What amperage fuse should I fit to the motor driver?
The fuse rating is 1.25-1.50 times the continuous current: about 20 A where 15 A is drawn. Character and cable cross-section decide as much as the rating.
The fuse rating is chosen as 1.25–1.50 times the continuous current measured under load. In an application drawing 15 A continuously that means a fuse of about 20 A. Two more things matter as much as the rating: the fuse's time-current characteristic has to pass the starting surge without blowing, and the cable cross-section has to carry the current the fuse lets through.
The three inputs that set the rating
- The continuous current under load. Measure it with a clamp meter, do not trust the label.
- The starting surge. Starting current is typically 5-8 times the rated current and lasts 0.5-3 seconds. The fuse's time-current curve has to pass that surge without blowing.
- Cable cross-section. The fuse protects the cable, not the driver. The cross-section has to carry the current the fuse lets through safely. Limit the fuse by the cross-section, not the cross-section by the fuse: enlarging the fuse without changing the cable makes the cable itself the weakest link in the protection chain.
Calculation and examples
| Measured continuous current | Fuse range | Typical application |
|---|---|---|
| 10 A | 12.5-15 A | Small hatch, blind, actuator |
| 15 A (KS250 at full capacity) | 19-22 A | Medium crane, lift movement |
| 25 A | 31-37 A | Heavy duty, PT500 class load |
The lower end of the range is for steady loads, the upper end for loads that start frequently. If the calculation does not land on a standard fuse rating, round down rather than up: a fuse that is too small will blow unnecessarily and you will notice, whereas one that is too large quietly leaves you unprotected. If you feed more than one load from the same line, putting a separate fuse on each load and leaving a single upstream fuse on the main line lets you tell which branch is at fault when something goes wrong.
Character and position matter no less than the rating
ATO/blade type fuses are generally of the fast-acting character and may blow needlessly on the motor starting surge. For motor loads choose a slow-blow (T/TT) character or a time-current curve that covers the starting surge; this is exactly where the difference between fast-acting and slow-blow fuses earns its keep. Place the fuse as close as possible to the supply source, preferably at the first point out of the battery terminal; what needs protecting is the whole cable in between.
Where this calculation does not help
A fuse is not an overload protection, it is a short-circuit and fire protection. If the driver has no current limit or thermal protection of its own, even a correctly chosen fuse will not stop the module running at its limit and ageing; the driver can fail without the fuse blowing at all. If the fuse blows again and again, enlarging the rating is not the answer: going up in rating only leaves the cable unprotected and makes the next fault more expensive. First eliminate the real causes such as mechanical binding, a winding leak, a chafing cable touching the chassis, or a loose connection. An aged fuse can also tire under repeated starting surges and begin to blow below its rated current; do not strike a fuse that has sat in place for years off the list of suspects.
What to do
Measure the continuous current under load, multiply by 1.25-1.50, take the nearest standard fuse to the result, and verify the cable cross-section against that fuse rating. Choose the fuse together with the cable, not separately. After installation, check over the first few cycles whether the fuse holder is warming up; a warm holder means a contact problem even when the rating is right, and a loose holder will in time both melt and blow at the wrong moment.
The full topic: DC Motor Driver Selection Guide