# What Is the Rule for a Step-Up Transformer?

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All transformers carry a KVA rating that identifies the transformer's electrical characteristics.

Step-up transformers are sold according to their kilowatt-volt-amp (KVA) rating, often referred to as the transformer's "size." When determining the proper size step-up transformer needed, there are two applicable rules: electrical codes and Watt's Law.

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## Electrical Codes

Under the National Electrical Code, a step-up transformer must have certain current carrying protections in place. There must be a main circuit breaker installed in a load center served by the transformer, and the main breaker can be no larger than 70 percent of the maximum current capacity of the transformer.

## KVA Rating

All transformers have a KVA rating. This rating represents the transformer's maximum capacity in kilowatts (thousands of watts). In calculating what KVA means in practical terms, Watt's Law is applied. In Watt's Law, power (P) equals the output voltage (E) times the ampere capacity (I). Using this formula, P = E x I, and its direct derivatives, I = P / E and E = P / I, all transformer attributes can be calculated. For example, if the transformer's rating is 10 KVA and has a 240-volt output, it has a current capacity of 41.67 amperes (10,000 watts / 240 volts = 41.67 amps).