I have a 30amp dryer circuit (4x10 gauge - 2hot, 1neutral, 1gnd) and a 20amp circuit (3x12 - hot, neutral, ground) coming into the same metal box. The 20 amp circuit continues on to another box where the 20amp outlet is and the 30amp outlet is in this box.

Do I need to splice all the grounds together in this box (and to the box)? Or, can I directly connect the 4x10 to dryer outlet (without any splices with the 20amp circuit), then splice the 12gauge ground together and to the box?

If I do need to splice them all together, do I need to use 10AWG to the metal box?

EDIT: Basically I'm wondering if doing something like this is acceptable, or if I need to also to connect the incoming line to the 30amp dryer outlet to the ground bundle. enter image description here

  • What wiring method is being used here? Apr 12, 2018 at 22:35
  • @ThreePhaseEel - not sure what you are asking exactly. It's 120v split phase with two separate circuits (1 120v 20amp circuit and 1 240v 30amp circuit) entering the same box.
    – astoffel
    Apr 13, 2018 at 0:02
  • Are you wiring this using plastic-jacketed cables? Metal-armored cables? Conduit of some sort or another? Apr 13, 2018 at 0:12
  • The wiring entering the boxes (on the right side in the image) is coming in through grounded FMC (all 6 conductors are actually in the same conduit even though my image doesn't show that) and the wire continuing on to the 20amp outlet is in MC
    – astoffel
    Apr 13, 2018 at 0:22
  • Is this a flush-mounted or a surface-mounted box, and how long is the FMC run? Apr 13, 2018 at 0:30

1 Answer 1


Splice the ground wires together and to a box ground pigtail

You'll need to splice the ground wires coming into the box to each other and to 10AWG pigtails to both the receptacle ground terminal and the box grounding screw, as per NEC 250.148. That way, there's a direct path for ground current from the MC's ground conductor to the ground wire in the FMC, instead of sending ground current from the MC cable through the box and the yoke of the NEMA 14-30R in order to get to the ground wire in the FMC (or equivalently, through the box and then the 5' of FMC to get to the next box).

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