How to Size an Overcurrent Device for a Transformer

Transformers are at the heart of power distribution systems
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Circuit breakers protect transformers from overcurrent situations and short circuits. They also protect circuits downstream from the transformer. Once the circuit breaker opens or "trips" due to a short circuit or some other overcurrent scenario, the circuit breaker blocks current flow to the circuit. Technicians must physically reset the breaker to allow the circuit to function normally. Circuit breakers are sized in amps; meaning once the overcurrent scenario reaches that current value, the circuit breaker will trip.

Size the Primary Side of the Transformer.

    Find the rated kilovolt-amperes or "KVA" of the transformer. Refer to transformer specifications. As a example, assume 20 KVA.

    Find the primary voltage of the transformer or "Vprimary." Refer to transformer specifications. As an example, assume the primary voltage is 480-volts.

    Calculate the primary current flow, or "Iprimary," using the formula Iprimary = KVA x 1000/Vprimary.

    Using the example numbers:

    Iprimary = (20 x 1000)/480 = 20,000/480 = 41.6 amps.

    Note: If you had a 3-phase transformer, the formula would be Iprimary = KVA x 1000/(Vprimary x 1.732). The 1.732 accounts for the 3-phase configuration.

    Find the circuit breaker size for the primary side of the transformer by multiplying Iprimary by 1.25.

    Continuing with the example:

    Primary circuit breaker size = 41.6 x 1.25 = 52 amps

Size the Secondary Side of the Transformer.

    Find the secondary voltage of the transformer or "Vsecondary." Refer to transformer specifications. As an example, assume the secondary voltage is 240-volts:

    Calculate the secondary current flow, or "Isecondary," using the formula Isecondary = KVA x 1000/Vsecondary.

    Using the example numbers:

    Isecondary = (20 x 1000)/240= 20,000/240 = 83.3 amps.

    Note: If you had a 3-phase transformer, the formula would be Isecondary = KVA x 1000/(Vsecondary x 1.732). The 1.732 accounts for the 3-phase configuration.

    Find the circuit breaker size for the secondary by multiplying Isecondary by 1.25.

    Continuing with the example:

    Secondary circuit breaker size = 83.3 x 1.25 = 104 amps.

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