Overcurrent Protection - SolaHD - #3

Transformers

Primary Overcurrent Protection

A transformer has all the same component parts as a motor, and like a motor, exhibits an inrush when energized. This inrush current is dependent upon where in the sine wave the transformer was last turned off in relation to the point of the sinewave you are when you energize the transformer. Although transformer inrush could run up to 30 to 35 times full load current under no load, it typically is the same as a motor...about 6 to 8 times normal running current. For this reason it is important to use a dual element slow blow type fuse - the same type of fuse you would use with a motor. If using a circuit breaker, select a breaker with a time delay - again the same type you would use with a motor. If the time delay is not suf cient, you may experience "nuisance tripping" - a condition where the breaker trips when energizing the transformer but when you try it again, it works ne.

Secondary Overcurrent Protection

Overcurrent devices are used between the output terminals of the transformer and the load for three reasons:

1. Protect the transformer from load electrical anomalies.

2. Since short circuit current is minimized, a smaller gauge wire may be used between the transformer and the load.

3. Per NEC, a larger primary fuse may be used to reduce nuisance tripping.

Secondary Fuse Recommendations

Fuse = 1*167% next size smaller if secondary current is less than 9 amp.

Fuse = 1*125% next size smaller if secondary current is 9 amp. or higher.

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