How is a fuse current rating chosen?
The fuse rating stays between two limits: above the device's continuous operating current and below the carrying capacity of the smallest cross-section on the line. A delayed type may be needed for the inrush current drawn at start. ATO/ATC blade fuses are available in the 1-40 A range; for currents above that range, larger-bodied types or a circuit breaker are used.
Step-by-step method:
- Determine the device's continuous current. Read it from the label or technical data; if you are going to measure, read it with a clamp meter during normal operation.
- Consider the inrush current. Motor and coil loads may draw several times the continuous current at start. A type that does not blow at this inrush, or a rating one step up, is chosen, but the upper limit cannot be exceeded.
- Find the smallest cross-section on the line. Ampacity is typically around 18 A for 1.5 mm² and 76 A for 16 mm²; routing and temperature reduce this, and a factor of about 0.71 applies in a 50 °C environment.
- Pick a standard rating between the two limits. In the ATO/ATC range there are steps such as red 10 A, blue 15 A, yellow 20 A, clear 25 A, green 30 A, orange 40 A.
If the two limits overlap, that is, if the device's current is very close to the cable's capacity, the solution is not to push the fuse but to increase the cross-section.
Final check: after fitting the fuse, run the device under normal conditions and observe that the fuse body does not overheat. A fuse that runs hot all the time is operating at its limit and will fatigue over time and blow for no apparent reason.
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