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
Start a Discussionhttps://teslamotorsclub.com/tmc/tags/

Calling All Electricians - NEMA 14-50 Power

Discussion in 'Model S: Battery & Charging' started by JPoldo, Aug 31, 2017.

  1. JPoldo

    JPoldo Member

    Joined:
    Aug 13, 2017
    Messages:
    23
    Location:
    Sudbury, MA 01776
    I'm installing a wall outlet 25 feet from the breaker box and wonder if #8 copper wire is sufficient to avoid a large voltage drop causing the charger to drop below 29 amps/hr. I read the charger senses voltage and if too low, reduces charging power significantly to avoid heating conductors. 40 amps through 50 ft (25 ft x 2 wires) produces a 1.24V drop at room temperature per common resistance tables. I prefer #8 over #6 because it's easier to install and lower cost. Will I get maximum charging with #8 wire?
     
  2. Rockster

    Rockster Active Member

    Joined:
    Oct 22, 2013
    Messages:
    1,183
    Location:
    McKinney, TX
    Voltage Drop Calculator

    Based on this calculator, you're okay. I tend to err on the side of caution, though. The cost of #6 versus #8 is pretty trivial, all things considered.
     
  3. Rocky_H

    Rocky_H Active Member

    Joined:
    Feb 19, 2015
    Messages:
    1,500
    Location:
    Boise, ID
    Don't use 8 wire gauge. Read Tesla's recommended install guide for the 14-50 outlet.

    https://www.tesla.com/sites/default/files/downloads/US/universalmobileconnector_nema_14-50.pdf

    "Conductors: 6 AWG, Copper Wire Only. Upsize wiring for installations over 150 feet"
     
  4. ColdRauv

    ColdRauv Member

    Joined:
    Oct 29, 2015
    Messages:
    66
    Location:
    Minnesota
    #6 AWG minimum is required by NEC unless it is installed in conduit.
     
  5. JPoldo

    JPoldo Member

    Joined:
    Aug 13, 2017
    Messages:
    23
    Location:
    Sudbury, MA 01776
    Yes, I am installing it in conduit and trying to understand why Tesla recommends #6 when #8 should work fine at 25 feet. Maybe they are covering much longer distances such as 100-125 feet. #6 wire cost is 50% more at HD / Lowes plus extra labor to pull 4 lines through conduit. Am I "penny wise & pound foolish" considering #8 cable?
     
  6. davewill

    davewill Member

    Joined:
    Feb 5, 2014
    Messages:
    537
    Location:
    San Diego, CA, US
    Probably. Hey, the #8 is to code, and it should work just fine, but how much extra is the #6, anyway?

    BTW, you probably just mistyped, but if you're installing in conduit, you shouldn't be using "cable". They should be separate wires.
     
    • Informative x 1
  7. mongo

    mongo Member

    Joined:
    May 3, 2017
    Messages:
    714
    Location:
    Michigan
    Just for reference, that is because #6 has > 50% more copper per foot/ cross section of 26.250 circular mills vs 16.510.
    There are online cut to length wire sources you can price shop.

    General tips:
    Get a permit.
    Use a slightly larger conduit if you are worried about pull, but 25 ft of EMT shouldn't be a problem.
    Less bends are easier, 360 degrees is max allowable.
    Can use lube to make it easier, don't get in on pull rope.
    If you have two people, one to pull on the rope and one to push it is even easier.
    Pull all 4 wires at one time.

    Minimum size for 50 Amps with 60 degree rating termination is 6 Gauge. 6 and smaller can't be remarked, so you will need a green, white, and either 2 black or a black and red.
     
    • Informative x 1
  8. JPoldo

    JPoldo Member

    Joined:
    Aug 13, 2017
    Messages:
    23
    Location:
    Sudbury, MA 01776
    You are correct as I used the wrong term. In conduit, I believe the NEC requires THHN which is about $0.60/ft for #8 and $0.90/ft for #6. Difference in cost is under $50. I simply don't like spending money unless there is some benefit.
     
  9. brkaus

    brkaus Active Member

    Joined:
    Jul 8, 2014
    Messages:
    2,427
    Do consider that if you use larger wiring, you can potentially replace the outlet with an HPWC at some point and charge at 48A (60A circuit) sometime in the future w/o pulling new wire. So, $50 now could save in the future.
     
  10. JPoldo

    JPoldo Member

    Joined:
    Aug 13, 2017
    Messages:
    23
    Location:
    Sudbury, MA 01776
    Thank you ! THAT is an excellent reason for using #6 wire. I am considering a HPWC if the included NEMA14-50 is too slow. If I'm reading the chart correctly, you only get 5 more mi/hr of charging (34 vs. 29) using the HPWC. I'm not sure if the 17% increase is worth $500 to me.
     
