I have an induction cooktop that calls for a 50 amp circuit and a double wall oven that calls for 40 amp circuit. Both call for 4 conductors. Contractor ran a 4-conductor 6 AWG SOOW wire rated for 90 degrees C/194 degrees F. Is this wire ok/safe to use generally instead of NM-B? Code wise? Contractor already ran the wire so I'd rather not make him re-run something else if I don't have to. He said he was having a hard time finding wire in general. Thanks for any help!
As far as I know (checked various sources but remember reading it previously somewhere on DIY SE), you can't use SOOW inside walls. Which is much the same as "you can't use NM (Romex) as cordage. They perform different functions, with different strengths/weaknesses.
If you can't get proper NM (8 AWG or larger for the 40A, 6 AWG or larger for the 50A) then your alternative is to use wires instead of cables, using THHN wire 8 AWG for the 40A, 6 AWG for the 50A. Actually, you may be able to use smaller THHN - depends on temperature ratings of the receptacles. The catch is that you need to run the wires through conduit. That could be metal, in which case you save on a ground wire, or it could be PVC or "smurf tube". There are all kinds of rules about how you run conduit, but if you have reasonable access into the walls/ceiling, or if you have a crawlspace or unfinished area below the kitchen then it is a good solution for a cable shortage.
Depending on the jurisdiction, you may be able to run aluminum cable and/or wire (again, has to be the right type) instead of copper. That requires using the right size (typically one size larger than the comparable copper) and the right kind of connectors (breakers are usually not a problem, but receptacles often are). That can save a lot of money. But beware, some jurisdictions do not allow aluminum wire for branch circuit connections.