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For reasons irrelevant to my question, I’ve been looking at the 100A subpanel in my house and noticed that my electric oven (which the manufacturer says needs 40A circuit) is on a 30A breaker. After briefly panicking that my oven was on an undersized circuit, I popped the panel cover off and measured the wire to be #8 wire (the wiring was redone by the house flippers we bought from and the sheathing is black too, while the dryer lines are #10 and orange sheathing). So it seems that the wiring is indeed the correct 40A circuit. Phew. I assume that they just found it easier to buy a box of 30A breakers and use them for the three 30A+ circuits in the panel.

So here’s my question: for $12 should I go down to the hardware store and pickup a 40A breaker for this circuit so that it is properly sized? In the year we’ve lived in this house the oven has never tripped the breaker, so maybe I should just leave it. Is there a standard way to mark that the circuit really is #8 wire so a future electrician/home inspector/future owner doesn’t also freak out?

Relatedly, the breaker in the subpanel itself is 200A, even though the breaker at the main panel is 100A. How much should I worry about that? In the event of pulling >100 many amps, the main panel breaker should trip and protect the circuit, so maybe that’s also fine? That feeder line is also the biggest wire I’ve ever seen in my life so maybe the wiring is sized for 200A?

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  • 3
    How did you "measure" the wire to be #8? the size should be imprinted on the wire insulation. Check that.
    – JACK
    Jan 9 at 15:00
  • Assuming #8 copper ? Jan 9 at 15:10
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    @JACK I couldn’t read the insulation. The breaker is right at the top of the box so there’s only about 1 inch of insulation in the panel and I crawled around the crawlspace for an hour, found the line, but it wasn’t clearly legible from a vantage point I could get to. I measured the wire literally with my calipers; the dimension of the individual leads (including the leads insulation) was .22 inches, which is what the Internet tells me is #8.
    – levineds
    Jan 9 at 17:24
  • @NoSparksPlease, yes, it is copper
    – levineds
    Jan 9 at 17:24
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    If it isn't broken, don't fix it. Having a breaker that's too small yet doesn't trip is not an inconvenience, it's overly safe.
    – Mast
    Jan 10 at 9:55
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It's fine and normal to have a larger main breaker on a sub-panel than the feed breaker. That is only serving the purpose of being a local shutoff, not as a current limiting device, the current limiting is the responsibility of the breaker upstream. Generally you end up with a 200A breaker because that's what a panel with a reasonable number of spaces comes with, and you want a reasonable (or even "seems like too many at first") number of spaces.

If the 30A breaker has not tripped in a year of using the oven, I would leave it alone. You can upgrade to a 40A if it ever does nuisance trip; in the most fussily technical sense you should make it a 40 because that's what the oven manual calls for, but undersized-yet-working is arguably a hair safer in the case of a fault and unlikely to upset an inspector. If you want to grab a labelmaker and put a note that the wires are 8Ga on there, feel free, but anyone actually looking at the circuit will see that, as you did, so no need, really.

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    Agreed. A 30 amp breaker would trip for an all-elements hosting type of cookup where the oven was working and at least two of the burners were on medium-high heat with the other 2 or 3 also at least averaging a medium heat. Most home cooks aren't even coordinated enough in the kitchen to keep two high-heat burners going at once, so unless the kitchen is seeing some regular and enthusiastic home cooking episodes then the 30A will likely never even break a sweat.
    – J...
    Jan 9 at 20:21

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