Since most single phase transformers have a secondary voltage of 120/240, they will be operated as a three wire system. Care must be taken in properly distributing the load as the transformer secondary consists of 2 separate 120 volt windings. Each 120 volt winding is rated at one-half the name plate KVA rating. For example: A 10 KVA transformer, 120/240 volt secondary is to service an 8 KVA load at 240 volts and two 1 KVA loads at 120 volts each. If the incorrect method is used, winding A will be loaded at 6 KVA, and winding B will be loaded at 4 KVA. These do total 10 KVA but since each winding is only rated at 5 KVA (1/2 of nameplate rating), we have an overloaded transformer and a certain failure.
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