Question on 3-wire Kitchen Range > 4 wire

[Sierra7]

Howdy All,

I have an older home (circa 1980) that has XHHW Type SE cable on a dedicated wall oven circuit. So, I have 3-wire (AWG 8) from a 40A breaker the service panel (NOT a sub-panel) to the wall oven box. Conductors = 2 black hot + 1 bare, and all 3 are copper.

Our old 240V wall oven is toast. We chose a replacement oven that has a 7 KW rating, so the AWG 8/ 40A dedicated circuit is still good.

The replacement is a JennAir JMW8527AAB oven/ microwave combo. Looking at the wiring diagram, separate neutral and ground are required for 120v components (here, the microwave). Per the attached wiring diagram, the ground conductor only goes to a receptacle for the microwave (on the top/ back of the oven cabinet). That is all that ground serves per this wiring diagram. The neutral takes care of the digital controls, clock, lights, etc. on the oven unit.

My understanding is that for code sake, I am "Grandfathered" by pre-1996 NEC for the Type SE cable containing the bare conductor. I know that red/ black does not matter for the hot legs. The bare copper conductor within the SE cable is currently bonded to the service panel neutral bar.

The replacement oven will be hard wired to the conductors in the wall box.

My question to you all is that with the new 4-wire oven, could I just run a ground conductor to supplement the NM type SE cable, So, I would leave the SE cable bare conductor on the panel neutral bar, thence connect the range white neutral to this bare wire. And run the new ground out to the service

With 40A, a #10 ground is size per NEC. A lot less cost and effort than replacing the old cable.

TIA for your advice.

 

[Stuff]

Unless you have a local requirement you are permitted to attach the range's neutral and ground to the SE cable's bare conductor in the junction box.

Running a separate grounding wire back to the panel should be acceptable but really doesn't make for a safer install. It just changes what things could go wrong.

 

[Sponsor]

 

[Sierra7]

Thanks Stuff,

While it is locally permissible, and fine per NEC, to combine the neutral and ground from the 240v to the Type SE cable bare conductor with a pre-1996 3-wire circuit, my concern is with the 120v microwave (1400 W) which feeds off one of the 120v legs ("red") via a receptacle atop the lower oven unit.

When the microwave is running, the neutral on this 120v would be carrying the return current? So if neutral and ground are combined on the 3rd (bare) wire of the Type SE cable, then the frame of the wall oven (connected to both neutral ground) would be energized at about 12A 120v (assuming a PF of 1), would it not?

Granted, when energized it would also have the return conductor to the neutral bar of the service panel. BUT, if one were to touch the oven frame AND the frame of another appliance that had true ground, then the person touching both could become a conductor for whatever current is flowing through the microwave 120v neutral ??? Maybe I am missing something here...

Or maybe I am just being overly cautious. With kids, I try to think these things through before the wire nuts are tight.

Thus, my question posed on what might be allowable to add true ground to the existing 3-wire type SE cable for this wall oven circuit.

What things could go wrong by adding a ground conductor?

 

[Reach4]

hen the person touching both could become a conductor for whatever current is flowing through the microwave 120v neutral ???

You could put a 40 amp 2-pole GFCI into the breaker panel for this concern.

I am not speaking about what is allowable for your proposed new wire.

 

[Stuff]

You are correct that you would become a conductor. If it was a 50' run of SE and everything was running you could see up to 2 volts potential between the ground/chassis and "true" ground. You won't feel it and it wouldn't trip a GFCI.

Running a separate wire for grounding has a tendency to get disconnected or broken in the real world.

 

[Jadnashua]

I do not think that you can make what you have work as you WILL have current running on your ground lead. Physically, it would work, but you should never utilize a bare wire as a current carrying conductor. On a typical stove, prior to the 4-wire requirement, you had three insulated wires: 2-hots, and a neutral. Your old oven did not apparently need 120vac for anything, so it saved some money by using the cable they did that had a ground wire. If I read things properly, and I'm not a pro, you are NOT grandfathered if you connect your existing cable to a new device that needs 120vac, and thus a neutral...only if you had three insulated wires could you then do it.