using two oven in one outlet

Question

I have two ovens. One is 20 amp at 240 V and Second is 30 amp at 240V. And I have one 240 V 50 amp outlet hook up to one circuit. Would this be sufficient enough to run two ovens without overheating the wire and overload the circuit breaker?

I been told the 1 circuit could handle two appliances. I been told electricity 20+30 amp is not equal to 50 amp. They use the average amperage withdrawal calculation to calculate the circuit could handle it.

My question would 50 amp 240 V circuit could handle these two ovens without heating the wire or blowing the circuit?

These ovens will be turn on at the same time.

Answer 1

NEC 220.55

The load for household electric ranges <...> individually rated in excess of 1.75 kW shall be permitted to be calculated in accordance with Table 220.55.

Based on note #3; since one range is rated 7.2 kW (30A x 240V) and the other is 4.8 kW (20A x 240V), we are able to simply add the total rated kW of both and multiply it by the demand factor as found in Column B -- you have 2 appliances that are both between 3.5 and 8.75 kW, so the demand factor here is 65%.

7.2 kW + 4.8 kW = 12 kW

12 kW x 65% = 7.8 kW or 7800 W

7800 W / 240 = 32.5A

Finally, since the oven circuits are under 600V and noncontinuous loads, the following code applies:

NEC 210.19(A)(1)(b)

The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

..., i.e. your conductors (wires) should be sized to at least 32.5A (they should be already) and your breaker should then be sized to your wires (which it is.)

Your current 50A breaker will suffice for this application.

Answer 2

Sizing branch-circuits for cooking appliances can be complex, and I don't pretend to be an expert in this area. After reading through the NEC, I wasn't clear on how demand factors would work in your situation. After doing a bit of research, I came across this article from ECMAG which states:

...Derating is not permitted when one branch circuit supplies two ovens and no cooktop.

Which means you'll have to simply add the loads

20A * 240V = 4800W
30A * 240V = 7200W
4800W + 7200W = 12,000W
12,000W / 240V = 50A

So you'll have to size the overcurrent protection and conductors to a 50 ampere load.

Answer 3

Derating doesn't work for you since it's based on the theory that you hardly ever use everything at the same time. In your case, the entire point of having two ovens is to use both at once. So you need to bring honest 50A of service to run the 2 ovens.

Remember, breakers protect wires. I would make sure the breaker is 50A and the wire is 6 gauge, remove the 50A outlet and install a 30A outlet and 20A outlet right next to it. That will make it impossible to plug in more than 50A of devices.

My guess is, 6 gauge wire will not fit on a 20A outlet, and may not fit on a 30A outlet either. In that case, pigtail both outlets (10-12 gauge for 20A and 8-10 gauge for 30A) and join them together with a splice technique rated for 50 amps. You may want to use an extra-deep electrical box for this.

I take it these ovens' heater elements will cycle on and off to hold set temperature? Reason I ask is, if they don't cycle and do run at max possible power continuously, for more than 3 hours, that is what the Code calls a "continuous" load and you must increase current capacity by 20%. In that case, or if you just want to overdesign it, you could install a 30A breaker and 30A outlet on the existing wire run, and pull a second wire run of 10 gauge, connect that to a 20A outlet and a 20A breaker. Copper is cheap, pulling wire is hard.