Use one leg of oven's 240v supply, to simultaneously power a 120v microwave?

Question

I just removed a double wall oven, because the control panel had partially melted. (I suspect the previous owner used the self-clean function, which in my opinion is an anti-feature for ovens.)

Rather than replace it with another double oven, I'm considering a single wall oven and a separate microwave. (As opposed to a combo; separate seems smarter to me, since if one breaks I only have to replace the one the broke.) But in order to accomplish that in a cost-effective way, I'm going to need a source of 120v, and (I suspect) on a circuit with at least 15 amps to spare.

In the attached photo, you can see the steel conduit for the original oven's 4-wire 240v supply. (Emerging from the conduit, it forks into 2 branches, one for each original oven. That's just how the old double-oven needed it.) I'm wondering if I can use just one of the legs of one of those forks, to supply my 120v microwave. Is that allowed by code? If not, what's the concern? (This is my preferred solution, if permissible. The oven was the only thing supplied by this breaker, which IIRC is rated for 50 amps.) This could be a hard-wiring or I could add in a recepticle, whatever is needed.

Answer 1

Generally speaking the NEC requires overcurrent protection to be closely aligned with the appliances in use. A 15A receptacle on a breaker larger than 20A is outside of the rules.

There are "tap" rules for cooking appliances that allow more leeway that are generally intended for a single circuit to supply multiple ovens, or an oven and detached cook-top with protection above the rating of the individual appliances.

An oven and microwave loosely fits within the code parameters of the code but the big catch is the NEC also requires complying with UL, which listing is only valid as permitted in the installation instructions. Generally those instructions will only allow a plug-in option.

The Code also prohibits feeding a 15/20A receptacle with a larger breaker, so to comply with the tap rules you would need to cut off the plug and "hard wire" it (and provide permeant lock-off device in panel to serve as disconnect). This is probably not allowed in those pesky microwave instructions.

So to provide and protect the proper sized 15A receptacle on a larger circuit you need to add overcurrent protection. Adding a small accessible electrical panel with required clearance and accessibility requirements is one ugly option, a fused receptacle like a Bussman SRU-BC is probably your best option. Not too ugly since it will be behind the microwave.

Where this can become ugly again is if your locality has adopted a modern edition of the NEC any receptacle in the kitchen will require GFCI protection, and the reset must be readily accessible, so it can't be behind the microwave. The easiest way is to change the breaker feeding this circuit to a two-pole GFCI. Alternately you would need to go from the fuse holder out of the cabinet space to a (dead front) GFCI, then back to the receptacle.

Answer 2

You cannot put an appliance intended to be powered by a standard 15/20A circuit on a circuit protected by a larger breaker, such as 30A or in this case 50A.

It might be possible to install a small subpanel in an accessible location, but that's heavily dependent on space.

You may be able to tap power from another nearby circuit - I am not sure on US rules for microwaves; I think they require a dedicated circuit.