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.