Transformer Fuse Sizing - The NEC is not the Last Word

It is widely accepted that transformer primary side fuses should be appropriately sized. Proper sizing involves consideration of fault and overload conditions that are possible. The basis for such sizing is commonly derived from NEC limits. These NEC imposed limits are the legal limits, but direct application of these maximum values to size transformer primary side fuses is ill-advised. The primary side fuse for medium voltage (MV) applications greater than 1000 volts, as with most MV protective devices, is not intended to provide overload protection. That is achieved by design (considering the intended loading on the transformer). The National Electrical Code recognizes this by permitting the fuse to be sized up to 300 percent of the transformer full load ampere rating. It is generally not a good idea to apply 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 (LV) transformers, although the content can be easily extended to liquid filled transformers by using the equations from the appropriate standard.