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Junction Box Grounding Question - pprobus - 01-18-2017
I am putting a 50 amp outlet for my welder in my detached workshop (detached workshop has 100 amp subpanel, welding circuit would obviously be coming out of that panel). I am running 4 #6 (2 hots, 1 neutral and one ground) wires in conduit to the junction box where the outlet will be. The box I have has a tapped bump out for a grounding screw. For all my other circuits (20 amp), I just used a piece of extra wire from the pull to ground the junction box so that the JB grounding wire was the same size as the grounding wire back to the panel. For code compliance, do I need to use #6 to ground the JB to the rest of the circuit or can I get away with using a #12 wire that I have short pieces of? Its not a matter of being cheap, its a matter of avoiding having to crimp a terminal connector onto the end of the 6 ga. wire, using a piece of solid #12 wire would be so much easier. Again, I will be running a full 6 ga. grounding wire from the panel to the outlet, I am only talking about the grounding of the JB.
Thanks,
Paul
RE: Junction Box Grounding Question - TDKPE - 01-18-2017
For starters, since you're using conduit and pulling individual conductors, you can use #8 copper THHN or THWN.* You don't need the neutral for 240V (only), but if you choose to pull it, it too can be #8 copper.** This is based on using the 75C column of Table 310.16, which is valid for insulation types I listed (among others), and the terminations being 75C rated, which they should be for the breaker and receptacle.
The equipment ground can be #10 copper for circuits up to 60A (Table 250.95).
Edit: If the conduit is metal, and if you use the correct hubs at the panel (assuming knockouts that aren't fully knocked out), you don't technically need to bond the box, as it already is bonded through the conduit. Also, if the receptacle is 'self-grounding', you don't need to ground the box. But I doubt a 6-50 receptacle is 'self-grounding', and if it's mounted in a raised surface box lid (4" square), where the receptacle mounts to the lid, then the lid mounts to the box, I don't believe it would qualify, as the bonding path relies on the cover's screws at that point.
*They can actually be smaller, even for 50A, based on duty-cycle of the welding machine and the number of welding machines on the circuit (Article 630), but for short runs and/or home use, most just stick with the Table 310.16 values.
**I ran a 4-wire circuit for my welder, and plug in a mini-subpanel to make life easier in my garage. I have four 15A circuits on half-height breakers, with one duplex GFCI receptacle on each, so I can run four 1200W space heaters (for instance) and/or use portable tools on that side of the garage. The 6-50 welder receptacle (2 hots, plus equipment ground) is also on that 'portable' electric panel, but with no breaker of its own, as it and the feeder are protected by the 2-pole breaker in my basement shop panel that supplies it. It's a little unusual, but not unsafe. I just have to shut off the big loads (normally heaters, and only in the winter) when welding.
RE: Junction Box Grounding Question - pprobus - 01-18-2017
Thanks for the info. I highly doubt that my welder pulls 50 amps, in reality, but it calls for 50 amps and the breaker is 50 amps and since I don't know if I may use that circuit for something else (I bought all the materials about a year ago, before I had the electric coop put in a generlink in my meter, I was thinking I would use the circuit to occasionally backfeed my house using a portable generator located out by my workshop rather than near the house, yes I would turn off the main breaker in my home's panel before doing this, but now with the generlink, that is pretty much moot, though I can back feed my workshop using the generator when I want to give it some runtime, again, turn off the main breaker in my subpanel in the workshop), I decided to go with a 14-50R receptacle and made up a patch cord to "convert" from the 6-50 plug on my welder to the 14-50 receptacle. Now I just need to, finally, get off my duff and install the circuit.
So, I take it that I just need to use a #10 wire for the grounding of the JB, then, correct? I don't know if that's considered an "equipment ground" since I will have the wire I am using for the ground pulled all the way from the subpanel to the outlet, the equipment would not be grounded directly to the JB except through the receptacle. I just thought that I would need to make sure the JB is grounded enough in case it gets energized by a loose hot wire, it has a path back to the panel, which I would have thought a #12 wire would work OK, especially since it should never carry the full 50 amps. I understand what you are saying about the JB should be grounded via the conduit, anyway, I had that thought too, but I guess I have been using "belts and suspenders" on all my previous electric installs in my workshop. Plus I am not sure if the zinc, set-screw couplings and JB fittings are acceptable for grounding the JB, so I just ground every JB to the circuit grounds for all the JB's I have outlets or switches in.
Thank you,
Paul
RE: Junction Box Grounding Question - TDKPE - 01-18-2017
(01-18-2017, 02:31 PM)pprobus Wrote: So, I take it that I just need to use a #10 wire for the grounding of the JB, then, correct? Correct. Junction box and receptacle. I don't know if that's considered an "equipment ground" since I will have the wire I am using for the ground pulled all the way from the subpanel to the outlet, the equipment would not be grounded directly to the JB except through the receptacle. It is. In fact, the heading of Table 250.95 is "Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment", and the column heading says, "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)". For a 50A circuit, it reads across to #10 copper or #8 aluminium or copper-clad aluminium wire. I just thought that I would need to make sure the JB is grounded enough in case it gets energized by a loose hot wire, it has a path back to the panel, which I would have thought a #12 wire would work OK, especially since it should never carry the full 50 amps. If a hot conductor were to contact it, the conductor or conduit (or both) would conduct thousands of amperes for a half-cycle or two before the breaker opened. That's why the breaker has a 10,000A short-circuit rating, or 22,000A, or more. I understand what you are saying about the JB should be grounded via the conduit, anyway, I had that thought too, but I guess I have been using "belts and suspenders" on all my previous electric installs in my workshop. Plus I am not sure if the zinc, set-screw couplings and JB fittings are acceptable for grounding the JB, so I just ground every JB to the circuit grounds for all the JB's I have outlets or switches in. I always pull an equipment grounding conductor. Even with multiple circuits in the conduit, you only need one, but it has to be sized for the largest circuit protection (breaker or fuse) serving conductors in the conduit or cable tray, so if you pulled some 14 gauge for a 15A circuit along with the conductors for 50A, you'd still need to use a #10 or larger equipment ground. But it's your call whether or not you want to bond the box to the equipment grounding conductor, as technically, it's already bonded to the subpanel.