I currently have a single regular microwave(120V 15amps) and a single oven (240V 30amps) I want to replace them with a Single new oven (240V 30amps) and a speed oven (240V 16amps)

The breaker of the oven should work fine. My question is for the speed oven now. I do have space in my electrical box for the 240V breaker. But the cable for microwave is a regular 12 cable with black, white, and bare. I now have attached the installation instruction for the speed oven (BOSCH HMC80242UC).

My questions would be:

  1. Why does it require conduit, could regular 12 wire work for this?
  2. Would the three-wire connection work for me? I'm in Austin TX Thank you so much in advance!

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  • 1
    Can you get us the make and model of the appliances in question? Commented Dec 22, 2021 at 5:09
  • It's BOSCH HMC80242UC. Thanks!
    – Ivan
    Commented Dec 22, 2021 at 18:15

2 Answers 2


Based on the installation instructions, the oven in question requires neutral (to supply 120V to the light at least, and possibly fan and control electronics as well).

If it didn't require neutral, it wouldn't have a white wire at all. In that case, its instructions might permit it to be connected to existing grandfathered 3-wire 240V feeds by connecting the oven green to feed neutral/bare.

You cannot connect this oven to your three-wire white/black/bare feed.

  • Gotcha, thanks so much! So I need to replace the cable with a 12/3 cable so I have two hots and one neutral, do I understand it correctly?
    – Ivan
    Commented Dec 22, 2021 at 21:42
  • Yes, exactly - a /3 feed.
    – nobody
    Commented Dec 22, 2021 at 21:44

I currently have a single regular microwave(120V 15amps) and a single oven (240V 30amps)

According to the pictures, your breaker for your microwave oven is 120V 20 Amps. It is quite likely that your microwave oven itself is rated for 120V 15 Amps, probably plugged into one receptacle of a duplex 15A receptacle. Which is all quite normal. That means you should have 12 AWG wire going to the microwave, which can support 120V or 240V with the appropriate receptacle (or hardwired appropriately).

There is, however, one catch. Many US ovens, dryers, HVAC, etc. that are rated for 240V usage actually use both 120V and 240V. They do that by connecting three wires - two hots and neutral - instead of just two hot wires. If your speed oven is rated at 16A then a 20A breaker and 12 AWG wire is all correct. But you need to find out if it needs a neutral wire. If it does then you need to add a 12 AWG neutral (if it is conduit) or replace the cable (far more likely, more below) with a 12/3 cable. If it only needs 240V then you find a place where you can connect a double breaker and put the black wire on one breaker and the white wire on the other breaker. (Unless you are in conduit, in which case you need to run another wire because you can't use white as a hot wire in conduit.)

If your wires are in conduit, which I doubt, then this is all relatively easy - you can upgrade the 30A to 40A by replacing the wires, add a new wire for a new hot for 240V for a speed oven, etc. But if your wires are not in conduit then anything other than a simple swap of hot vs. neutral for the white wire in a 2-wire circuit requires running a new cable, which may be easy or may be very hard.

  • Hi, thanks for your reply. I have just checked my breaker and cables. I don't think my wires are in conduit. So replacing or upgrading wires won't be one of my options, correct? I have also checked the wires my microwave has right now. It's just regular black, white, ground wires. I have sent an email to BOSCH to see if the speed oven requires two hots and neutral, if it does need a neutral, I think there is no way for me to have a speed oven?
    – Ivan
    Commented Dec 22, 2021 at 0:11
  • None of this is "can't". It is a question of "at what cost". When I replaced my single oven with a double oven, I had to upgrade the breaker, and new cable along with it. In my case, the cable was easy (I helped my electrician run the wire, but really it was easy) the hard part was knocking out bricks from the wall to make space for the new oven (seriously...it was an adventure). YMMV. Commented Dec 22, 2021 at 0:18
  • LOL yes, it's not actually "can't". But since I don't want to spend a lot and don't want to break the wall. My cabinet has space for either a speed oven with a single oven or an oven comb, so my only problem would be the electricity. Would you think the only way I can have a speed oven is by buying a new double breaker and replacing the current microwave breaker? (only when the speed oven only needs 240V)
    – Ivan
    Commented Dec 22, 2021 at 0:33
  • Correct. The key question is whether it needs a neutral or not. Commented Dec 22, 2021 at 0:37
  • Thank you so much! I'm still waiting the response from the tech support
    – Ivan
    Commented Dec 22, 2021 at 1:46

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