Transformer Fuse Sizing—The NEC is not the Last Word

It is widely accepted that transformer primary side fuses should be appropriately sized to protect load side transformers. Proper sizing involves consideration of fault and overload conditions. The basis for such sizing is commonly derived from National Electrical Code (NEC) limits. The NEC imposed limits are the legal limits, but direct application of the NEC maximum values to size transformer primary side fuses is ill advised. The primary side fuse for medium voltage (MV) applications greater than 1000 V, as with most MV protective devices, is not intended to provide overload protection for the transformer, that is achieved by design (considering the intended loading on the transformer). The NEC recognizes this by permitting the fuse to be sized up to 300% of the transformer full-load ampere rating. It is clearly not a good idea to apply the maximum limits of the NEC as an absolute standard for fuse sizing to protect transformers in the absence of the application of other techniques (such as plotting the ANSI/IEEE thermal-mechanical damage characteristic for transformers on a time current curve along with the primary side (MV) fuse time current curve characteristic). These considerations are discussed in the course of this paper. The focus here is on dry type, low voltage transformers, although the content can be easily extended to liquid-filled transformers by using the equations from the appropriate standard.