For a 10kVA single-phase transformer with a 30 amp circuit breaker on the primary, what is the minimum size allowed for secondary conductors?

To determine the minimum size allowed for secondary conductors of a transformer, one must consider the output capacity of the transformer and the associated standards for conductor sizing as per the National Electrical Code (NEC) requirements.

A 10kVA transformer operating on a single-phase circuit will have a maximum secondary current that can be calculated as follows:

The secondary current can be derived from the formula:

[ I = \frac{VA}{V} ]

Assuming a standard voltage level of 240 volts on the secondary side, the calculation would be:

[ I = \frac{10,000 VA}{240 V} \approx 41.67 A ]

This indicates that under full load, the secondary current can be around 41.67 amperes. According to the NEC, conductors must be sized adequately to handle expected load continuously. Specifically, conductors are typically required to be rated for at least 125% of the continuous load to ensure they can handle potential overload scenarios without overheating.

Calculating 125% of the secondary current:

[ 41.67 A \times 1.25 \approx 52.1 A ]

This suggests that the conductors should be sized for at least 52.1 amperes