only allowed for "cooking appliances" that are "fastened in place" per 210.23(C).

    So basically my reading of NEC is that for our car charging purposes, you should just have a single receptacle per circuit.

    Personally, for vehicle charging, I think the most "standard" and "practical" solution is a NEMA 14-50 with 6 gauge copper wire on a 50 amp breaker. That is about as vanilla as you get - though clearly other things will work and can be code compliant.

    I see no issue using a Gen 2 UMC on a NEMA 14-50 plug that is only backed with a 40 amp breaker (32 amps being the 20 percent derate of a 40a circuit and that is what a Gen 2 UMC can draw max). But yeah, I likely would never install a NEMA 14-50 that way since someone later may have a Gen 1 UMC or other EVSE that tries to draw a full 40 amps on that NEMA 14-50 which may work for some time until that breaker blows. Cars are considered "continuous load" and you have to oversize your conductors and breakers by a multiple of 1.25 to be code compliant. So that is where you get in trouble with just a 40a circuit... The Gen 1 UMC is capable of 40amps of actual draw and so it would have no way of knowing it is 100% maxing the circuit (and due to the way breakers are tested in "standard test conditions" there is a good chance you would blow that breaker regularly).

    As to what your "breaker panel can handle" - I have not heard of any panels that can do a 40a breaker but not a 50a breaker. If you have sized the conductors such that they could handle 50a then I see no reason not to do a 50a breaker rather than a 40a breaker. Either way your UMC gen 2 is only going to draw 32 amps max and so that is the number you would use for your load calculations to determine if you have a large enough electrical service to allow an EVSE charging circuit in the first place... (so I don't think the breaker sizing is actually relevant to the load calcs discussion - the loads are what is important).
     
  15. EVnut

    EVnut Darell, the EVnut

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    Thanks Eprosenx.

    Your understanding of the NEC is more logical and complete than most* licensed electricians that I know.
    (Much the same way that I know many "civilians" who know and understand the vehicle code better than our law enforcement officers. Reading comprehension counts for a lot!)

    This should be etched in stone somewhere:
    The most "standard" and "practical" solution is a NEMA 14-50 with 6 gauge copper wire on a 50 amp breaker. That is about as vanilla as you get - though clearly other things will work and can be code compliant.​


    * The best electrician I ever knew was a close friend. His command of physics was outstanding, and he could quickly explain every choice he made... and by explain I sure don't mean "because that's what the code says." I worked for him for years, and he has sadly passed far too early. The silver lining is that my home is full of his work. Including those 50A circuits with 8 gage wire. :)
     
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  16. Rocky_H

    Rocky_H Active Member

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    This is the thing I was getting at, @EVnut . I was disagreeing with the absoluteness of your statement, but my counterpoint isn't quite absolute either. This temperature rating is where it spans a borderline in the ampacity tables about whether you can use 8 gauge or 6 gauge, depending on how the installation is done. Different temperature ratings are required, depending on whether you are using romex or separate wires, in-wall versus outside of wall in conduit, etc. So the 8 gauge I think can be used, but only in some types of installations, not all.
     
  17. EVnut

    EVnut Darell, the EVnut

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    Ah yes. Goes to prove that two Absolutes don't necessarily make a good vodka.

    Or something.

    I wasn't quite sure what you meant about my absoluteness, so I just re-read my post. I guess my "should" (that still isn't absolute though, is it?) was intended to be *could.*

    Cheers,
     
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  18. Rocky_H

    Rocky_H Active Member

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    Oh, you're right. Now that I re-read it, I do realize you used "should", so there's room for variation. I came on too strongly then.
    It just hit me a little wrong, recommending 8, when I know there are installations where that is not allowed, and Tesla recommends 6 in their install guide.
     
  19. eprosenx

    eprosenx Member

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    Oh, I also need to call out that after I posted that last night I found NEC 625 which covers "Electric Vehicle Charging System". There are a lot of good nuggets in there. None of it negates what I said above, but it does provide clarity.

    Specifically 625.40 makes it very clear "Each outlet installed for the purpose of charging electric vehicles shall be supplied by an individual branch circuit. Each circuit shall have no other outlets".

    There is an interesting callout for "disconnecting means". Basically for equipment over 60 amps or that is over 150v (which all level 2 style chargers or the Tesla Wall Connector is) you have to have a disconnecting means in a "readily accessible" location and it must be able to be locked in the open position (i.e. to lock a breaker off so that you can work on the EVSE safely without fear of someone flipping back on).

    What is unclear to me is if my install I just did is now compliant (I installed my wall connector outside without a disconnect switch). I presume that my breaker on the inside of the garage less than 10 feet away counts - but I am unclear). For sure I need to add one of those metal clip things that allows the breaker to be locked in the "off" position I guess.

    This was 2017 code just FYI which is the latest.
     
  20. EVnut

    EVnut Darell, the EVnut

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    Hmmm. Skipping the "outside" part for a moment, it bears pointing out that everything related to "disconnecting means" (readily accessible, locked open) seems to be solved with a receptacle and plug.

    I bring this up due to all the discussion of the downsides of adding a pigtail to the HPWC, and how scary that can be. This is *not* a recommendation, merely a mention.

    The plug is going to be right there, readily accessible. And once it is unplugged, it isn't going to accidentally plug itself back in. An additional benefit is that everybody knows how and where to break the circuit if needed.

    (for the record, I'm using a NEMA 14-50p on my HPWC, and my 14-50R is on a breaker that's a a foot away from the HPWC. And did I mention my 8 gauge, non-Romex, properly temp-rated, up-to-code wire for the 50A circuit? :)

    Over 20 years ago, every EV charger (yes, these were chargers and not EVSEs) was "required" to be hard-wired. And for the past 20+ years, I've plugged in my chargers (and now EVSEs). Call me crazy, but I can't be without the flexibility that this provides. I have three 14-50R's in my garage, and being able to reconfigure my parking and charging instantly has proven to be worth it's weight in gold over the years, and many cars. The only downside for me is that I'm limited to a 50A circuit. But considering that every EV I've owned before the Tesla topped out at about 17 mph charge rate, I'm OK with the 37 mph of the Model 3 on a 50A circuit!
     

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