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I am replacing electric range in an older house with a legacy 3-wire supply. Based on reading posts in this forum, I realize that I can enhance safety by upgrading to a 4-wire supply.

One things I'm trying to sort out is: right now, the range is wired to a 50A breaker. However, the new range is rated for 40A.

Should I replace that 50A breaker with a 40A? I see at least one advantage: I can use slightly lower gauge wire, which would be a little easier to work with. But is there any other advantage or reason to change? The panel is about 7 years old, so it's not an especially old breaker.

Thanks!

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  • Can you post pictures of the inside of the box for the range outlet please? Jul 23, 2020 at 23:05

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First of all, note that if your current wiring is in good condition, you don't need to remove it to install a 4 wire cable. Instead, you can retrofit a ground by using an appropriately sized wire (depending on your breaker choice), running along any route you please, that connects the stove to the panel it's served from, or to any ground wire at least that big that goes to that service panel (i.e. the wire from your ground rods).

As far as the breaker size, check your new range's instructions: if they say it requires a 40A breaker, then yes, you need to change it to that. If instead it says you must use a minimum of a 40A breaker, it should also list a maximum (which will almost certainly be 50A), and in that case you're free to use either.

As far as the wire size, you could downgrade to a lower gauge if you're using a 40A breaker, but since you already have 3 of the 4 wires sized for 50A, personally I'd recommend sticking with the current 50A wire and adding a 50A ground wire as well -- that way if you (or whoever buys your house someday) wants a more powerful range in the future, you'll be ready. It's always allowed to use bigger wire, so putting wire that's good for 50A on a 40A breaker is no problem.

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  • This is a great tip and I didn't realize that. That would make this much, much easier. Can I just run a single green TFFN wire?
    – Jacob
    Jul 23, 2020 at 19:32
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    Yes, that would work. You can also use bare copper if you prefer.
    – Nate S.
    Jul 23, 2020 at 19:35
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    TFFN stops at 16ga, for a 40A (or 50A) circuit you need at least a 10AWG grounding conductor. naffainc.com/x/CB2/Elect/EHtmFiles/Table%20250-122.htm One other advantage to leaving the 50A circuit conductors in place is if you (or someone after you) ever end up changing again to an appliance needing 50A, all you need to do is change the breaker back.
    – JRaef
    Jul 23, 2020 at 20:09
  • Wait sorry OP, I thought you wrote THHN, not TFFN. As JRaef mentions, TFFN won't come in the size you need, but THHN will.
    – Nate S.
    Jul 23, 2020 at 20:14
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    Well said about the retrofit. However if OP has certain cables that were used illegally at the time of install, such as black-white-bare NM or UF cable, the whole cable must go. Black-black-mesh SE cable could be retrofit, but with care keeping the mesh neutral away from ground. Jul 23, 2020 at 20:21

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