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My cooktop uses 120v from a 240 split phase 30 amp breaker.

  • Can I use the the other phase from same breaker to power a dedicated 120v circuit for my microwave?
  • Will the cooktop current flow in the microwave phase?
    • If so what is minimum wire gauge for said microwave circuit?
FreeMan
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Gregory
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  • Does this answer your question? https://diy.stackexchange.com/questions/129764/splitting-a-240v-line-into-two-120v-lines – P2000 Jan 31 '22 at 20:49
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    Your microwave is expecting to be protected by a 15 or 20 amp breaker maximum, guess what happens when there is a problem, but it is on a 30 amp breaker. The problem gets bigger. Must have 10 gauge(or bigger) wire on 30 amp circuits. – crip659 Jan 31 '22 at 20:49
  • Let’s check code the above comment links a 20 amp double pole not a 30 amp. If the microwave instructions do not state the max the tap rule can be used and #12 wire can be run in flex directly to the microwave (no receptacle) code allows this for a cooking unit, see NEC exhibit 210.24 a nice picture for those that don’t have to find and read all the tap rules and the requirements of 15 & 20 amp receptacles. – Ed Beal Jan 31 '22 at 21:49
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    Does your cooktop really use 30A? It's possible but it would be unusual to have a cooktop that demands 30A but not 240V. If your cooktop in fact requires a 20A circuit this is easy, you put in a 2-pole 20A breaker and follow the link in the first comment. – jay613 Jan 31 '22 at 22:06
  • What make and model is your cooktop even? – ThreePhaseEel Feb 01 '22 at 02:04
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    To answer this question properly we need to see: 1) The breakers feeding this 2) The outlet feeding this including its inside, clearly showing all wires (are there 2, 3, 4?), and your confirmation of the size (gauge) of those wires 3) The nameplate of your cooker clearly showing the voltage and current requirements. 4) Where will the microwave be and how will you get power to there from the existing outlet? – jay613 Feb 01 '22 at 15:23
  • Further to the comment by @jay613, you can upload pictures of the panel, the receptacle, the stove and the microwave & its cord, and ideally a picture or drawing of the kitchen. Or ask an experienced DIY friend to dig out all this info and add it to your post. – P2000 Feb 01 '22 at 17:47

2 Answers2

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You cannot run a #10 (10-gauge) wire from a 30A breaker to 15A/20A receptacle.

You also cannot run that wire to a junction box for a 15A/20A appliance hard-wire, unless the appliance allows it and it is mentioned in the manual (rarely).

Whether you need to wire to an outlet with or without receptacle depends on the microwave. Does its cord have a plug?

  1. You can run a #10 to a sub panel, then from a 15A or 20A breaker by #14 or #12 wire to an outlet box with a receptacle for a plug-in, or just a junction for the hard-wire.

  2. You can replace the breaker with a 15A (or 20A), use the existing #10 to where it terminates, and use #14 (or #12) wire to continue to a 15A receptacle.

The sub-panel will require dedicated wall space free of obstructions, and it's more work as it requires a wall cutout. All this depends on your circumstances in the kitchen or a nearby space. For instance, is there a garage behind the kitchen?

  1. If your microwave has a plug, you can also run a #10 to a 4-prong stove receptacle (NEMA 14-30), then a "gas range adapter plug" with built-in 15A protection to provide a 15A 120V receptacle for the microwave plug.

  2. You can wire a 15A appliance via #14 "tap wire" from a #10 protected by a 30A breaker if the tap length is 10ft or less, and the appliance has built-in overcurrent protection. The manual would state so, and it wouldn't be with a plug.

In general

  • You can wire a 15A receptacle to a 15A or 20A circuit, and you can over-gauge the wire, e.g. use an existing #10.
  • You cannot wire a 15A receptacle to a 30A circuit, regardless the wire gauge.

The appliance also has a cord and internal circuitry that require protection at the 15A or 20A level by a breaker. Any allowable exceptions would have to be listed in the manual.

