Guidelines for Selecting the Correctly-Rated Circuit Breaker
- Aug 02, 2017
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There are literally thousands of circuit breakers available today from many manufacturers. They vary greatly in their intended use, functionality, wiring methods, and power/amperage ratings.
How can you select the correctly-rated breaker for your particular application? Read on for some guidelines on making the selection accurately to get the right circuit breaker you need.
Determining the Circuit Breaker Rating Needed
Consider these factors when evaluating circuit breakers to find the right one for your needs:
1. Function of the Circuit
What will the breaker be servicing? Requirements will be different for breakers intended to protect wiring installations vs. those intended to protect equipment such as motors. This will include the decision about whether or not thermal overload protection is required.
2. Rated Current Requirements / Features
Evaluate the protection setting range required for the equipment to be protected. Breakers overlap in their ratings. A requirement to support 80-amp service could potentially be satisfied with either a 100-amp or 125-amp breaker, as an example.
Voltage rating and amperage requirements for the total load expected on the circuit are basic values that initially determine the breaker sizing.
Features needed will come into play in the selection process, as well. Depending on the mounting or panel enclosure for the breaker, the installation may call for a rotating handle breaker or a toggle switch construction.
Mounting requirements are of course an additional factor. Standard panels will accommodate certain types of circuit breakers, whereas modular systems or those utilizing special mounting plates will require different breaker types.
3. Environmental Factors
With hot temperatures, high altitudes, or unusually high frequencies exceeding 120 Hz, special capacity reduction factors must be taken into consideration. Each of these conditions will cause the capacity of the circuit breaker to be derated.
4. Maximum Interrupting Capacity
Maximum interrupting capacity is the highest level of fault current the breaker can tolerate without damage to the device itself. The interrupting capacity of the breaker must exceed the fault levels generated on the circuit. Damage to the device will otherwise occur.
5. Continuous Operating Current Rating
Standard breakers are rated by amperage, based on the capacity of the conductors in the circuit they protect. Breaker selection will therefore be dependent on not only the circuit function, but the wire rating as well.
NEC defines continuous operation as a condition where “the maximum current is expected to continue for three hours or more.”
Additional Circuit Breaker Considerations
Environmental factors affect many types of breakers, such as exposure to heat sources. Since heat is one of the contributing factors to tripping a circuit breaker, breakers located close together, as in a panel enclosure, can generate additional heat that impacts the sensitivity of the trip functions.
This is why the National Electrical Code (NEC) dictates that breaker sensitivity include a factor of 80% instead of a full 100% of listed breaker capacity. Be certain that the breakers installed into service takes this into account.
Testing and reliability are also considerations. All circuit breakers purchased should be rated as tested by UL Laboratories. Used or refurbished breakers should be verified as tested and include guarantees of functionality.