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I need to install a 4 wire subpanel to accept a 240V GFCI breaker for a new spa installation. My main service panel does not isolate neutrals from grounds.
I have an existing 3 wire 240 volt circuit (2 hot and 1 braided bare conductors) for an electric range that is not in use. Can I wire the new subpanel from the end of the existing 3 wire circuit by bonding the new hots to the existing ones and the new ground AND neutral wires to the existing braided ground?
It seems like this would be the same as landing the new neutral and grounds to the same bus at the main service panel.

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    Main service panels don't need to isolate neutrals and grounds. All other panels do.
    – Ecnerwal
    Nov 10, 2023 at 23:08

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Basically NO.

Neutrals and grounds can only be bonded at the main panel. Sub panels they must be separated.

I have a feeling your stove circuit is the old, now banned, two hots and neutral type with no ground (fake ground). A NEMA type 10 circuit

Your sub panel will require a separated neutral and ground wire with an unbonded neutral and ground bus.

Possibly you can run a ground wire (only ground wire) outside of the cable.

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