# 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:

And here are pictures of 20 amp plugs and receptacles:

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. Mark Wilson says:

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!!

https://mikeaurelius.wordpress.com/2008/03/27/kiln-power-cord-too-short/

3. Mark Wilson says:

i sure wish you could post at WC and LE. i grow tired of having to deal with dennis on my own.