How to Calculate a Transformer Size

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Electrical engineers are trained to design and implement electronic staging, devices and wiring satisfying the needs of the intended household, business or industrial structure. Within these responsibilities, engineers utilize their knowledge of circuitry, voltage, power and subsystems to achieve these goals. According to Tech-FAQ, transformers are devices transferring electrical energy between circuits. Transformers usually consist of two wire coils wrapped around a core, with one serving as the primary and the other secondary.

Determine the load. Tech-FAQ states transformers are sized based on the connected load on the secondary side, and then aligned with the best available kilowatt rating.

Acquire a sizing chart. Standard transformer charts can be obtained from a licensed electrician, or you can review the National Electric Code.


Compute the transformer size. Transformer size can be manually derived from load voltage, load current amps and line voltage. According to Jefferson Electric, use the formula "Volts x Amps / 1000" for single-phase transformers. For three-phase transformers, use "Volts x Amps x 1.732 / 1000."


Use an online calculator. ETAP, Jefferson Electric and CSG Network offer calculators on their websites to assist users in efficiently determining the right transformer.