A practical sizing guide for 12V and 24V installations
Start with the fuse specified for your exact light-and-harness combination. For custom wiring, this guide shows how to turn the total current into a preliminary fuse size and check whether it suits the circuit.
Quick example: two lights drawing 5A each total 10A. Using the 75% continuous-load method, 10 ÷ 0.75 = 13.33A. From common blade-fuse sizes, the initial choice is 15A—not a final approval until the wiring, temperature and startup checks are complete.
1. Confirm the total current on the circuit
Add the maximum continuous current of every light that will operate together on the same circuit. Use data for your supply voltage and intended operating conditions. Check whether the specification is for one light or a pair. If only actual input power is available, watts ÷ volts gives a preliminary current estimate.
Missing the current rating or harness specification? Send the light model and a photo of the harness to Primelux using the support link below before choosing a fuse.
2. Calculate a starting fuse size
The examples here use the 75% continuous-load method described in Littelfuse’s automotive Fuseology guide for approximately 25°C ambient conditions. This leaves normal running current at no more than 75% of the fuse’s nominal rating. A specific fuse manufacturer’s instructions take priority. [1]
Starting fuse rating = total continuous current ÷ 0.75
Choose the smallest available rating at or above that result. For the hypothetical 10A pair above, 13.33A rounds up to 15A from the sizes listed below. Do not round down to 10A or jump to 30A for extra headroom.
Quick reference using the 75 percent method
| Total continuous current | Calculated rating | Initial fuse size |
|---|---|---|
| 4A | 5.33A | 7.5A |
| 6A | 8.00A | 10A |
| 8A | 10.67A | 15A |
| 10A | 13.33A | 15A |
| 12A | 16.00A | 20A |
| 16A | 21.33A | 25A |
Examples at about 25°C, using common blade-fuse sizes. These are preliminary calculations, not Primelux model recommendations. Check your selected fuse series. [3]
3. Check the circuit before fitting the fuse
The calculated size must pass all three checks:
Wiring protection. The main cable, smaller branches, fuse holder and connectors must be suitable for protection by the chosen fuse. Cable capacity depends on its construction and installation conditions. A large relay rating does not establish the rating of the whole harness. [4]
Temperature. Use the selected fuse’s temperature guidance for where it will be mounted. Engine-bay heat can reduce its continuous current capacity. A published allowed-current table may already include temperature adjustment; do not apply the same correction twice. [1, 3]
Startup current. LED drivers can briefly draw more current when switched on. Check that the fuse can tolerate that pulse while still protecting against faults. Use the correct fuse type, DC voltage rating and interrupting rating as well as amperage. [2]
If the calculation gives 15A but the harness specifies a maximum 10A fuse, do not fit 15A. Use a suitable harness or have the circuit design checked. A larger fuse cannot make undersized wiring suitable. [4]
Common questions
Why do some guides use a different multiplier?
Multiplying current by 1.25 assumes 80% continuous loading: 10A ÷ 0.80 = 12.5A. Eaton’s general guidance uses 80% at 25°C. The 75% method in this article divides by 0.75, equivalent to about ×1.33. Neither multiplier overrides the selected fuse’s datasheet or the circuit limits. [1, 2]
Can I use a larger fuse to stop it blowing?
Not without finding the cause. A short circuit, damaged wire, excess load or unsuitable fuse can cause repeated failures. Switch off and investigate; do not bypass the fuse or repeatedly increase its rating.
Is the method different for a 24V vehicle?
The method is the same, but use the actual current at the relevant voltage. Do not automatically reuse a 12V current figure or assume every component in a 12V harness is compatible with 24V.
Planning the rest of the circuit? See our auxiliary LED light wiring guide for an independent-switch relay diagram and installation steps.
Get help with your Primelux setup
Send your light model, quantity, vehicle voltage and harness model. For custom wiring, include cable size, length and fuse-holder location so we can understand the installation you are planning.
Contact Primelux for installation support
Technical references
1. Littelfuse — Automotive Fuseology
