The rule of 80% and kilns

For some time now, I’ve been watching a running debate about how much power a kiln uses on a given circuit. Over on WC there is a thread about the circuit breaker always kicking out, and it turns out that the person has a kiln that draws 14 amps and is running it on a 15 amp circuit (along with other appliances and devices).

Under the National (USA only) Electrical Code (NEC), a device that draws 14 amps needs to be on a 20 amp circuit. The reason for this is what is referred to as the Rule of 80%. In brief, a given circuit should have a load of no more than 80% of its marked value: for a 15 amp circuit, that’s 12 amps, for a 20 amp circuit, that’s 16 amps.

I have seen kilns sold that have a standard 15 amp plug, yet draw 14 amps, clearly violating the Rule of 80%.

Plugs and receptacle (outlets) are classified by the NEMA (National Electrical Manufacturers Association) code. For normal household devices, we are concerned about 2 particular codes: 5-15 (P/R) and 5-20 (P/R) [P=plug, R=receptacle).

Here are pictures of the standard 15 amp plugs and receptacles:

NEMA 5-15PNEMA 5-15R Standard 15 amp wall outlet


And here are pictures of 20 amp plugs and receptacles:

NEMA 5-20PNEMA 5-20R 20 Amp receptical

Note that the 20 amp plug and receptacle have the neutral line plug/outlet opening turned 90 degrees from the hot line plug/outlet opening and that the receptacle also allows the use of standard 15 amp devices.

All kilns that draw 12 amps or more on a 125 volt circuit are required under the NEC to have a NEMA5-20P plug on them. This is to ensure that the device is plugged into a circuit that can handle the amperage of the device and to prevent a possible fire from an overloaded circuit. But many kiln builders totally ignore this requirement and install 15 amp plugs to make it “easier for the user to use on a normal electrical circuit”.

Yes, it is certainly easier, but for the user who keeps popping circuit breakers or blowing fuses, it is nothing more than a hazard that could lead to a fire.


8 thoughts on “The rule of 80% and kilns

  1. i think you missed the point of the debate at WC. the kiln in question has a 15 amp plug on it, but it is blowing the circuit breaker because the circuit also has a freezer, garage door opener, and some lights on the circuit. the main issue is the suggestion that or friend dennis made stating that you could run a 14 gauge extension cord for 100 feet to hook up the kiln. there is no need to take the 15 amp plug off the kiln and install a 20 amp plug, that won’t help as you know.

  2. Actually, what I’m saying Mark is that if the kiln had the proper plug on it in the first place (and assuming the kiln owner didn’t cut it off and replace it), this issue most likely would not have happened.

    Then the post becomes:

    “What the heck??? I have this weird plug on my kiln and it won’t fit into the plug in my wall!!!”

    And everything else after that is moot. AIM, Arrow Springs, and others who build kilns should know better — if the kiln uses more than 12 amps, put a 20 amp plug on it!!

    As far as extension cords, I’ve already address that issue here:

  3. Interesting. My 15amp kiln is on a 20amp circuit alone, but the plug is like the top one for both the kiln and the receptacle. In fact the only plugs I’ve ever seen that were not the standard design are the 220 outlet/plug for my big fusing kiln. I’ll check more closely going forward.

  4. I’m not at all surprised, Gwacie. The code exists for a reason: to not overload circuits and circuit breakers, but it is undermined when kiln manufacturers ON PURPOSE put a connector (plug) on a kiln that is designed for a lower amperage rating.

    They do it so you (the user) doesn’t have to have a special circuit/outlet installed, but by doing it, they set up a condition where the possibility exists to create a fire with the potential of loss of life. Electrical fires happen a lot of the time inside the wall where it is very hard to discover until smoke and flame start shooting out of the wall. And of course, by then, you have a fire that is already out of control.

  5. Pingback: More on amperage ratings for kilns « Chaotic Glass by Mike Aurelius

  6. Thanks for this post. I just bought a house and tried running my Paragon kiln tonight and tripped the circuit breaker a bunch of times. I was totally baffled until I found your post. Guess I’ll be calling an electrician tomorrow!

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