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I have a double wall oven (40 amps) on a separate circuit and want to replace with single wall oven (20 amps) and wall microwave (120v, 2000 watt). How can I use the 240v feed to supply the wall oven (240v) and microwave (120v)? Is this code compliant. The reason for asking is that a new wire is difficult to lay in existing home.

  • How many wires are in the existing cable? Are they hot-hot-neutral-(ground?) or is there no neutral? – Harper Nov 22 '17 at 19:12
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If the microwave is going to be plugged in to a receptacle, which almost all are, then the following articles apply:

406.4 General Installation Requirements. Receptacle outlets shall be located in branch circuits in accordance with Part III of Article 210. General installation requirements shall be in accordance with 406.4(A) through (F).

(A) Grounding Type. Except as provided in 406.4(D) [this refers to replacing old two prong receptacles and doesn't apply to your case], receptacles installed on 15- and 20-ampere branch circuits shall be of the grounding type. Grounding-type receptacles shall be installed only on circuits of the voltage class and current for which they are rated, except as provided in Table 210.21(B)(2) and Table 210.21(B)(3).

This is a crude recreation of the Table, notice 40 amp circuits require 40 amp receptacles.

Table 210.21(B)(3) Receptacle Ratings for Various Size Circuits

Circuit Rating (Amperes) : Receptacle Rating (Amperes)

15 : Not over 15

20 : 15 or 20

30 : 30

40 : 40 or 50

50 : 50

This means a receptacle on a 40 amp circuit has to be rated 40 or 50 amps.

If your microwave is hard-wired, then there is an exception to allow taps on a 50 amp circuit but not on anything smaller so this exception does not apply to your 40 amp circuit.

210.19(A)(3) 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 the microwave would have to be hard-wired with minimum #12 wire.]

So, after reading through these articles, my opinion is that you will need to run a separate circuit for the microwave. And as long as you are doing it, you should make it a 20 amp circuit to allow for a higher powered microwave. In this case, doing the difficult thing is the safer and Code compliant thing.

Good luck!

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    Code fails to mention 40A receptacles don't exist on the market, and NEMA refuses to define them, so that exception is really about allowing a 50A receptacle on a 40A circuit with 40A wire (#8). A 20A circuit is a minimum for a 2000W microwave, as a 15A can only supply 1800W in any case. Even a 20A can only supply 1920W continuous, but hopefully a microwave is not a continuous load. – Harper Nov 22 '17 at 19:08
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If you no longer need the 240V 40A circuit. You should be able to convert it to a 20 ampere circuit, by simply replacing the 40 ampere breaker with a 20 ampere breaker. This will give you a 240V 20A circuit.

Since you need two 120V circuits, you're going to have to supply a neutral. I'm assuming the oven was supplied by only two wires (+ground). If that's the case, you'll have to pull new cable/wire. If the circuit was run through conduit, you may have enough space to simply pull a neutral conductor. If it's a cable, you're likely going to have to pull a new 12/3+ground cable from the panel to the oven.

Once you pull the new cable/wire, you'll connect it up as follows... The two ungrounded (hot) conductors will attach to the double pole 20A breaker. The grounded (neutral) connects to the neutral bar in the panel. And the grounding conductor connects to the grounding bar in the panel.

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To make a very long story short, you can't. The code does not allow it and it would result in a hazardous situation. You have to run a second set of wires for the microwave over. Happy wiring. P.

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