  11. JPoldo

    JPoldo Member

    Joined:
    Aug 13, 2017
    Messages:
    23
    Location:
    Sudbury, MA 01776
    Mongo, thanks for all the tips as most have worked well for me in the past. Sorry, tip #1 is has no value to me.
     
  12. brkaus

    brkaus Active Member

    Joined:
    Jul 8, 2014
    Messages:
    2,427
    It is 20% faster, so it is statistically significant. But if it isn't needed, it isn't needed. Still, I like to enable the future opportunity given the minimal extra cost. I'm currently on a 14-15 and doing just fine.
     
  13. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    5,463
    Can I jump in on this discussion to have someone bring me up to speed with respect to the newer HPWCs and multiple-car charging?

    Do I understand correctly that-
    • The new HPWCs are set up so that two EVs of differing capacities, charging hardware &c can be ganged onto one HPWC?
    • Can one supply 3 vehicles?
    • Would a 100-amp circuit still be the appropriate size for same (surely it can't be higher....)?
     
  14. SageBrush

    SageBrush Active Member

    Joined:
    May 7, 2015
    Messages:
    4,383
    Location:
    Colorado
    30 cents per foot * 25 feet = $7.50

    Benefits:
    Less heat losses -- money in your pocket, less pollution (or more PV.)
    Less heat losses, faster charging
    Less heat losses, safer operation, better reliability

    This is a no-brainer
     
    • Like x 2
  15. davedavedave

    davedavedave Member

    Joined:
    Aug 5, 2017
    Messages:
    67
    Location:
    NoVA
    And to add to this - OP: I'd wager that based on your hourly income, pondering this question alone cost more than $7.50 worth of your time ;)
     
    • Like x 1
    • Funny x 1
  16. ucmndd

    ucmndd Member

    Joined:
    Mar 10, 2016
    Messages:
    462
    Location:
    California
    To be fair it’s $7.50 x 4 wires, so we’re talking a whopping $30 - but the point still stands.
     
    • Informative x 1
  17. brkaus

    brkaus Active Member

    Joined:
    Jul 8, 2014
    Messages:
    2,427
    There are a few threads around on how the HPWC operates. Let me see if I can cover it quickly -

    - Each HPWC can only be connected to one car at a time.
    - An HPWC can support different types of Tesla cars, and will charge at the rate the car accepts... up to the configuration of the HPWC (set by a dial inside based on breaker/wiring).
    - If you have limited power capacity, multiple (up to 4) can be wired together to share a specified amount of power
    (example - if you only have 50a available in your house, you could connect 4 HPWC to that breaker, wire the HPWC together with a control wire, and they will ensure they max out at 40a for all 4 at any given time).

    I'm not sure on the algorithm on how it shares.
     
    • Informative x 1
  18. jsmay311

    jsmay311 Member

    Joined:
    Apr 22, 2016
    Messages:
    232
    Location:
    Chicago suburbs
    Not an electrician, but I'll hijack the call with some snooty responses anyway.... :p

    "Amps/hr" isn't a thing. :D

    Is it really significantly easier? Over that short of a distance?

    I think you can install a smaller ground wire. So maybe only save $25 or so. :)

    And the 6AWG would reduce losses by ~0.4%. So you could recoup that $25 over, say, 200,000 miles worth of driving and charging. Then after that it's all profit! ;)

    And lastly, OP, cool zip code! :)
     
  19. gregd

    gregd Active Member

    Joined:
    Dec 31, 2014
    Messages:
    1,321
    Location:
    CM98
    I'd recommend the #6. High power stuff is best over-built if you can, just for peace of mind if nothing else.

    I had a 35' run to the panel for a new 14-50 outlet, and chose to pull #4-3+ground cable even though #6 would be adequate. It was annoying stringing it (the run was almost all under the house, tacked up to the joists), and very annoying dealing with it in the box with the outlet. But the result is that I have virtually no voltage drop, and I have the ability to simply upgrade the breaker and add an HPWC (or even two, if I were to get a second EV) without having to re-string any wires.
     
  20. ColdRauv

    ColdRauv Member

    Joined:
    Oct 29, 2015
    Messages:
    66
    Location:
    Minnesota
    OK, if you're in penny-pinching mode, you could eliminate the ground wire and use the EMT as the equipment ground (not recommended) or go with a #10 THHN green ground.
     

Share This Page