How can I upgrade a 3 wire appliance receptacle to a 4 wire receptacle?

Question

I have a cabin, wired for 220 and even has a sub-breaker near the outlet. There are 3 wires to the connector (two hots and the neutral). The range-top that I want to hook up is a 4-wire model (2 hots, neutral and ground.)

When I put a tester's negative to the neutral, of course either of the hots shows 120v. When I touch the negative to the metal box, the same result with each of the hots (though I can't see any ground wire adapter on that metal box. Touching wallboard with the negative does NOT ground it; when I touch the negative to drywall and the positive to either of the hots, I get nothing.

Questions:

1. does this mean that the metal box is itself grounded somewhere/how (so I can just attach the ground wire to it?)

2. Is there any cause for concern about wiring these directly, without a plug?

Answer 1

Both of the previous answers are completely accurate. New ranges all come with the new four wire plug, however you can convert to a three wire configuration if your box is grounded (in this case your's is) by connecting the ground lug to the box in the female plug. Although this would not meet current code requirements, it is perfectly safe and this three wire configuration has been used for over 50 years before the new 4 wire config was established. This method is a "common sense fix", but the totally proper way would be to run a new 8/3WG Cu or 6/3WG Alu wire to the panel, assuming a 50 Amp breaker is being used.

Answer 2

You might check and see if the wire is in conduit. If so, take all doubt away and fish a ground wire through it. If it metal conduit and no ground that means more places for the ground to fail, like if a connector is not tightened or corrosion. If you can run a ground run the ground. Latest code asks for these to be 4 wires now. If an electrician is doing it, he would have to bring it up to code.

Most of the high end homes here hardwire their ranges. Maybe that's what the higher end range manufacturers expect. It just takes the ease out of removing or changing appliances.

Answer 3

Is the box connected to the main panel with some sort of metal conduit(like EMT) In that case it is probably used as ground

Answer 4

Given that there is a breaker near the outlet and assuming the breaker is in a panel and the panel has a ground bar, he should be able to make the receptacle totally compliant by simply running a protected ground wire the short distance to the panel.

According to 250.122, a 10 gauge ground should be sufficient.

250.120(C) Equipment Grounding Conductors Smaller Than 6 AWG. Equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by a raceway or cable armor except where run in hollow spaces of walls or partitions, where not subject to physical damage, or where protected from physical damage.