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After waiting 15 month, we finally received our Bosch Microwave/Oven combo unit model HBL8743UC. The combo unit is rated at 9600W, and per manual, requiring a 240V 40A circuit. This combo unit actually comes in two boxes, microwave and the oven. The Microwave is to be mounted on top of the oven with four wires going into a panel on top of the oven. Then there is a set of four wires to be connected to the wall electrical box. If these units were to be purchased separately, the oven is rated at 240V 30A, and microwave at 240V 16A.

The circuit for the old oven was 30A with 10AWG wires (~75 feet run). The old microwave had a 120V 20A on a separate circuit.

My first thought was to add new 8AWG wiring connected to a 40A breaker. Then I noticed the wiring coming of this unit are marked 10AWG. How is the unit requiring a 40A circuit has 10AWG internal wiring? If I follow the manual, I would have to connect the unit's 10AWG wires to my newly run 8AWG wires. That doesn't sound right too me.

  1. Is this normal that a unit requiring a 240V/40A has 10AWG connecting wires?
  2. What if I just use the existing 30A circuit with 10AWG wiring for the whole unit? Worst case if I use the oven and microwave at the same time, it would trip the breaker, right?
  3. If the answer to 1st question is no, should I wire the microwave separately on a different circuit?
  4. The manual says I could do a 3-wire connection if allowed by local codes (white and green connected to green). How do I find out about the codes in Los Angeles?

Thanks a bunch in advance!

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  • You have to go by the instructions. The instructions say use 40 amps and code says you need to use 8 gauge on a 40 amp circuit. The manufacturer as had that oven tested to use a short length of 10 gauge wire for hook up only. I have a 5HP motor. I need thick wire to carry the power to it. The wires going into the motor are about 18 gauge for the half foot needed.
    – crip659
    Aug 29, 2022 at 19:39
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    Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
    – Community Bot
    Aug 29, 2022 at 19:56
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    3-wire connections (neutral and ground connected together) is only approved for grandfathered applications where you are replacing a 3-wire oven. There is no way code in California is going to allow a 3-wire install replacing a previous 4-wire install. Also, keeping 3-wire installs is stupid and potentially dangerous. See many other questions on that subject.
    – DoxyLover
    Aug 29, 2022 at 19:58
  • Thank you all. While this unit is sold as a combo unit with its own model number (HBL8743UC), the oven and microwave individually have their own model numbers. It's written on the individual boxes, and in fact there is a separate manual for each in their boxes. When you search for those model numbers, you can easily find the product specs on each unit. It's from the products specs sheets and their manual that I found out the oven has a 30A, and the microwave has a 20A rating. I also found this response from Bosch to this question: Q: Can the microwave oven be replaced if it goes out without re
    – user156024
    Sep 1, 2022 at 2:49
  • @Ari it appears that you've now created two accounts and that's why you couldn't edit your original question. Please follow the instructions here to get the accounts merged. Once that's done (if someone hasn't done it for you), please edit your question to add in all the info in that comment.
    – FreeMan
    Sep 1, 2022 at 11:50

3 Answers 3

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  1. Is this normal that a unit requiring a 240V/40A has 10AWG connecting wires?

The rules for "wires in walls" are often different from "wires in appliances". There are a few reasons, including:

  • Wires in walls may be inside all kinds of different materials - wood, insulation, drywall, etc. Current (which determines heat) is limited to avoid problems in the worst-case scenarios. But keep in mind that "worst case" here has been determined over the last 100+ years based on actual fires.
  • Wires inside (or outside but assembled with) appliances may be of higher quality (i.e., insulation rated for higher temperatures) than standard building wires.
  • Wires may be inside an enclosure that has been determined through design & testing to be able to dissipate heat sufficiently.
  1. What if I just use the existing 30A circuit with 10AWG wiring for the whole unit? Worst case if I use the oven and microwave at the same time, it would trip the breaker, right?

Breakers don't trip immediately, by design, except at several times the rated current. Plus the rules are based on continuous use at 80%, so with a 40A breaker requirement, the expected current is no more than 32A except for brief periods (not likely applicable to an oven, though possibly to a limited degree for the microwave oven as microwave ovens are not used for hours at a time the way conventional ovens are used). Plus there is variability in breakers. You could easily have 32A continuous on a 30A breaker and not trip for a very long time and have overheated - possibly burning - wires as a result.

