Fuse Rating Selection
The fuse rating is placed between two limits: above the load's continuous current and below the ampacity of the cable it protects. If that band is narrowing, the solution is not a bigger fuse but a thicker cable. For motor loads that draw high current at start, a delayed characteristic is chosen.
Make the selection in three steps. First determine the load's continuous current; if there is no catalogue value, measure it with a clamp meter. In the second step, calculate the cable's ampacity together with the temperature and bundling corrections. In the third step, choose a standard fuse rating that falls between these two values.
- Continuous current margin: placing the fuse just above the continuous current leads to nuisance blowing on hot days; leave a reasonable margin.
- Inrush current: DC motors draw several times the continuous current at start. Handle this peak with a delayed characteristic, not by enlarging the fuse.
- The cable comes first: if your calculation does not find a rating that will protect the cable, increase the cross-section.
In automotive applications a single universal table of the form "this fuse for this cross-section" is not reliable; routing method and temperature change the values. Applying the principle and calculating for your own line gives a more accurate result than copying a ready-made table.
Context in which this term is used: Installation, redundancy and field continuity