I am running a few tools in my temporary workshop in my large balcony: air compressor, couple of space heaters, router, circular saw and some other tools. I have one wall outlet rated at 15 amps and clearly it is not sufficient.

So I am thinking of running a 10 AWG romex cable, 25' length, from the 40 amp cooking range outlet about 20 ft away. Now this will have two hots, one ground and one neutral. I am going to avoid using a power strip/surge protector and just wire 4 20 amp wall outlets in parallel. Now the outlets have two inputs for the hots and the inputs are connected via a brass plate, now I am guessing I will connect one hot to each of the two inputs, do I keep the brass plate or take it out? By taking out the brass plate I am hoping each of the 3 prong sublet on the outlet will be able to provide 20 amps separately?

Also, if I wire the 4 outlets in parallel will each of them give me 40 amps or will I only get 40 amps in total from all the outlets on this extension combined? Also, this might be a 220V since it is a 40 amp breaker, will check with a multimeter but if it is, how can I step down to 110V so I can plug in my tools safely?

  • 2
    You're overthinking this. Don't you have other circuits in your apartment? Plug two 12AWG extension cords into receptacles on different circuits. That should be sufficient for your load. Or buy a 240V space heater of the type used in residential construction before the furnace is installed (typically orange metal boxes around 12"x10"x8" with grills on each end). This can be plugged directly into your range outlet, leaving the existing 120V outlet for your power tools. Commented Nov 16, 2019 at 0:57
  • @user128216 Agreed. Since it's all temporary wiring anyway, the simplest course is to use 240V heaters and run extension cords from other circuits in the house. The heaters draw way more power than OP realizes they do, and even a cheapie "heaterfan" type pulls 12.5A, dominating any circuit it's plugged into. Commented Nov 16, 2019 at 17:01

4 Answers 4


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.

enter image description here

  • 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.

  • Thank you Harper. This is going to be a temporary setup only and my main goal is to be able to run two space heaters, air compressor and a couple of other tools all at the same time in the same space. This is a rented condo so I do not wish to make any permanent changes or wiring, sub panel isn't viable hence. My goal was to use the 40 amp Range outlet to provide multiple 20 amp receptacles to power my tools since there will be no other appliances or devices running on this line except for my tools.
    – user109141
    Commented Nov 15, 2019 at 17:56
  • 4
    You're not allowed to use 10 AWG for that. Wires don't magically limit current to safe levels; they catch on fire if overloaded. Commented Nov 15, 2019 at 19:16
  • 2
    @user109141 Forget the 10 AWG,, this deal requires 8 AWG. Also you need flexible cordage: Romex is not allowed here. You are not allowed to alter the service panel in a rental unit. Your only choices are to plug into the oven socket, using the cable I mentioned, or another flexible cord going with a subpanel. Commented Nov 15, 2019 at 19:19
  • 1
    Thank you for all the suggestions guys, I looked up some of the code, lots of NOs and for good measure, so am gonna skip all the complexities and just make an extension cord out of the 10 AWG I have and plug it into the 20 amp washer outlet (line & outlet are dedicated to washer only) and connect two 20 amp receptacles in parallel at the other end, hopefully this will help power my heaters which should be well under the 20 amp max. I will use the other two existing 15 amp outlets to power other tools so this solves my problems without any legal issues. Cheers!
    – user109141
    Commented Nov 15, 2019 at 20:02
  • 1
    I quit reading after, "many other problems with this plan" +1. OP needs to start over, asking how do I add outlets to my shop.
    – Mazura
    Commented Nov 16, 2019 at 0:26

This may be an unpopular answer, but due to the information you've provided I strongly suggest you call an electrician to perform this work.

40amps needs 8 AWG, not 10.

Extension cords can't be permanent (per OSHA, NFPA, and NEC).

You can't put 20 amp plugs on a 40 amp breaker, what happens if you put 35 amps on that plug? It never trips but it catches on fire.

Since there are two hots, you have 220v. You could buy a transformer to step it down to 120v, but why would you when you can just add a single pole breaker to your panel.

Please think about this, you could save some money trying to do it yourself or you could pay up and get it done correctly.

My suggestion is to add new 220v circuits on whatever breaker size your equipment requires, and to add a new 110v circuit for duplex receptacles. If you have access to your attic or crawlspace this won't be all that difficult to do.

EDIT: In regards to Harper's comment I'll explain myself a bit more on why I think OP should call a professional. I'm a licensed electrician, I've seen a variety of damage caused by illegal electrical installations from a destroyed outlet to an entire floor being burned. Adding circuits with the correct wire size, strapping the conductors appropriately, protecting them from damage, and landing the conductors in the panel are things I'm not confident the OP should do on his own. I'm sure he could do it, but why risk his safety or his home.

  • 1
    FYI "hire a professional" is the only unpopular thing; because it could potentially be trotted out for every question. The cure to that is to explain why it's warranted in that particular case. Commented Nov 15, 2019 at 16:55
  • Thank you for your input Michael, I do not disagree but hiring an electrician is out of the question for various reasons.
    – user109141
    Commented Nov 15, 2019 at 17:56
  • Appreciate that thought Harper. :)
    – user109141
    Commented Nov 15, 2019 at 17:57
  • @Mikael if I use the 40 amp outlet to extend two lines of 20 amp 120 v each (each hot wire on a separate line) and attach two receptacles on each of the lines (I get 4 receptacles in total), I am guessing the total amperage I will get on each line is 20 amps no matter how many receptacles I connect, correct? The two receptacles on each of the lines will provide a sum total of 20 amps though each will provide 120 v, going by laws of electricity. Please correct me if I am wrong.
    – user109141
    Commented Nov 15, 2019 at 18:04
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    @user109141 I understand your line of thinking but that won't work. The reason why is you'll (in essence) have 20amp receptacles landed on a 40amp breaker. So that receptacle can see 35+ amps without tripping the breaker. If that happens then the receptacle will fail and potentially cause a fire.
    – Mikael
    Commented Nov 15, 2019 at 18:09

Assuming you have a 4-prong 220v outlet (vs 3-prong):

The "right" way to do this is to get a stove pigtail, run it into small secondary breaker panel ("subpanel"), and install in the panel some 20A breakers. Feed those to your 20A duplex outlets. Probably best to mount the panel and outlet boxes on a piece of plywood.

You might be able to throw it together with about $100 in parts.

If you have a 3-prong outlet then fugitabowdit.

  • Appreciate it, thank you. :)
    – user109141
    Commented Nov 16, 2019 at 15:37

Are you sure it is 40A? Here the large A outlets come in 16,32,64,125A three-phase.

But then, you failed to mention in which country you live.

enter image description here

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