I am installing an outlet on the front of my house to charge my Kea Niro. The current charger I am using requires a 120v 20A circuit. In the future I expect to get a charger that will require a 240v 50A circuit.

Can I run #8 wires thru the conduit for the 240v 50A circuit but cap the RED wire and just hook the BLACK to a 20A Breaker and on the other end install a 20A recepticle in a box large enough for the future bigger outlet? Then when I upgrade I can install the 50 A double breaker and the proper 50A outlet. Does all of this make sense and seem proper?

  • 1
    You can do this, but it is possible that either the 20 A breaker or the 20 A receptacle will not accept #8 wire. If the wire is stranded, you could trim off some of the strands to allow it to fit. If the wire is solid, you might have to splice on #12 pigtails. May 28, 2019 at 21:45
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    Thanks. On the outlet side I plan to connect a #12 pigtail between the outlet and the #6 wires with wire nuts, may have to do the same on the breaker side.
    – Snobound2
    May 28, 2019 at 21:50
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    Do NOT "trim down stranded wires to fit" - join all the strands to a pigtail that fits, if the wire does not fit.
    – Ecnerwal
    May 28, 2019 at 23:13
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    You never take of strands..1995 ran into this problem had to run #8 thhn because of derating factors .Ideal had a pigtail had wire nut and fork connector on it.That state AHJ allowed and went to circuit breaker.
    – user101687
    May 29, 2019 at 3:42

2 Answers 2


There is really nothing wrong with having a bigger wire. Not knowing how long your future is. Would put in a over sized conduit in emt to handle ,future wire. 1 inch emt. pull in your 12 wire. when you do change it out .Use wires to pull in new ones. Codes may change by the time you upgrade.And wires may not meet new code and be waste of money spent. Or pull your 8 wire add pigtails fine to .

  • With electric cars just starting to get popular, this is great advice. There's no telling what the future will bring.
    – JPhi1618
    May 29, 2019 at 3:15

Yes and no.

Yes, you can run 20A on #8. You are always allowed to upsize wire, and you can run 2000kcmil on a 20A circuit if you really want to. You will need to pigtail going into the receptacle, because it won't accept #8.

No, you cannot run 50A on #8 unless the wire is rated for 75C operation (such as THHN or XHHW individual wires in conduit), and the terminations at both ends of the wire are rated for 75C. The breaker will be; the challenge is the receptacle.

Separately from that, you are required to pull from the 60C column if the wire is NM (which shouldn't be outside) or UF.

On the upside, if you are forced to #6, that is actually good for 55A of planned load, and you can breaker it for 60A (you're allowed to round up to the next available breaker size).

  • table 310.15B16 i.stack.imgur.com/DwtK0.png but I'm fine w/ running #6
    – Snobound2
    May 28, 2019 at 23:36
  • @Snobound2 Yeah that isn't labeled for the temperature, just type of cable, which is misleading... May 28, 2019 at 23:39
  • Yeah, the issue is that receptacles can't handle 75degC at their wire terminations... May 29, 2019 at 0:09
  • Thanks for all the helpful information. One more variant, suppose I run the #6 wires and just cap them and also run 2 #12 wires for the 120v 20A outlet. I am thinking that even after I install the 50A car charger I may still want a 120V outlet on the front of the house. Can both sets of wires go in the same conduit? Can both receptacles go in the same box? Does the ground wire have to be #6 too or can it be lighter?
    – Snobound2
    May 30, 2019 at 19:40
  • @Snobound2 I forgot you were in conduit. Multiple cables in conduit is weird and requires enormous conduit, min 2" here. However if you are usIng individual THWN wires, they will fit in 3/4" and very easily fit in 1". With THWN there is no reason to have a different color for each hot, just use two purples or whatever and call it good. Metal conduit is the ground, otherwise one #10 grounds both circuits. May 30, 2019 at 20:59

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