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Will this wiring setup work? NEMA 14-50 with 60 amp panel...

Discussion in 'Charging Standards and Infrastructure' started by zzzzdoc, Oct 8, 2014.

  1. zzzzdoc

    zzzzdoc Member

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    Well, the electrician came out today to wire my house for the NEMA 14-50 for my upcoming Tesla arrival. Only charged $350.00 to run the wire from the breaker panel to an outdoor NEMA 14-50 using 8 ga wire. I was originally skeptical about this, as I thought you needed 6 ga or 4 ga wire, but he assured me that 8 ga is sufficient wire size for the outlet.

    Now, my concern. This breaker box is a secondary one, in the workshop in my house (second garage converted to workshop). I had originally thought that the panel was 100 amp service, but he showed me that it is only 60 amp, run from the main panels with 6 ga wire. The run is at least 100 feet.

    I'm also not terribly happy about the prospect of not being able to use much at all in the workshop while the car is charging (things like my cyclone are 20-30 amps at 240V, others 15 amps at 115V, but this equipment has real 5HP motors, so they really do draw sizeable power), but realistically, if I set the car to charge from midnight to 6am, I'm hoping that the car will have enough time to charge at 30-40 amps (how much will I realistically get in amperage from this setup? I thought that breakers are rated to blow at 80% rated, so that immediately lowers things to 40 amps. Are there any other substantial losses in 5 feet of 8 ga wire to the outlet?)

    So, long-winded explanation, but will this work with only 60 amp service to that panel? I can't imagine what it would cost to run >100 feet of 4 ga cable through a circuitous route in my attic to make this happen).
     
  2. dhrivnak

    dhrivnak Active Member

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    I think you found a good low cost solution by having the charging of the car start AFTER you normally quit in the shop. I charge at 2 and it is very rare I do not have a full charge in the morning.
     
  3. Cosmacelf

    Cosmacelf Active Member

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    Yes, your car will charge at 40 amps, and if you set it to charge at midnight, you'll be fine.
     
  4. N4HHE

    N4HHE Member

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    I'm confused by some of your math. Is it a 60A circuit or not?

    Circuit breakers do not trip until some level over their rated current, not under. If you pull 50A through a 50A breaker it will eventually trip but not immediately. For continuous duty one is expected to limit current to 80% of rated. For 50A that is 40A.

    6 gauge copper for 60A at 100' is reasonable. My secondary distribution box is overkill with 02 aluminum on a 70A breaker from the main box to a 125A secondary distribution box. But my Model S charges off a 50A circuit in the primary box. Its a matter of having to buy wire for an HVAC upgrade so I simply bought longer than the HVAC needed. 02 aluminum was the best bargain and nominally good for 90A. Its a <female dog> to cut and thread into terminals.
     
  5. zzzzdoc

    zzzzdoc Member

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    I'm just going by what the electrician told me, which was that breakers are rated for 80% of their marked current, or 40A for a 50A breaker. He installed a 50A breaker for the NEMA 14-50 outlet, which certainly sounds appropriate. And I'm assuming that a Tesla charge would qualify as continuous duty, not peak - so the car should charge at 40A.

    To clarify, the circuit breaker in the main box which controls the subpanel is a 60A breaker. The circuit breaker on the individual circuit going to the outdoor NEMA 14-50 outlet is a 50A breaker.

    To make things worse, I just realized that 1 month ago we installed a well, which draws max 20A on the same subpanel, and usually starts after midnight. So now I have to make sure not to charge the car on the two days a week that the sprinklers are going off. Boy this is getting complicated. The sad part is, I could have easily installed the pump on the other side of the house and avoided this. Sigh...
     
  6. davewill

    davewill Member

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    In your shoes, I'd let the car charge all the time, and just unplug it whenever I was working in my shop. I would also reprogram the sprinklers to run in the early morning after the car should be done charging. Another solution might be to simply set the car to charge at 30a, which should be enough to keep you charged (unless you really rack up the miles) but be low enough to keep you from overloading the 60a circuit.

    P.S. Did you get a quote on upgrading that panel? :smile:
     
  7. LMB

    LMB Member

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    Have 14-50 on 50 amp breaker but typically dial the charge down to 24 amps. We started this when there were said to be issues with the old 14-50 Tesla adapter, but it worked so well we continued after Tesla replaced everyone's 14-50 adapter.. Still charges overnight and dissipates less power in the 125 foot run to the garage. We occasionally dial it back up to 40 amps if we need to charge faster, but this happens a few times a year at most.
     
  8. Cosmacelf

    Cosmacelf Active Member

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    Yeah, when you have to juggle this many things (holy cow, a 5HP motor??), maybe you might want to think about upgrading the panel...
     
  9. shelbri

    shelbri Member

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    Why not change the run from the main panel to the sub panel so that you have 100A service in the sub panel. My main is 200A and my sub is 100A. I installed the 50A breaker on the sub with a 25' run to a NEMA 14-50 and my car charges at 40A every night. I believe I am charging at 28mph so no problem with overnight charging.
     
  10. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    #10 FlasherZ, Oct 9, 2014
    Last edited: Oct 9, 2014
    #8 is sufficient only if it's not NM-B (Romex) wire. If it's wire-in-conduit, #8 is sufficient for 50A.

    Breakers are not rated to blow at 80%. Breakers must trip within 2 hours when current exceeds their rating by 135%. That "size breakers at 80%" rule of thumb that people throw out is a myth and is derived from a misunderstanding of the "continuous load" provision of the NEC. If a breaker is rated at 60A, then it's ok to put 60A of intermittent load or 48A of continuous load on it.

    It is fine to charge your car overnight and leave your load capacity when you're in the shop.

    Also, since your panel is served with #6 copper (assuming 75 deg rating for terminations / wet location -- significantly older cable will not meet this), you could legally replace your 60A breaker serving that panel with a 70A breaker (since 75 deg termination of #6 is 65A and you're allowed to size the breaker one step up when rating doesn't fall on a standard breaker size). That would give you just a bit more headroom.

    - - - Updated - - -

    If your electrician told you this, you should get a new electrician - breakers are not rated for 80% of their marked current; they're rated at their marked current. As I mentioned above, they are required by standards to trip at 135% load within 2 hours (and shorter time periods at more load), and that 80% myth is a misunderstanding of the continuous load rule. (Some people assume that all measurements and ratings on a main breaker or feeder breaker should be considered "continuous" because it's an aggregation of multiple loads, but that's not what the code says. A load is a load at the edge, and breakers are rated at 100% of offered intermittent loads + 80% of offered continuous loads at the edge.)

    Upgrading that breaker in your main panel to 70A (check cable type on the #6 out to the shed to make sure it qualifies for the 75 deg or greater rating) will give you a bit of headroom. The well pump will likely not draw [email protected] - that would be a 6 HP motor. The well load will likely be intermittent enough that it's not going to be an issue, and there are special rules for motor sizing w/ breakers because of their heavy start-up current.
     

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