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My house took considerable damage to the roof and electrical supply to my home in a wind storm. The main breaker panel was in a sorry state and so while the damage did not technically reach the panel I took this as an opportunity to replace an old tired panel with hard to find parts with something that would be newer and hopefully safer. One issue was the electrician put like-for-like circuit breakers in the box, and that concerned me. There were obviously 20 amp breakers feeding 14 AWG wires. That is an issue I have already halfway rectified, not discovering this issue immediately at the time. To fix the other half I'm just looking for a good time to have a "spotter" on hand for when the main panel is open. I don't like working with 240 volts alone. Perhaps that's a distracting aside. I'll get to the oven outlet.

As the electrician was here he said something about a shortage of 40 amp breakers for my oven. I asked if a 50 amp breaker would be suitable. He agreed and so my oven is connected by a 50 amp breaker. The outlet is 50 amp but the wire is #8. Is this safe? Looking at ampacity charts it is safe if the insulation is rated to 75C. How do I know it is rated to 75C? I'm not opposed to running a new wire, I would just rather not have to run wire if not required.

I do not see an ampere rating on the oven. It shows 10 kW for 120/240, and 7.5 kW for 120/208. That can be used to derive an amp rating, and depending on how one runs those numbers one can get 40, 50 or 60 as the minimum rating. The oven was second hand and so there is no manual, and finding one has been difficult. I plan on some kitchen remodeling soon and so there will be a discussion on keeping the stove, and if it goes then the replacement will likely be gas over another electric.

I can see this going down some rabbit holes so I'll see if I can keep this simple. If I keep the oven then what, if any, wiring changes should I do? If the oven goes then I'd simply plan making what I have safe with minimal work and extra materials, as that would likely mean I end up with a gas oven. I believe city code requires an electric oven outlet in place, so a 40 amp breaker to #8 wire and a 14-50 outlet should be fine. I'll just install the box in the wall, rather than bolted to the floor as it is now, run the cable a bit more neatly to the main panel, and finish the drywall work in the basement so the cable is out of sight.

Any comments? Is the 50 amp breaker fine? Should I use 40 amps instead? Is 40 amps enough for my current oven? Should I plan on a 60 amp outlet for some reason?

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  • There should be writing on the the cable. It might mention the temperature rating, 8awg(60). Check and edit your question with info.
    – crip659
    Commented Jan 7, 2022 at 10:57
  • Is the oven wire a cable or in conduit? Upload pictures of anything printed on the insulation. Commented Jan 7, 2022 at 11:31
  • Our 30-year-old GE range (4 surface burners, single std oven) is rated at 13.3 kW at 240 V and 10.0 kW at 208 V, but instructions allow connection through a 40 A breaker. 13300 W / 240 V = 55 A . Would current codes allow a range with this power rating to be connected through a 40 A breaker? The cable is a 4-conductor aluminum cable, original 50 year old, and the breaker is the original 50 A from 1970. Commented Jan 7, 2022 at 19:14
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    Depends on the #8 wire. Can you give us data off the wire labeling? (repeated every 12"/300mm). We're particularly looking for the letters NM, UF or TW (or other letters). Commented Jan 7, 2022 at 19:59
  • @JimStewart you have to punch those numbers into the provisioning formula for ranges. It's a little complicated. Commented Jan 7, 2022 at 20:20

2 Answers 2

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The outlet is 50 amp but the wire is #8. Is this safe?

Depends on the wire. You didn't specify what kind of wire you have. If it's THHN-2 (90C rated) in conduit, you're fine. If it's NM cable (by NEC definition 60C), it's not. If you're not sure which it is, assume it's NM (which it probably is).

Should I use 40 amps instead?

40 amps would be safe on any 8-gauge wire.

Is 40 amps enough for my current oven?

Depends on the appliance itself. You'll want to consult the owner's manual (find your model number, probably behind the over door, and search for online manuals) or any information plates the device has to see how many amps it needs. Some modern convection ovens need 60A.

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  • Don’t forget that the wire being rated for 90C is not sufficient. All the terminals must be similarly rated.
    – nobody
    Commented Jan 7, 2022 at 15:10
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    @nobody True, but 8-gauge THHN is rated for 50A at 75C, which is likely in the tolerances of the terminals
    – Machavity
    Commented Jan 7, 2022 at 15:19
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    I didn't specify the wire type because I don't know the type. The labeling on the wire was not something I could decipher but it is clearly not in conduit, so I will assume 40 amp max. I did finally track down the manual and it specifies 40 or 50 amp service. Unless someone can convince me I need more power I'll make a plan to simply fix the electrician's mistake and put in a 40 amp breaker.
    – MacGuffin
    Commented Jan 7, 2022 at 16:51
  • Are these modern convection ovens that require a 60 A breaker in the category of commercial/industrial oven? The instructions for our 30 inch GE electric range (four surface burners and one oven) allows connection via a 40 A breaker. Commented Jan 7, 2022 at 16:59
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    @MacGuffin you may not be able to decipher the labeling on the wiring, but there are plenty here who can. Copy the lettering exactly and edit it into your original post. Someone will know exactly what it means. If you can't make out the writing for certain, take a good clear, focused picture and include that. People here make their living interpreting it, and they're willing to share their knowledge with you for free! Take advantage of it.
    – FreeMan
    Commented Jan 7, 2022 at 18:20
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40A will just about always be fine. Most ranges say to connect to a 40A or 50A circuit, but the most interesting thing most say is about cord size. For instance Whirlpool page 5 says

If connecting to a 50 A circuit, use a 50 A rated cord with kit. For 50 A rated cord kits, use kits that specify use with a nominal 1³⁄8" (3.5 cm) diameter connection opening.

Many people don't notice that some range cords with NEMA 14-50 plugs are only rated for 40A. GE recommends a 40A cord.

Circuit temperature rating is the next question. We know NM-B uses higher rated conductors, but 334.80 restricts you to the 60°C rating, so 40A. Older editions of the NEC also restricted type SE cable to 60°C, but modern editions only apply that to #10 in contact with insulation, so the 75°C applies if the receptacle has 75°C rated terminations. Some big box store versions only have 60°C terminations so you will still be limited to 40A. But if you have SE or other 75°C copper cable, receptacle, and breaker rating (and 50A cord) then you could use a 50A receptacle.

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  • Are all "4-prong" cords rated for 50 A? (I suppose all 4-prong cords are nema 14-50, right?) Commented Jan 7, 2022 at 19:45
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    @NoSparksPlease the derating of type SE cable to 60C was a "momentary lapse in judgment" that appeared in one recent edition of NEC due to an oddball cable supplier's petition being made just as change suggestions were closing, and going un-noticed for that edition. As many others said, "We have 50 years of (conspicuous lack of) evidence that it's not a problem at 75C". Commented Jan 7, 2022 at 20:00
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    @JimStewart No, click the link in my answer. 40A is an add provision in the NEC/NEMA specs,. NEC 210.21(B)(3) specifies 40A or 50A for NEMA 14-50, and NEMA doesn't designate a NEMA 14-40. Commented Jan 7, 2022 at 20:02
  • NoSparksPlease I think you meant 14-50 when you wrote "some range cords with nema 6-50 plugs are only rated for 40 A" Commented Jan 7, 2022 at 22:27
  • Does a cord with a NEMA 14-50P have an indelible marking indicating whether it is rated for 40 A or 50 A? Commented Jan 8, 2022 at 2:34

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