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I am replacing a 15-year old advantium oven and wall oven with updated units.

The existing ovens are hard-wired into a junction box that runs to a single 40 amp breaker.

The new Advantium is 6.3kW at 240V and the oven is 4.5kW at 240V. I have read the installation recommendations/requirements online and a 30 amp dedicated breaker is supposed to be used for the Advantium and 20 amp for the oven. That would require cutting holes in the wall to add wire to run a new breaker.

So, do I really need to do that, or can I use a 50 amp breaker. The wire is 6 gauge. They would be running at the same time (Not every day, but a few times per week)...

  • That might work, and probably would, but if you ever try to sell your house, that kind of work might cost you a lot of money and ruin the deal with a detailed home inspection. It's better to go by the required local building codes. .... ounce of prevention. – Kurt Heckman Apr 25 '18 at 16:55
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This isn't as straightforward as it seems...but it turns out you're OK in the end

The original installer of your oven setup leaned on the NEC provisions for tap conductors in circuits feeding cooking appliances, found at 210.19(A)(3) Exception 1:

(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8.75 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

Exception No.1: Conductors tapped from a 50-ampere branch circuit supplying electric ranges. wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than 20 amperes and shall be sufficient for the load to be served. These tap conductors include any conductors that are a part of the leads supplied with the appliance that are smaller than the branch-circuit conductors. The taps shall not be longer than necessary for servicing the appliance.

This means that the general outline of your setup, while not recommended by the installation instructions for your new oven, does comply with the NEC. To figure out whether the new oven is allowed, we then turn to Table 220.55, applying Note 3 to the combination load of two ovens, one 6.3kW and one 4.5kW:

  1. Over 1.75 kW through 8.75 kW. In lieu of the method provided in Column C, it shall be permissible to add the nameplate ratings of all household cooking appliances rated more than 1.75 kW but not more than 8.75 kW and multiply the sum by the demand factors specified in Column A or Column B for the given number of appliances. Where the rating of cooking appliances falls under both Column A and Column B, the demand factors for each column shall be applied to the appliances for that column, and the results added together.

Given that both appliances fall within Column B, and that the table gives us a 65% demand factor for 2 appliances in that column, this means that while the two appliances sum together to 10.8kW, the Code-computed load on the circuit is actually 7.02kW due to the demand factor. This means that the 40A breaker is sufficient for the circuit, even with the new ovens installed; however, since your wire is 6AWG, you can bump the breaker to 50A safely.

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