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I’m installing a 240v 50amp outlet directly off my service panel. I used a metal box, with 3/4” PVC conduit, and 6ga THHN wire.

Do I need to ground the box in any way, and if I do, can I just run a 12ga ground wire from the lug in the box, directly to the ground bar in the sun panel?

isherwood
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JAS
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  • No this not for a EV. It’s for a welder. I’m wondering how to ground the outlet box, without having to cut and splice the 6ga wire. – JAS Jun 20 '23 at 19:49
  • Where would "cut and splice" come in? You need an *additional* wire for ground. – manassehkatz-Moving 2 Codidact Jun 20 '23 at 20:18
  • I guess cut and splice was a poor description. More like adding a pigtail, like you would on a standard 120v outlet. Obviously you can’t do that here. I’m planning to use #6 single strand THHN wire. Another commenter said I should have used #8, if using single strand, and then a #10 wire for the ground, but since I went with #6, my ground needs to be #8. The outlet is most definitely 75c rated and it is metal on metal with the outlet box, but I can’t find anywhere where it says it’s self-grounding. My plan is to get a strand of #8 and add an eyelet to attach it to the #10 box terminal. – JAS Jun 20 '23 at 20:47
  • On a standard 120V receptacle, if you have a metal box and a good quality receptacle then you don't need a pigtail there either - just ground to the box and receptacle gets it automatically. Make/model of 50A receptacle? – manassehkatz-Moving 2 Codidact Jun 20 '23 at 20:56
  • I’m not sure what brand it is. I bought it at Lowes. I made sure it was AL-Cu 75c rated. It has “NEMA 6-50R 250v 50a” on the face of it. I try to buy quality parts when I can. – JAS Jun 20 '23 at 21:29
  • That sounds like the right stuff. Which really just leaves open the question of why it said 6 AWG. Wait a second...are you using **copper** or **aluminum**? Because 6 AWG is correct for aluminum 50A. – manassehkatz-Moving 2 Codidact Jun 20 '23 at 21:52
  • I am using copper. I have no problem going back to lowes and getting #8 wire, for the conductors. I haven’t run any wiring yet. I have only installed the outlet box, and the plumbing. – JAS Jun 20 '23 at 23:04
  • Does this answer your question? [Proper way to ground a 240v metal outlet box?](https://diy.stackexchange.com/questions/222295/proper-way-to-ground-a-240v-metal-outlet-box) – ThreePhaseEel Jun 21 '23 at 03:03
  • How far is the box from the panel? – manassehkatz-Moving 2 Codidact Jun 21 '23 at 03:06
  • The box is connected to the panel with a long sweep 90° elbow, so I’d say a foot or so. It is a 100 amp sub panel that’s connected to my house main 200 amp panel and that’s about a 60’ run. – JAS Jun 21 '23 at 16:53

2 Answers2

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Yes, the box must be grounded and since you're running PVC conduit, you'll have to run a separate ground wire from your panel to the box. You'll need a #8 AWG copper wire.

JACK
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  • Thank you. Does it have to be AWG, or can it be THHN, in green? – JAS Jun 20 '23 at 19:53
  • @JAS Yes, in green. – JACK Jun 20 '23 at 21:28
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    The AWG stands for American Wire Gauge which is a standardized wire gauge for North America. THHN stands for Thermoplastic High Heat-resistant Nylon-coated wire which is the type of insulation. So you'll want #8 AWG THHN wire. – JACK Jun 20 '23 at 21:41
  • I understand. I guess I was a bit confused because Romex has AWG stamped in the casing, and the wires are one strand of said gauge, whereas THHN seems to be multiple strands of smaller gauge twisted wire. – JAS Jun 20 '23 at 23:11
  • @JAS that's because the smaller AWG sizes of Romex have solid wire and the smaller AWG sizes of THHN come with either solid or stranded wire. – JACK Jun 20 '23 at 23:33
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Yes, the ground needs to go to the metal box first. In most cases the socket will pick up ground off the metal box and no ground wire is needed. The conditions for that involve a receptacle marked "Self-Grounding", or hard flush metal-on-metal contact between receptacle yoke (metal frame) and receptacle box.

The metal box will either have a ground screw, or a site designed to accept a machine screw (bolt) threaded #10-32.

For a 50A circuit using THHN wire in conduit, and with 75C terminals at the outlet (most are)... you need #8 copper conductors.

For a 50A circuit, you need #10 ground with your #8 conductors.

You need a #8 ground, actually.

Whenever you use larger wire than is required for the conductors, you need to enlarge the ground in proportion.

#8 would suffice for your conductors and you chose #6 instead. Since you bumped the conductors by 2 wire sizes, you must bump the ground by 2 wire sizes as well. So #8 ground with your #6 conductors.

Harper - Reinstate Monica
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  • Okay. Everything I read said to use 6 gauge wire with 50 amps and it’s even written on the face of the outlet. That’s why I went with it. Not arguing, just giving my reason. I can get an 8 gauge ground, but then I’m going to run into the issue of attaching that big of wire to the outlet box ground terminal. Would it be your recommendation to use #8 wire, for the outlet? I have everything installed except the wire, because I’m waiting until I’m sure how to safely ground everything. – JAS Jun 20 '23 at 20:03
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    [See this chart](https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/) "Everything" was expecting you to use NM cable, which would be 6 AWG. But you used individual wires. The breaker was almost certainly 75C rated. If the receptacle is 75C rated (and if it isn't then it is probably relatively speaking junk) then 8 AWG is all you need. – manassehkatz-Moving 2 Codidact Jun 20 '23 at 20:16
  • I must say that I've never heard of bumping up the neutral because you used a larger size wire than was needed. Always thought it went by breaker size. – JACK Jun 20 '23 at 21:37
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    @JACK not neutrals, grounds. See 250.122(B). Note that although that describes cross-sectional area, the AWG system is log-based, so if any 2 wires have a difference of 2 AWG sizes, they are always in a 1:1.261 proportion. Insert math stuff here. – Harper - Reinstate Monica Jun 21 '23 at 01:14
  • Thanks, and yes, I meant grounds. – JACK Jun 21 '23 at 01:43