Existing doorbell transformer in 1950s era California house is sitting in open on a wood shelf in unfinished garage, supplied by non-metallic fabric-jacketed, plastic insulated 2-conductor cable in good physical condition. No ground, no conduit, no strapping, no junction box. I am replacing the transformer with a higher VA model for video doorbell, and wish to make the install better.

Replacement transformer will be mounted on knockout in a 4x4 metal junction box secured to framing, with existing 2-conductor ungrounded cable entering jbox via NM cable clamp. Because of cramped area in/around service panel, ground will be separately supplied to jbox via wire clamped on to Grounding Electrode Conductor.

Question: Can I extend this circuit from the new grounded jbox using metal conduit + THHN hot and neutral wires, or do new outlets supplied by extension have to have standard ground-with-conductors in same raceway all the way back to the service panel? That is, does the 2-wire + separate ground routing for the existing wiring from service panel to jbox prevent further extensions?

  • As far as I understand, metal conduit equals a ground wire. I have read newer code allows for a separate added ground to circuits without ground. Extending the circuit should be good, grounding the whole house with one 14 gauge ground wire to GEC, not so good.
    – crip659
    Sep 7, 2023 at 20:42
  • 1
    @crip659 Grounding a few extra 20A outlets with a 10 gauge ground wire to GEC :)
    – Armand
    Sep 7, 2023 at 21:01
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    Any chance of replacing the existing cable with three conductor cable? Do you know what size it is?
    – JACK
    Sep 7, 2023 at 21:36
  • @JACK Not at this point. Trying to break/make connections inside the untouched-since-1968 service panel is not something I want to rush into.
    – Armand
    Sep 8, 2023 at 1:15

1 Answer 1


You can do this

NEC 250.130(C) explicitly states "branch-circuit extension" as one of the things that can have its ground connected to a retrofit EGC, so your plan with the retrofit EGC is fine.

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