Transformer overcurrent protection is required to protect the primary windings from short circuits and overloads and the .secondary windings from overloads. Section 450.3 of the National Electrical Code (NEC) contains the requirements for overload protection of transformers. Overcurrent protection requirements depend on the following factors:
Rules for sizing overcurrent protection for a transformer operating at more than 600 volts are covered in Section 450.3(A) and Table 450.3(A) of the NEC. When only primary protection is provided in a supervised location, the maximum current rating of the fuse is 250% of the full-load current (FLA) of the primary. If a circuit breaker is used for protection, the maximum size is limited to 300% of the full-load primary current. In both cases, when the calculated current value is not equal to a standard size of fuse or circuit breaker the next larger size may be used.
Rules for sizing overcurrent protection for a transformer operating at 600 volts or less are covered in Section 450.3(B) and Table 450.3(B) of the Code. Where only primary protection is provided, the general rule is that the fuse or circuit breaker shall not exceed 125% of the full-load primary current.
For transformers 600 volts and less, the overcurrent device protecting the primary of a transformer is permitted to be rated as large as 250% (or the next smaller size) of the primary full-load current, provided the transformer secondary winding is protected. The transformer secondary over-current device rating is not permitted to be greater than 125% (or the next larger size) of the secondary full-load current.
Transformer overcurrents, like that of other devices, may be classified as being due to overloads or short circuits. A transformer overload condition is said to exist when it is delivering from one to six times its normal current rating. In this case the current is confined to its normal path and a temperature rise takes place in the transformer.
When a transformer short circuit condition exists, current is not confined to the normal wiring channels to the load. Short circuit currents can reach levels that are hundreds of times greater than the normal full load operating current. Damage can be extensive if protective devices do not react in milliseconds to open the current path created. As discussed previously, the impedance rating of the transformer is used to calculate short circuit current and to determine the interrupting current capacity required by the protective device.