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Wire Gauge Size for 50 Amps

Discussion in 'Charging Standards and Infrastructure' started by n00b, Sep 12, 2012.

  1. n00b

    n00b Member

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    Hi,

    After checking out the article here <American wire gauge>, it appears that AWG #6 is sufficient for a 50-amp setup (for NEMA 14-50). However, I recall reading some posts here on TMC saying that it would be better to go bigger (#4, not sure if #5 is common). To my mind, a thicker wire would incur a smaller amount of energy lost to heat (therefore increasing the charging efficiency). Would it be worthwhile to go this route?

    Code:
    AWG#6 - 1.296  mΩ/m
    AWG#5 - 1.028  mΩ/m (21% less than #6)
    AWG#4 - 0.8125 mΩ/m (37% less than #6)
    
    At 40 A over a wire run of 5 m for 9 hours, total energy lost over the wire run for a 300-mile full charge is:
    for AWG#6: (40 A)^2 x (5 m x 1.296 mΩ/m) x 9 hr = 0.093 kWh
    for AWG#5: (40 A)^2 x (5 m x 1.028 mΩ/m) x 9 hr = 0.074 kWh
    for AWG#4: (40 A)^2 x (5 m x 0.8125 mΩ/m) x 9 hr = 0.059 kWh

    After 300,000 miles, the AWG#4 will have saved around 34 kWh over AWG#6.
    At 0.08 cents/kWh, that's only $2.72, over several years at least.

    So it seems getting a thicker wire will not save money, but it will reduce the temperature of the wire itself during charging.

    I guess my questions are:
    1) How much more expensive is AWG#4 / #5 over AWG#6?
    2) How much cooler will the thicker wire be?
     
  2. cinergi

    cinergi Active Member

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    Wow, great question and detail there. I don't know the answers (tho pricing should be easy to find online), but one thing came to mind: some 50A device (the circuit breaker or the plug) might not accept the thicker wire -- so just be sure it does if you decide to go thicker.
     
  3. efusco

    efusco Moderator - Model S & X forums

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  4. teslasguy

    teslasguy MSP P#1117

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    Standard for 50amp is 6 gauge for copper. Use #4 for 100 amp. I'm running 100 amp to subpanel on the garage for future addition of another 50 amp outlet for my wife. For me it cost around $200 extra for the #4 vs #6 for about a 40' run. #4 is much thicker wire to run.


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  5. smorgasbord

    smorgasbord Active Member

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  6. teslasguy

    teslasguy MSP P#1117

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    Unless using aluminum wire. I think #8 is ok for aluminum for 50amp.


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  7. mitch672

    mitch672 Active Member

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    8/3 NM-B is rated for up to 45A, since the Tesla mobile connector will be drawing 40A maximum, they can get away with it. Yes, technically it's on a 50A breaker.

    See the chart here: Application Charts

    With that said, I used 6/3 NM-B on my 50A 14-50, I'm also using 3AWG THHN for the 75A OpenEVSE I am building on a 100A breaker, because it was less money than 4AWG (believe it or not, $0.75 per foot for the #3 vs $1.20 for the #4)

    600 Volt THHN


    Mitch
     
  8. drees

    drees Active Member

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    Depends on the insulation temperature rating.

    efusco's picture is this product: 125 ft. Black 8-3 Romex NM-B W/G Wire-63949202 at The Home Depot

    If the insulation is rated for 75C, it's good for a 50A circuit. If it's only rated for 60C, it's only good for a 40A circuit.

    Wire ampacity chart: Wire Chart

    The product datasheet indicates that while the insulation is rated for up to 90C temperatures, that comes with the note that ampacities are to be rated at 60C temperatures from NEC.

    http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet6

    So yeah - I'm afraid that efusco's Plug and 100Amp line sticker shock is not yet over - this wire is only good for a 40A circuit which means you can only pull 32A from it with a continuous load like charging your car.
     
  9. vfx

    vfx Well-Known Member

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    A longer run (mine is 180 foot) requires an increased size.
     
  10. efusco

    efusco Moderator - Model S & X forums

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    Well I don't like to hear that! Let me contact the electrician &amp; see what he says.


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  11. efusco

    efusco Moderator - Model S & X forums

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    My run is about 18"


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  12. smorgasbord

    smorgasbord Active Member

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    Then it'll be cheap and easy to fix. If you're getting an inspection (which for insurance and resale purposes you should), the inspector will know.
     
