In order to make that work, there are several hoops to jump through. Because the OP mentions a receptacle, I'm assuming that this microwave is 120V and <= 15A and cord-and-plug connected.
First, the range circuit has to be 50A, not 40A.
Second, the microwave would have to qualify as a "wall-mounted electric oven." That rules out a portable microwave sitting on a counter, but is plausible if it installed as a built-in.
Third, that section requires that the tap conductors, including "the leads supplied with the appliance" to have an ampacity of at least 20A. So the microwave cord would need to have at least #12 copper conductors.
Lastly, 210.21(B)(3) still applies, as the 50A branch circuit supplies at least two outlets, the "wall-mounted electric oven" and the range. So the receptacle would need to be a 50A receptacle, meaning either a 5-50 (do they exist?) or a 14-50. The microwave cord would need to have the corresponding plug.
As far as I can see, if you satisfy all of the above, the install could be compliant. But I may be overlooking some other provision that would throw a wrench into the works.
Cheers, Wayne
Thanks to Wayn for considering tap conductors on a microwave
Regarding your points with NEC 210.19
Household Ranges and Cooking Appliances, Exception 1.
First:
It appears the language for range circuit "
not exceeding 50 amperes," probibits any tap to circuits rated > 50A.
Second:
* Since "
wall-mounted electric ovens" are not defined in Article 100, microwaves may rely on permissible interpretation:
-- Range hood microwaves have back-plate mount for walls, and redundant bolts secured thru bottom of cabinet / top of microwave
-- A wall mount shelf, or wall-mount cabinet, supporting a microwave oven, may be indirect wall support that relies on permissible interpretation
* Since article 100 defines counter-mounted cooking unit:
"
A cooking appliance designed for mounting in or on a counter and consisting of one or more heating elements, internal wiring, and built-in or mountable controls. (CMP-2)"
-- If no wall mount exists, only microwave dog houses for fancy models using heating elements for steam, or browning, would meet this definition
Third:
*
These tap conductors (shall)
include any conductors that are a part of the leads supplied with the appliance"
-- So receptacle taps wont work, if cord & plug (appliance leads) wont be included in tap-to-receptacle wiring
-- Any hard wiring may need 105C appliance leads, or cord, listed for 20A ampacity per NEC 405, if cord reaches the tap J-box
Lastly:
* Table 210.21(B)(3) wont allow receptacles on a tap, if rated for smaller tap conductors.
-- Receptacles rated for the larger branch circuit before tap, using RV adapters, may fail to use the same leads from appliance to tap.
It appears any microwave using taps, per NEC 210.19 exceptions, must have:
* Appliance J-box terminals listed for hard wiring, or
* A cord & plug listed for 20A ampacity, and only if long enough for direct splice to larger branch tap
-- If not sold as an appliance whip, cutting off a cord cap may violate equipment listings, per NEC 110.3(B), or rely on permissible interpretation.
-- If fixed appliance whip/cords are available with 20A rating, the disconnect must comply with 422.31(B); panel within sight, or lockable breaker.
