MV HRC FUSE CALCULATION

If the fusing is immediately at the transformer primary (transformer protection only), then the defining parameter is the thermal and mechanical damage curve per IEEE C57.12.00. For this size transformer it falls in Category II. The fuse should be chosen so that the maximum clearing time falls below the damage curve for the rating of the transformer if possible, based on full-load amps (FLA, which another poster calculated as 52 amps) and the % impedance.

Typical fusing for a transformer would be on the order of 150% to not more than 300% of FLA, depending on the installation, the frequency of transformer exposure to external faults, fault current levels, transformer % impedance, etc. The poster who suggested an 80 amp fuse was probably using a 150% recommendation. However, it is best to plot the transformer and fuse curves (in software or on paper) to determine the protection and coordination.

If the fuse is upstream of the primary cables, then the damage curve of the cables can be included on the plot and an attempt made to protect them as well.

You can use the NEC Table 450.3(A). Per NEC Table 450.3(A), the primary fuse rating is not more than 300% of primary rated current (for 600V and over) and the secondary fuse rating is not more than 125% of secondary rated current (for 600V and less). However, do the device coordination for the proper selection of the fuse or get it suggested by the transformer manufacturer.

This link also helps:

https://www.littelfuse.com/data/en/White_Papers/Littelfuse-Protection-Relay-Transformer-Protection.pdf

- MS