Old 3 prong oven plug has 2 hot and 1 stripped aluminum wire. How do I convert this to a 4 pronged oven plug?

Question

The original 3-prong oven plug has the normal 2-hot wires. However, I don’t know if it was a neutral or ground wire that was the third wire, as it’s aluminum and completely stripped. I’ve looked up what to do in this situation but can’t find anything anywhere to fix it. (The wiring in this house was terribly done about 60 years ago)

Answer 1

This particular installation is SEU type cable - 2 black hots and one "wrapped mesh" neutral. Weird that bare is neutral, but it is with SEU.

Advice in this answer Does Not Apply to "/2+ground" (black white bare) NM or UF cable; that cable was illegal the day it was installed, and cannot be used for an appliance which needs 240V+neutral. At all.

Any 3-wire range connection has 2 hots and a neutral. There is no ground. This is a dangerous condition, because a simple, common problem with the neutral wire can cause the chassis of the range to be electrified!

If you want to install a 4-wire receptacle, there are 2 options.

Option 1. You can use a GFCI circuit breaker

This is simply a matter of fitting the appropriate breaker, then marking the receptacle "GFCI Protected / No Equipment Ground".

Option 2. You can retrofit a ground wire

This is a "simple" matter of running a #10 ground wire from the receptacle location to anywhere there also is a #10 or larger ground going back to the panel. That could be the panel itself, a grounded water heater or A/C unit, the grounding electrodes out to the ground rods, or non-flex metal conduit going back to the panel.

Answer 2

What you likely have is Type SE cable with 2 hots and a neutral. There was a time when the NEC allowed to install just those three wires and ground your range or dryer to the neutral conductor (called the grounded conductor in the NEC). Existing 3 wire circuits that were installed when legal are still legal. See the exception portion of the NEC quote below. If UL or other lab has approved your appliance for this type of grounding then the installation instruction will tell you how to connect the ground to the neutral in the appliance and connect a 3 wire cord. Here is the applicable code section:

250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.

(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.