How to Size a Heater Circuit Breaker


Selecting and installing the proper size CB (circuit breaker) is one of the keys to installing a safe and efficient electric heating system. Underwriter's Laboratories tests CBs at 100 percent of their rated current and approve them to carry their rated current for an indefinite period. The NEC restricts CBs to supply a maximum load that doesn't exceed 80 percent of their rated amperes. Why is this? The real life application of a CB seldom matches laboratory conditions and the NEC considers this factor. The heat build up during a real life application exceeds the heat build up under ideal conditions.

Things You'll Need

  • The NEC (National Electric Code), NFPA (National Fire Protection Association) Publication 70.
  • Pocket calculator
  • Pen
  • Paper
  • Locate the specifications plate on the heater and determine the FLA (full load amperes). The information on this plate may give this data as volts and watts instead of volts and amperes. You can calculate amperes by dividing the watts by the volts. P (watts) = E (volts) X I (amperes) therefore I =P/E.

    If you are sizing a CB that protects several individual, fixed in-place heating units, use the total wattage to calculate the FLA.

  • Calculate the continuous duty load that will be placed on the CB. The NEC defines a continuous duty load as any load that will draw maximum current continuously for three hours or more. Circuit breakers protecting continuous duty loads must be rated at 125 percent of the load's FLA. Keep in mind that 125 percent is the mathematical reciprocal of 80 percent. 1/1.25=0.8 or 80 percent.

    Using the FLA calculated in the above example, the continuous duty load would be 1.25(100) = 125 ampere

    Size the heater CB to have a 125-Ampere rating.

  • According to Article 240.6(A) of the NEC, which gives the standard ratings for fuses and for fixed-trip circuit breakers, 125 ampere is a standard rating so select a 125-Ampere CB. If the computed rating was 135-Amperes, move up to the nearest higher rated CB, which would be a 150-Ampere CB.

Tips & Warnings

  • If you are also running the branch circuit conductors for this circuit, they too must be rated at 125 percent of the FLA of the heater load.

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