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What is the appropriate way to bond a metal junction box containing a receptacle wired with 6 AWG wire? It seems that most metal junction boxes have a 10-32 tapped hole to accept a ground screw but almost all of the pre-built pigtails that I've seen that attach to this screw are 12 AWG. Using some sort of ginormous wire nut to connect 2-6 and 1-12 AWG wires doesn't seem like the right move.

Is there some sort of ground lug that attaches to the 10-32 hold and then accepts the two 6 gauge wires? How is this normally done?

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  • A lug that accepts two conductors would probably work, something like this.
    – Tester101
    Commented Feb 7, 2016 at 16:20
  • What size is the ground wire coming into the box? A 6AWG ground wire would be mighty odd.
    – ThreePhaseEel
    Commented Feb 7, 2016 at 21:43
  • I'm probably going to be using 8AWG with the loads I have in mind, but worse-case I'm thinking I could go up to 6AWG for a welder or a monster air compressor. I assume 6/3 Romex w/ground would have a 6 gauge ground?
    – John Hodge
    Commented Feb 7, 2016 at 22:10
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    Conductors are sized for continuous use. You're allowed to use a smaller ground on 6ga, because the notion is a ground fault is unlikely to flow current long enough to overheat the wire before it trips the breaker. But if you want to spend more on copper for a bit more safety margin, your call. I would.
    – Harper - Reinstate Monica
    Commented Feb 8, 2016 at 2:18

3 Answers 3

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First off, why is the ground #6? Seems big unless the circuit is over 60A.

Personally, as Tester commented, I'd use a double lug screwed to the box via a machine thread screw, that is if you can find one small enough. The one Tester linked is quite large for the application. Smaller ones might be hard to find though. This one is the smallest I could find: http://www.cesco.com/b2c/product/Burndy-K2A25U-Two-Conductor-Terminal-Lug/159535

Another alternative is to splice the two grounds to a third tail and connect that to a single small lug screwed to the box. http://www.cesco.com/b2c/product/Blackburn-Elastimold-L70-Type-L-Slot-Screw/3978

Blackburn mechanical lug

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  • Common boxes already have a tapped hole for grounds. He wants to use that. It is 10-32 thread because common metal boxes aren't thick enough to reliably hold a coarser thread from a larger bolt. Like the ones you'd need to use with the lugs you are recommending, which are made for larger wire than 6 ga. Given that space inside the box is at a premium, why not use parts made for that size of wire and bolt?
    – Harper - Reinstate Monica
    Commented Feb 8, 2016 at 2:40
  • @WolfHarper, the smaller lug will certainly fit with a 10-32. A larger lug can be installed with a 1/4-20 screw and nut into one of the wholes in the box.
    – Speedy Petey
    Commented Feb 8, 2016 at 12:56
  • Interesting, did not know how much wiggle room (heh) they gave you on lug hole vs bolt size. I've used nuts in industrial work where the back of the box is accessible. My concern is places where it is not.
    – Harper - Reinstate Monica
    Commented Feb 8, 2016 at 17:31
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Another solution is a lay-in grounding lug. It allows you to bond up to 4AWG wire to the junction box without splicing or pigtailing the ground wire. You can fasten it to the JB with the existing grounding bonding screw in the box.

enter image description here

From http://www.solar-electric.com/gbdbtsopagrl.html

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the receptacle has steel tabs on it which bond to the box when you fasten it. this should qualify as bonding. I am not a licensed electrician though so you might talk to one.

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    The receptacle CANNOT bond the box. ........ In some instances, such as with self-grounding receptacles, the box can bond the receptacle.
    – Speedy Petey
    Commented Feb 7, 2016 at 16:23

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