grounding j-box on new range

odo53May 8, 2012

I want to update the range outlet from an older style 3 prong (10-50R) surface mount box to an inwall 4 prong (14-50R) for my new range. The new range is tighter to the wall so the inwall connection makes better use of space. The existing cable is non-metal sheathed 3 conductor 8 awg with a 10 awg ground. I have put in a double gang metal box for the outlet and run the cable into it with proper clamping.

What is the required grounding procedure for the metal box? I have 14 awg grounding pigtails and I have the special green #10 screws for 20 amp, 120V service but this seems inadequate compared to the 8 awg wires that are carrying current in. What is the required wire gauge for grounding the box? How do I attach that cable to the box? Can I use the pre-threaded hole with a #10 screw?

Thanks much! And apologies for double posting, I put this in the appliances forum before I realized this is the better place for this question.

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The ground must be 10 awg or larger for this circuit. It must be connected to the box (under that screw that is threaded into your metal box) and also to the 10-50R receptacle. You can use a pigtail to connect this ground, as long as the ground wires are at least 10 awg.

Under normal circumstances the ground will carry no current. It must be large enough to trip the circuit breaker quickly should a fault occur.

    Bookmark   May 8, 2012 at 9:54PM
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"I have put in a double gang metal box for the outlet and run the cable into it with proper clamping. "

Sounds like the wrong box.

Range outlets are normally installed in a 4x4 inch box with a cover plate to carry the receptacle.

You may have exceeded box fill badly if you used a pair of device boxes.

    Bookmark   May 9, 2012 at 11:37AM
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Thanks for the advice. I apologize if I have the terminology wrong. The metal box I am using is 4" x 4" x 2". I have 3 current carrying wires and a receptacle, but the receptacle is double width. That should be 3 + (2 (for receptacle) x 2 (for double width)) = 7 conductors for the box volume calculation, right? 7 x 3 in^3 = minimum 21 cubic inches. I thought I was fine going with a box with over 30 cubic inches.

Is there a trick to bending these 8 gauge wires? I'm thinking about switching to a bigger box just because I can't bend the wires sharply enough to get the receptacle mounted.

    Bookmark   May 9, 2012 at 9:06PM
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The wire count is 3 (hot, hot, neutral) + 2 (device) + 1 (all grounds).
If an internal cable clamp was used add another allowance.

You are at least 6 allowances of #8 wire at 3in^3 each, for a total of 24 in^3 of box volume.

A 4x4x2.125 square box is 42 in^3 (14 #8 allowances).

See Table 314.16(A) and 314.16(B)and 3 314.16(B) for box fill calculations.
If the ground is #10 you could roll that in to reduce the volume slightly (2.5 in^3) for the ground fill, but it does not appear to matter.

Even with adequate fill, larger conductors can be a real PITA to position.
The biggest 'gotcha' is damaging the insulation as you try to shape them.

Smooth jawed pliers work best here.
Flat nose and round nose allow shaping with less chance of gouging insulation.

At least some of it comes down to muscle.

Make sure you keep strands tight after stripping and use no-corrode on any aluminum.

    Bookmark   May 10, 2012 at 10:09AM
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