I have a heater that draws a continuous 18 amps and has a 20 amp plug. I created an extension cord to plug into the wall, however on one end it's a 20 amp receptacle - heater plugged into extension cord - and the other end of the cord is a normal 15 amp plug, plugged into a 15 amp receptacle in the wall. The extension cord is 12 gauge.

I used a kill-a-watt to measure the draw at the wall (the 15 amp outlet) and it's drawing 18 amps.

Is this ok for long term use? i.e. plugged in 8 hours a day straight.

UPDATE: In my haste to get this posted as I worried about the setup, I mis-spoke and need to make a correction - The outlet itself is a 20 amp outlet in the wall, BUT the make plug on the end of the extension cord is a 15 amp plug (was the only part I had at the time). Does this still pose a large risk? Here's the picture:

image 3

Update: It's no longer plugged in at the top where it was for several years. Image of extension box: Image

Here's a photo of the plug that goes to the heater: image 2

  • 4
    What is the circuit breaker rated at? What is the in-wall wiring? If there's a 15A receptacle, I would expect wiring for 15A circuit, protected by a 15A circuit breaker. If the circuit breaker is not tripping, it's defective. If the circuit breaker/fuse was replaced with a 20A and your wiring is rated for 15A (eg, 14/2) then you've created a dangerous situation -- this is how electrical fires start, and is the type of situation where the insurance claim can get denied.
    – gregmac
    Apr 21, 2015 at 16:41
  • It's on a three breaker circuit, each 30 amps. This is in a shop with machinery running. I don't know enough about electricity to know how much the other machine is pulling, but I think there's ample amperage to go round. Here's the other thing, the plug was plugged into one of the outlets on the extension cord for awhile before I noticed this (image) - posted in the original question at the top. I changed it to the other receptacle just below and haven't seen anything yet. Could some lint or something else just been caught in there to cause that?
    – v15
    Apr 21, 2015 at 17:30
  • 5
    Custom extension cords and undersized extension cords and ignoring the rating labels on your devices are good ways to get insurance claims denied, too.
    – longneck
    Apr 21, 2015 at 17:57
  • Can you include a photo of the plug that is connected to the equipment you're using? I'm interested to see the pin configuration.
    – Tester101
    Apr 21, 2015 at 18:57
  • 1
    Your photo is of a 20A outlet (R5-20) which obviously overheated. Since the draw is 18A, the problem should not be with the outlet. Most likely, the wire feeding the outlet came slightly loose, causing high resistance and overheating. This is regardless of your original question with the cord leading to this outlet being plugged into a 15A outlet.
    – DoxyLover
    Apr 21, 2015 at 20:16

2 Answers 2


No it is not safe.

A 15 ampere rated receptacle should not be installed on a circuit protected by a 30 ampere breaker.

You should not be plugging devices that draw more than 15 amperes, into a 15 ampere rated receptacle.

The receptacle in your photo is a 20 ampere receptacle, so you shouldn't have a problem plugging in a 20 ampere device. They make receptacle configurations different, so that you can't plug things into the wrong receptacle. By making a bootleg extension cord, you're bypassing that safety feature.

You're going to burn the place down.

Also note that since the device is active for longer than 3 hours, it would be considered a continuous load. In which case the circuit (overcurrent protection, wiring, and devices) should all be rated for 125% of the load. That would mean the wiring, overcurrent device, and receptacle should be rated to handle at least 22.5 amperes (18 × 1.25). That means a 30 ampere breaker, 10 AWG copper conductors, and a 30 ampere receptacle.


Your extension cord is nope, but you know that and it's the least of your problems. Fortunately this is fixable.

First, search all three branches of this circuit (I gather it is 208 wye 3-phase) for the smallest wire in use. If it is 10 AWG, then it can be a 30A circuit and the 30A triple breaker is OK. If any wires are 12 AWG, you actually have a 20A circuit, either change the breaker to 20A or the wires to 10AWG.

The only receptacles that can be on a 30A circuit are 30A receptacles. So all those NEMA 5-20 receptacles must go away and be replaced with 30A receptacles. You are allowed to have as many 30A receptacles as you want on a 30A circuit.


As Tester101 says, your heater needs to be rated for continuous running at 125%, or 22.5 amps. That means it also needs a 30A receptacle.

So, this is a mismatch made in heaven, convert this circuit to 30A in all respects, and you are safe, legal and ready to go.

  • Id say if tbe heater was sold with a 20 amp plug and is drawing more than that he should ditch or repair the heater.
    – agentp
    Nov 11, 2017 at 14:13

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