10 AWG is too small for a 40A breaker, and many other problems with this plan as originally proposed.
Further, you cannot do any of this if your range receptacle is a NEMA 10-50 type (3-wire). Since you have ground present at the location, you should convert receptacle (and oven plug, if necessary) to NEMA 14-50 by retrofitting a ground wire if needed.
Further, this is a rental property. Only licensed electricians can do electrical work (other than plugging in of cords), however your local authority may make an exception for simple changes of sockets (e.g. changing a NEMA 10-50 to 14-50).
You are seriously underestimating your power needs. Fortunately, it is there.
You seem to think of power as "number of things plugged in". Actually, you should be thinking about the power drawn by each individual device.
- Heaters are huge loads. Those cheapie "heater-fans" are 1500 watts (12.5 amps @ 120V). They are all this magic number because that is the maximum UL will list on a plug-in 120V heater. You can get larger heater-fans as large as 6000 watts (25 amps @ 240V) but they are 240V-only.
- A compressor is also a huge load. How big varies by nameplate, so you have to read the nameplate.
- Table saws and radial arm saws - another huge load, again you have to visit the nameplate.
- Routers can be quite large as well depending on capacity.
- Other hand tools vary wildly, and can be as demanding as a table saw, or just a few amps.
Your power available from that socket is (in 240V mode) 40A obviously, and (in 120V) two independent poles of 40A each. So you can still power a fair amount of stuff if you arrange it properly; the trick is doing so.
Plan A: One MWBC containing two 20A circuits
In this case, you change the circuit breaker to 20A 2-pole, and change the oven receptacle to NEMA L14-20. The wire in the wall can stay the same.
How to do this in permanent wiring is a gory mess. Fortunately, this is temporary wiring.
Then, you use common, pre-made, pre-molded cords intended for generator distribution, which has a NEMA L14-20 plug on one end, and some number of 120V/15-20A receptacles on the other end.
- I note that Home Depot also sells these with a L14-30 plug on the pointy end, or NEMA 14-50 plug on the pointy end. I suspect this may not be legal, but normally Home Depot is reliable about selling things which are UL listed - don't use it if it's not. The UL listing means the agency has satisfied itself that the cord is safe, and have done torture testing and destructive testing to confirm it. The UL listing must be on the cord as a tag, not molded in the cord (that only applies to the cable) or in the sockets (that only applies to the sockets).
Regardless, the socket must match the breaker:
- 15A breaker = 15A socket
- 20A breaker = 20A socket or 2 or more 15A sockets (special rule)
- 30A breaker = 30A socket
- 40A breaker = 50A socket (40A sockets are not made)
Assuming your house's internal wiring is 8 AWG, you are not allowed to use a 50A breaker.
Plan B: Subpanel
This is the viable way to run all your loads. In this case you mount a subpanel somewhere and fit 15-20A breakers to feed common receptacles, or any size of breaker to feed a specialized load, including 240V. The subpanel must separate neutral and ground!
The connection from subpanel to receptacle can be done with flexible cord with an appropriate NEMA 14-50 plug, since it is temporary and will be frequently interchanged; therefore legal under NEC 400.7. Cord will need to be 8 AWG for a 40A breaker or 6 AWG for a 50A breaker. You can leave the breaker at its current value, just make sure your wire is sized for it.
Cable must be cordage e.g. SJOOW, not in-wall wiring such as Romex, NM or UF.
I would go with an 8-12 space panel, as they're not too expensive an this is a temporary setup. 100A or 125A busing is fine. A main breaker is not needed in the panel. You can mount the panel and nearby outlets on a piece of plywood and place it out of the way.
