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Temporary charging at 40A on 40A circuit

Discussion in 'Charging Standards and Infrastructure' started by ccutrer, Jun 20, 2016.

  1. ccutrer

    ccutrer Member

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    Long story short, I remembered my parents have a NEMA 14-50 in their garage on a 40A circuit breaker, run with 8-3 NM. I'm pretty sure it's only been used once or so in the last ~20 years since it was installed.

    If I were ever in a pinch, would there be a problem just letting the car do its thing and pull 40A, for an hour, maybe two. Since this would be a maybe once-in-a-year thing, I'm not worried about doing a load calc figuring in a continuous load. What I'm wondering is since the definition of continuous load is 3+ hours, and I don't intend to get near that, and definitely not a regular basis, can I count on the full 40A?

    Of course, if the breaker trips, I'll dial it down to the correct 32A for the 40A circuit.
     
  2. AEdennis

    AEdennis Active Member

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    Safer to dial it down and work your way up to see what load that circuit can take.

    However 32A and 40A is pretty close to each other and really not much gain for that last 8A
     
  3. cman8

    cman8 Member

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    I installed a 40A breaker and charge at 40 amps no problem over night. Never have tripped the breaker or anything. I think you will be fine.
     
  4. TexasEV

    TexasEV Active Member

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    Why take the chance? Take the few extra minutes to charge at 32A.
     
  5. Rocky_H

    Rocky_H Member

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    Please don't give bad/dangerous advice like that to people. This is serious, and what you are doing, running it overnight like that, is dangerous. Electric code is written the way it is for real reasons. Long term loads need to be 80% of the circuit level. And it is with that expectation that they specify what size wire goes for what size circuit. In most of Europe, they do it differently, where they write their electric code where circuit levels and wiring sizes are specified for expecting constant 100% usage, but not in the U.S.

    As to the original question, I guess an hour or two would comply with the expectation of the NEC, but what if you forget to turn it off and it does run for several hours? It doesn't seem like a good risk to take. In the NEC, electric car charging is actually defined now as a permanent load, regardless of the actual time, because it doesn't cycle on and off, but does stay constantly on for quite a while.
     
  6. ccutrer

    ccutrer Member

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    You should probably get your breaker replaced. It's obviously not functioning like it's supposed to. If there ever is an actual fault condition, it's likely that the breaker won't trip at all.
     
    • Helpful x 1
  7. FlatSix911

    FlatSix911 918 Hybrid

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    I would charge at 32 Amps ... not a big difference compared to 40A when charging overnight.

    Miles per charge.PNG
     
  8. ccutrer

    ccutrer Member

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    The whole point of the thread is for not overnight charging. Being at parents house for an hour, two at the most.
     
  9. TexasEV

    TexasEV Active Member

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    So at 32A an hour or two of charging is a difference of 6 or 12 miles of range vs. 40A. If 6 or 12 miles range matters, then maybe you're cutting it too close.
     
  10. ccutrer

    ccutrer Member

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    No, the only reason I would be doing this at all would be because somehow I went on a trip, and accidentally cut it too close to get home, so could stop by my parents for an hour and gain enough to not be worried. The whole discussion was theoretical as in "since it's theoretically not a continuous load do continuous load rules still apply". I have to agree with others - the speed at which we jump down each other's throat on this forum for doing something "wrong" or "unsafe" is saddening.

    Btw, I recently found out at the Canadian 14-50 adapter automatically limits the car to 32A. I've ordered one and I'll label both of mine clearly, then I don't have to worry about forgetting to manually dial down the car for any similar situation (I also have an actual 14-30 tesla adapter, rather than a homemade 14-30 to 14-50 adapter, for the same reasons). So everyone can chill about cutting it too close or not being safe. I'm definitely not one to flirt with safety margins, but in order to do that, I like to understand where they are, and how they work.
     
  11. davewill

    davewill Member

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    I agree with the others. Use 32a and stay for dessert to make up the difference.
     

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