Electrical – change a 240 volt 40 amp outlet for an oven to two 20amp 240 volt outlets

electricalwiring

I am installing two wall ovens that each require 20 amp 240 volt outlets. I currently have wired for a single 40 amp 240 outlet. To convert without pulling new wire from the panel can I put in a sub panel an add two 20 amp 240 volt breakers that can feed the outlets I need? Is there any easier way?

Best Answer

Normally, you can never split a 40A supply to serve two loads rated for a 20A circuit. Except here.

This is an allowed exception for supplying oven/range loads. It's in NEC 220.55, referring to Table 220.55, Note 4:

The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

NEC 220.55 and Table 220.55 also includes some derating (or more accurately, permissive up-rating) that overrides the normal "125% for continuous" derate found in NEC 210.19(A)(1). It appears to be permissive. Speaking of 210.19, ThreePhaseEel points out

210.19(A)(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 3/4 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.

Exception No. 2: The neutral conductor of a 3-wire branch circuit supplying a household electric range, a wall-mounted oven, or a counter-mounted cooking unit shall be permitted to be smaller than the ungrounded conductors where the maximum demand of a range of 8¾-kW or more rating has been calculated according to Column C of Table 220.55, but such conductor shall have an ampacity of not less than 70 percent of the branch-circuit rating and shall not be smaller than 10 AWG.

This also overrides the 125% rating by saying a 40A breaker can definitely supply two 20A ovens, and saying a 8.75KW-9.6KW oven is allowed on a 40A circuit.

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