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. Continue reading

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