If all this sounds too complicated, you may be better served with help from an electrician on-site or a DIY-friend with electrical experience.

P2000
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  • I would just say no , a standard 120 volt receptacle can only be fed by a 15 or 20 amp circuit. There are ways to (tap, see NEC exhibit 210.24 in that case it could be 12 awg) and hard wire but the microwave instructions usually state a maximum of a 20 amp circuit. You have the information in there , the adapter used as premises wiring I think would be a stretch, absolutely not if built in. – Ed Beal Jan 31 '22 at 21:38
  • @EdBeal these adapters are UL listed and they have a plug on one side and a receptacle (socket) on the other, in which a gas stove -or in the case of the OP- a microwave can be plugged. They include 15A protection, as I wrote. What "premises wiring" ? – P2000 Jan 31 '22 at 23:11
  • Check the premises wiring this is the same as using an extension cord. Cordage can not be used for built in devices. – Ed Beal Feb 01 '22 at 13:26
  • @EdBeal OP writes that he wishes to wire a "dedicated circuit". OP does not mention hard-wire. For hard-wire, a receptacle is not an option and he can use the other options (sub-panel, lower breaker) to wire to a junction box. In any case, I'll edit. – P2000 Feb 01 '22 at 15:01
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Both halves of the circuit have to be 100% good with 30A. For the wiring that's easy, use 10 AWG. But if the microwave oven requires a 15A or 20A circuit then you can't put it on a 30A breaker.

You can replace the 30A double breaker with a single 30A and a single 20A breaker, but then you will need to run a new 12AWG cable for the 20A circuit.

manassehkatz-Moving 2 Codidact
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  • A 30A and 20A breaker can be handle-tied and used on a MWBC with 10/3 cable, as long as the MWBC does not serve any 240V loads. However I rather doubt this is a 30A range, I think that's just how they hooked it up when they converted to gas. – Harper - Reinstate Monica Feb 01 '22 at 00:58
  • @Harper-ReinstateMonica Good points. I was thinking of a 30/20 MWBC but it just seemed a bit too strange. But electric 30A -> gas just needs ignition, *that* makes a lot of sense, and of course shouldn't then be on a 30A breaker any more. – manassehkatz-Moving 2 Codidact Feb 01 '22 at 01:11
  • Look at exhibit 210.24 #12 can be used if in flex and hard wired even fed from a 50 amp. – Ed Beal Feb 01 '22 at 16:33
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    @EdBeal Interesting. So essentially a 10 ft tap can be much smaller, but can't be NM, and provided the stuff attached to it is OK with being on the larger circuit. – manassehkatz-Moving 2 Codidact Feb 01 '22 at 16:44
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    @manassehkatz-Moving2Codidact "the stuff attached to it is OK" it must have its own overcurrent protection, which would be listed in the manual. Receptacles won't work since anyone can plug anything in, but hardwire could be ok. – P2000 Feb 01 '22 at 16:57
  • @P2000 So in OP's current situation, a big question is whether the cooktop is hardwired or plug-in. – manassehkatz-Moving 2 Codidact Feb 01 '22 at 17:27
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    well, I think if OP opts for a lower gauge tap to the microwave, the question is whether the microwave is plug-in or hardwire. OP mentions "dedicated circuit" but that could still be either. Frankly, I think OP needs to get an electrician or electrically experienced DIY friend. I will put that in my answer. – P2000 Feb 01 '22 at 17:38
  • Agree re: microwave, and the vast majority that I have seen in recent years, including many with an integrated hood/fan designed to be permanently mounted over a cooktop, are plug-in. I'm saying *even the existing cooktop connection* may be a code violation. – manassehkatz-Moving 2 Codidact Feb 01 '22 at 17:43
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    @manassehkatz, it could be nm inside of flex but it requires a hard wire to the tapped device, so THHN is what pros use or metal clad. Thanks for being one of the few to check the code and respond, + – Ed Beal Feb 01 '22 at 20:15