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I removed an electric slide-in range. The box was wired with 8-3 stranded copper on a 60a circuit breaker. I am installing a cooktop with a built-in oven. All I have is the one 8-3 wire in the wall. What are my chances of using that wire for both the cooktop and oven? Cooktop is a 40 amp unit. Not sure of the oven. Edit.. oven is a 30amp appliance. Neither will be used even close to their full potential.

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  • It is only a three wire conductor in the wall. both appliances give a 3 wire hook up chart. Either way I need to do something. 30 amp oven and 40 amp cooktop does not equate to my 60 amp breaker nor the 8ga wire. I did see a post on the common junction box. Ref. 220.19 (4) and 210.19 a (3). Not excited right now. Does it look like just running another 8ga 50a circuit separating the two appliances? Also I would be removing the original 60 amp breaker and replacing it with a 50 amp breaker. Do you see a way I could replace the 8ga wire and 60a breaker so that I can have a single junction box Sep 25 '20 at 22:48
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    Can you get us the actual kW ratings of the cooktop and oven in question? Also, can you post a photo of the inside of the existing junction box please? Sep 25 '20 at 23:46
  • The oven is 5000 watts and the cooktop total 8500 watts. I'll get a picture of the panel when I see it again. You guys will surely have a stroke when you see it though. My plan will be removing the 60 amp and replacing it with a 50a on the 8ga wire. Installing a 40a on a new 8-3 . Sep 27 '20 at 2:07
  • While a photo of the panel would be interesting, we need a photo of the inside of the box for the existing range receptacle/outlet though, that'll tell us if that 8AWG can really handle 50A or if it's limited to 40A (which matters in your case) Sep 27 '20 at 2:20
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That 8/3's overbreakered no matter what

The 8AWG copper you have in your walls is rated for 40A at 60°C or 50A at 75°C as per Table 310.16, so you'll have to swap that 60A breaker out for a 40A (NM cable) or 50A (armored cable, SE cable, wires in conduit) no matter what else you do.

40A is adequate for most ranges though

However, due to the Table 220.55 range load demand factors, which also apply to a separate cooktop and oven on a single circuit, 40A is adequate for the vast majority of range setups, and even those that don't fit on 40A have no trouble with a 50A circuit, unless you have a very large cooktop. In particular, a 12kW nameplate range load only is treated as 8kW of actual load for circuit sizing purposes, and every additional kW or major fraction thereof adds 5% to that 8kW. So, knowing that a 40A circuit gives us 9.6kW and that ranges are not treated as continuous loads by the NEC, we get 10% over 12kW, or a 13.2kW maximum range load on a 40A circuit. Going up to 50A gives us 12kW, which translates into a 14.4kW maximum range load.

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  • Sorry it took a while to get back. Appliances were back ordered until just 2 weeks ago. Jan 24 at 21:59
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#8 wire on the 75 degree table Is good for 50 amps but of the range is rated as a 50 amp load you need to go up to #6, a 40 amp load rated range should be put on a 50 amp breaker as that is 125% of the load. If your circuit rating is 40 amps the receptacle can be 40 or 50 amp per table 210.21.b.3. If the range requires 4 wire you may need to pull new wires if you have black, red, white a new separate ground can be pulled as long as it taps a circuit from or goes back to the same panel. If you have 3 & a bare copper and your new range requires 4 wire most locations require a new cable to be pulled.

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  • So the existing circuit breaker is oversized?
    – isherwood
    Sep 25 '20 at 20:59
  • Yes that’s why I tried to give the appropriate sizes
    – Ed Beal
    Sep 25 '20 at 21:24
  • I thought range/oven calcs were more complicated than that? Sep 25 '20 at 21:41
  • You are thinking of load calculations for the entire system load you still have to have the wire sized for the actual load unless multiples on a tapped feed a range is a oven and cook top Combined and will have 1 value. but have 10ea 12kw loads and the total for the system is only 25kw table 220.55 the wires are still have to be sized for 12kw. But where there are multiple different loads on a tap it gets a bit more complex but not for a single device calculated at 8kw if under 12kw and +5% for each kw over 12.
    – Ed Beal
    Sep 25 '20 at 22:21
  • The panel is a mess. I ended up removing the 70amp breaker and replaced it with the 60 amp from the range circuit. The cooktop now has a 40amp with 6/3. The oven has a new 6/3 cable with a 30 amp breaker. The hot water heater had a 50amp breaker with 10/2. It now has a 30 amp with an 8/3 wire. All of the loose connection at the breakers were secured. Two electrical tape splices got wire nuts and tape. I will get around to addressing that later. The new cook top pulled 29.8 amps with all burners on boiling water. 6/3 wires remained cool. The oven pulled 16 amps set at 400. Jan 24 at 21:58

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