  13. drees

    drees Active Member

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    Definitely. It's generally recommended that you keep voltage drop under 4-5% (although personally I'd aim for half that if possible - if for no other reason that you may want to pull more current in the future). So for a 240V circuit where you expect to pull 40A you could have a maximum run using these wire gauges if running copper (used this voltage drop calculator):

    8 AWG: 160ft
    6 AWG: 250ft

    Once you get to 6 AWG size wire the cost of copper really makes you want to consider using Aluminum instead, but you typically need to go up 1 wire size to compensate for aluminum's lower conductivity. Max AL run assuming max of 4% voltage drop:

    6 AWG AL: 175ft
    4 AWG AL: 275ft

    You can see that the next wire size up in AL is slightly more conductive than copper.
     
  14. Todd Burch

    Todd Burch Electron Pilot

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    Everyone here seems to be talking about 8/3 or 6/3 wire (3 strands). I used 6/4 (4 strands) on my installation (Red for hot, phase 1...Black for hot, phase 2...white and ground both grounded). I thought 4 strand was required for a NEMA 14-50?
     
  15. mitch672

    mitch672 Active Member

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    The ground is not counted, it is assumed. So 8/3 or 6/3 is 3 conductors plus a ground, usually 2 gauges smaller, for example, 6/3 and 8/3 commonly come with a #10AWG ground... The current carrying conductors what the 2nd number refers to. The neutral is technically a return current conductor, and is sometimes also reduced 1 wire gauge size (because a balanced 240V circuit should have equal half cycle return current on the Neutral), so for exmaple if you buy 6/3 at Home Depot, you would typically get (2) 6AWG conductors (Red/Black), (1) 8AWG conductor (white) for the Neutral, and (1) 10AWG conductor for the ground, usually this is a bare conductor (no insulation)
     
  16. Todd Burch

    Todd Burch Electron Pilot

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    Thanks...I guess I bought 6/3 and just forgot that it was called 6/3 instead of 6/4 (installed it all a while ago!)
     
  17. sp4rk

    sp4rk Banned

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    Ok, doing our install today for the NEMA 14-50. Home Depot wiring chart clearly states that AWG 8 is fine for 55 amps.
    I'll be pulling 50 peak, 40 continuous. Right?
    We have a run of < 70'.
    So we're going 8. As otherwise I'd have an extra $250 bill for wire alone.
    And originally the circuit was for a range in a kitchen; with a 50amp breaker on it already.
    What would the symptom be if we're too "thin"? Reduced charging? Or smoke?
     
  18. FlasherZ

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

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    Your load is a 40A continuous load, which requires a 50A branch circuit, appliance, and receptacle rating.

    For wire size, the electrician's favorite answer: "it depends".

    8 AWG wire is good for 40 amps for 60 degC systems, 50 amps for 75 degC systems, and 55 amps for 90 degC systems. To qualify for the 90 degC rating, *every* point in the system must be 90 degC rated -- and that can be tough. Typically, for ampacity purposes, you look at 60 degC for Romex (NM-B) and 75 degC for wire-in-conduit.

    If you're using Romex (type NM cable), it's only good for 45 amps because by code it must be de-rated to the 60 degC rating (NEC 334). AWG 8 is good for 50 amps only if all connection points, breakers, conductors, termination boxes, etc., are all rated for 75 degC or higher. If you're running through an attic that has a high ambient temperature in the summer, you have to de-rate / correct for that per NEC tables 310.15(B)(2).

    This is why many electricians will just forego #8 for 50A and install #6, which gives plenty of headroom for all de-rating needs.
     
  19. Lloyd

    Lloyd Active Member

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    FlasherZ is the expert here, but you will get a 5 volt (2.1%) drop using #8 for a 70 foot run, and it will get warm. Be sure that you use wire rated to 90 degrees C. If you are pulling wire I would pull #6 then drop is 3 volts.
     
  20. sp4rk

    sp4rk Banned

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    Our run is in the basement of the house ... coolest spot. Never goes about 70f ambient.
    Insulated Copper wire, in 3/4" conduit. With 50amp breaker.
    Do I care about the additional 2 volt drop?
    And I should mention that 1/3rd (final 1/3rd) will be #6 as we already bought it.
     

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