  1. If the answer to 1st question is no, should I wire the microwave separately on a different circuit?

That is a possibility. Unfortunately, since it is a 4-wire circuit you can't reuse the existing 120V circuit. If it was a pure 240V circuit then it would only need 3 wires and you could turn the 120V circuit into a 240V circuit by changing to a double breaker. But that won't work here - new cable needed.

  1. The manual says I could do a 3-wire connection if allowed by local codes (white and green connected to green). How do I find out about the codes in Los Angeles?

No! That is allowed in many places for old stuff - i.e., a straight replacement of a previous 3-wire connection. Not allowed for new stuff. And in any case, that would not solve the 10 AWG/30A vs. 8 AWG/40A problem.

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The NEC code does not allow under most circumstances to protect #10 AWG CU with more than a 30 amp breaker or fuse for normal use. However, #10 AWG CU can handle much more than 30 amps depending on the insulation. So your oven has fallen into that design category and has been UL approved. You need the 40 amp breaker because your oven could easily be pulling 35 to 40 amps and would need the 40 amp breaker specified in the instructions. You really want to follow those direction exactly as written because your warranty and insurances could be affected if you don't. Run the new 40 amp circuit and #8 AWG wire if that's what the instructions call for.

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To answer your point #3, there is a very specific situation where you could do this with the wire already in the wall.

If the units are listed to be wired separately (which I am assuming is true because you were able to find circuit ratings for each separately), and -- and this is the important part -- the existing 120V/20A circuit for the microwave was only for the microwave (i.e. no other outlets on the circuit), you could use the existing 240/120V 30A circuit on #10 AWG for the oven and convert the 120V 20A circuit on #12 AWG to be 240V by switching the 20A single-pole breaker out for a 20A 2-pole breaker.

Depending on the requirement of the microwave, you'd terminate that in a box in the cabinetry behind with either a junction box for hardwiring, or a NEMA 6-20 receptacle for cord & plug connection. This is a pretty unusual setup, but not completely unheard of, especially in CA as we're all of a sudden all trying to cram support for 240V appliances into buildings that were never designed for them.

Note that that would only work if the following were true:

  1. The new microwave requires straight 240V, not 240/120V
  2. The old microwave circuit doesn't serve any other receptacles
  3. The old microwave circuit is using #12 AWG wire
  4. The old microwave circuit wiring can be moved to the new location (physical location will make this either a snap or a terrible project - shortening the existing cable run is ideal - extending isn't because you'll leave a junction box that needs to stay accessible.
  5. You have enough panel space for another 2-pole breaker.

If any of those points aren't true in your situation, you'll have to pull a new circuit for the oven/micro.

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  • Actually, no, the OP cannot wire them to separate circuits and meet code. The UL listing requires installation in accordance with the provided instructions, and in the case of the oven/microwave combo the instructions call for a single 40A circuit. The fact that he could have purchased and installed them separately doesn't count, since the instructions he has in hand don't allow for that.
    – FreeMan
    Aug 30, 2022 at 14:55
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    @FreeMan I'll delete this answer if necessary, but I am hung up on the OP's sentence If these units were to be purchased separately, the oven is rated at 240V 30A, and microwave at 240V 16A. I'd like to know how he got those nameplate ratings. If they are in fact sold separately, then we can infer that they are also UL listed to be installed separately.
    – Chris O
    Aug 30, 2022 at 15:25
  • Combo unit has its own model (HBL8743UC), the oven and microwave each have their own models. It's written on the individual boxes, and in fact there is a separate manual for each in their boxes. It's from the individual products' specs sheets & manuals that I found out the ratings. Oven: HBL8443UC + Speed Oven/Microwave" HMC80242UC. The product spec for microwave HMC80242UC says 'Circuit breaker (A) 16A', while its installation manual says: wire conduit cable 208 V, 60 Hz, 20A, 240 V, 60 Hz 20A. I guess there is no 16A breakers in the US, so next size up is 20A.
    – user155930
    Sep 2, 2022 at 19:48

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