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Bought a house built in 1928 (in Chicago area - everything original to the house is in metal boxes with armoured (BX) cable. Any newer work has EMT). All the wiring is the cloth wrapped wires and no ground wires. All receptacles are 2 prong. Inspector said you can find the first outlet in the circuit, put a GFCI on it, and then not worry about ground on anything else in the circuit. A multimeter says I have a strong 120 V from a hot wire to the box, so common sense tells me I can just skip the GFCI and put in a 3-prong outlet. I wouldn't even need a pigtail jumper ground because the green screw has continuity with the 10-32 screw and ears of the receptacle anyway, right? Am I missing anything?

  • Are we talking about a box that recesses back into the wall, or sticks out from the wall? – ThreePhaseEel Jan 15 '18 at 0:39
  • Recessed a little. The ears on the receptacle rest against a layer of plaster. – Byergs Jan 15 '18 at 2:13
  • If the EMT provides a continuous path back to the service neutral, then that can be used as a grounding conductor. However, the armor on old "BX" cables, likely cannot be used as a grounding conductor. Check with your local building department, to determine exactly what they'll allow. Just because you're reading 120 volts from hot to the box, does not mean you have a sufficient ground-fault current path. You wouldn't want to have a ground-fault, that turns your BX sheathing into a heater. – Tester101 Jan 15 '18 at 16:03
  • Recessed = cannot use the receptacle yoke (ears) as a grounding path. Plaster isn't a grounding path, nor is the screw-head contact of the mounting screws. The yoke would have to bottom hard, flush and clean on the steel box to count. – Harper - Reinstate Monica Jan 15 '18 at 16:12
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I wouldn't even need a pigtail jumper ground because the green screw has continuity with the 10-32 screw and ears of the receptacle anyway, right? Am I missing anything?

Since they used metal raceways you could do it this way only if the receptacles are of the "self-grounding" type. They are more expensive and have a metal clip on one end of the mounting yoke. Or, if your boxes are surface mount and you remove one of the plastic screw keepers. See (A) below.

Chicago has their own Electrical Code but I would be surprised if it is any more lenient than the National Electrical Code. They are usually more strict. The NEC allows self grounding receptacles to be used for grounding in this article, attention to (B):

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.

(A) Surface-Mounted Box. Where the box is mounted on the surface, direct metal-to-metal contact between the device yoke and the box or a contact yoke or device that complies with 250.146(B) shall be permitted to ground the receptacle to the box. At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke or device that complies with 250.146(B) to ensure direct metal-to-metal contact. This provision shall not apply to cover-mounted receptacles unless the box and cover combination are listed as providing satisfactory ground continuity between the box and the receptacle. A listed exposed work cover shall be permitted to be the grounding and bonding means when (1) the device is attached to the cover with at least two fasteners that are permanent (such as a rivet) or have a thread locking or screw or nut locking means and (2) when the cover mounting holes are located on a at non-raised portion of the cover.

(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.

(C) Floor Boxes.......

(D) Isolated Ground Receptacles......

Check with your inspector to see if you are under the Chicago Electrical Code and if it allows this.

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Although this may sound good and show up on a meter is a good quality ground, all is not well. The circular design of the BX jacket presents some particularly sticky circumstances when it is trying to clear a fault. On longer runs and runs that may not be clamped as tightly today as they once were, the jacket acts like a coil and creates a resistive field. If the fault is not cleared in a short period of time the jacket can actually get hot and under worse case scenario start to glow like a resistive element. Can anyone say 911.

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