I live outside of Chicago, and my house has older wiring. The wiring to the kitchen oven is armored cable with two hot conductors and a neutral. My wiring system uses the metal sheathing of the AC as a grounding conductor. I want to convert my current three prong plug to a four prong one. Is it safe and within code to use the AC sheathing for a 60 amp circuit? I thought I read somewhere that 20 amps is the limit.

My second question is: Is the grounding hole in in a 50 amp plug bonded to the surrounding metal plate like in a 15 amp plug? So, if the metal plate is connected to my metal box, which is grounded through the AC sheathing, I don't need a jumper cable from the back of the plug to the box, right? The plug I bought is a Lengrand 3894CC6 and here's a couple of pics. Thanks.

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    Is the box flush with the wall, or mounted so it sticks out from the wall? May 7 '19 at 0:56
  • @ ThreePhaseEel I haven't installed it yet, but I was going to make it flush with the wall. May 7 '19 at 1:01
  • The armored cable's an existing run, correct? May 7 '19 at 1:04
  • @ ThreePhaseEel yes May 7 '19 at 1:21

You'll need to use a 10AWG ground pigtail here

For a flush mounted box, NEC 250.146 and in particular 250.146(B) form the controlling Code on how to ground the device(s) inside:

250.146 Connecting Receptacle Grounding Terminal to Box. An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 250.146(A) through (D). The equipment bonding jumper shall be sized in accordance with Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors.

. . .

(B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish equipment bonding between the device yoke and flush-type boxes.

Since your receptacle does not have a self-grounding clip on the mounting yoke (I know of no receptacles >20A that do, even), you will need to use a bare or green 10AWG copper wire from the receptacle ground terminal to a 10-32 screw in the designated grounding hole on the back of the box. This will ground the yoke, faceplate, and receptacle grounding terminal, without relying on the receptacle mounting screws as a grounding path.

As to that sheathing...

Since this is an existing run of cable, and the straight NEC does not limit the ampacity for which AC sheathing can be used as a ground path, I would use this sheathing as a ground, considering the obvious alternative (NEMA 10) is quite a bit worse here. There is also the option of running a 10AWG bare copper wire back to the panel or grounding electrode conductor/system by any route possible to serve as a retrofit ground conductor as per NEC 250.130(C).

(The subpoints to NEC 250.118(8) you mention are Chicago-area specific amendments to the NEC.)

  • I think you forgot to answer the original question of whether using the cable sheath as a ground is acceptable, or if it needs to be a legacy NEMA 10. May 7 '19 at 4:11
  • Yes, I would indeed like to know if I can use the sheathing as a ground here. NEC 250.118(8)(a) says, "The circuit conductors contained in the tubing are protected by overcurrent devices rated at 20 amperes or less." My stove is on a 60 amp circuit. Maybe Chicago or more specifically Oak Park, IL has an exception? Also, while reading this I noticed250.118(8)(b) says that the return path can't exceed 6 ft!?? Every run in my house is longer than 6 feet! Maybe I'm reading it wrong. Any input would help. May 7 '19 at 11:23
  • @user3080392 You say you are NOT in Chicago? Chicago proper requires conduit. That 6' requirement doesn't apply to in-wall wiring, obviously as that would be unworkable. May 7 '19 at 15:58
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    @user3080392 You want to use 4 prong. That way ground & neutral can be separated in the oven, which is safer than having them combined. May 7 '19 at 22:25
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    @user3080392 -- you would be reading it correctly for a new run of cable, but I'm considering it basically grandfathered since it already exists, and having ground and neutral separated is basically always better than having them combined May 7 '19 at 23:04

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