A Kiln's Electrical SystemVIEW 85% RULE BELOW 

The 85% Rule The single most important rule to observe in laying out your kiln electrical system is The 85% Rule. It keeps you from burning the house down. You design a system so that the maximum amount of power you use is no more than 85% of the power ratings for every component. 

Example: Our kiln above is powered by 4 elements that draw 10 amps each at 240 Volts AC, single phase. So we calculate the power use of the kiln to be 40 Amps (4 x 10 = 40). Now apply the 85% rule →  
What does this mean? Well it means that all the components should be rated above 47.06 amps. Fifty amps is the closest nominal unit. This applies to the breaker in the breaker box (#1) as well as all the other components: The main switch (#5), plug (#2), and socket (#2) also the mercury relay found in the power connection board cabinet (#3 above). And all the wiring (#6) connecting these components should be rated for 50 amps. 