12 Volt Encyclopedia Volume III
Quick answer: amps = watts ÷ 12, fuse at 125% of the load, relay at 150%
A 120 W light bar draws about 10 A at 12 V, so it needs a 15 A fuse, a 30 A relay and wire sized to keep voltage drop under 3%. Always switch high-current lights through a relay, put the fuse near the battery, and size the wire for both current and length.
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The 4-step sizing method
- Add up the watts. Use the total for every light on the circuit (for example, two 60 W pods = 120 W).
- Convert to amps: amps = watts ÷ 12. Using 12 V instead of the 13.8–14.4 V of a running engine gives a safe, slightly high number.
- Pick the fuse: amps × 1.25, rounded up to the next standard fuse size. A 10 A load gets a 15 A fuse.
- Pick the relay and wire: relay rated at least 1.5 × the load, and wire thick enough for the current, the fuse and the run length.
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Light bar load calculator
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Quick-reference table
Copper wire, 12 V, grounded near the light, 3% maximum drop. Wire size is the larger of what the fuse requires and what the length requires.
| LED load | Current | Fuse | Relay | Wire · 6 ft run | Wire · 10 ft run | Wire · 15 ft run |
|---|---|---|---|---|---|---|
| 36 W | 3.0 A | 5 A | 30 A | 18 AWG | 18 AWG | 18 AWG |
| 60 W | 5.0 A | 7.5 A | 30 A | 18 AWG | 16 AWG | 16 AWG |
| 120 W | 10.0 A | 15 A | 30 A | 16 AWG | 14 AWG | 12 AWG |
| 180 W | 15.0 A | 20 A | 30 A | 14 AWG | 12 AWG | 10 AWG |
| 240 W | 20.0 A | 25 A | 30 A | 12 AWG | 10 AWG | 10 AWG |
| 300 W | 25.0 A | 35 A | 40 A | 10 AWG | 10 AWG | 8 AWG |
| 400 W | 33.3 A | 50 A | Split into 2 relays or use a high-current relay | 8 AWG | 8 AWG | 8 AWG |
Why the wire looks thickLong runs to a roof or bumper bar lose voltage. LEDs stay bright, but the driver works harder and runs hotter. If your kit's harness is thinner than the table, keep the run short or upgrade the power lead.
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Wire it through a relay
The switch only carries the small relay-coil current. The heavy current goes straight from the fused battery feed through the relay to the light.
| Relay pin | Connects to |
|---|---|
| 30 | Battery positive through the main fuse |
| 87 | Light bar positive |
| 86 | Dash switch output (from a fused accessory or ignition source) |
| 85 | Chassis ground |
Don't skip the fuseAn unfused feed from the battery can burn the wire and start a fire if it chafes against the body. Fuse it within about 7 in. of the battery.
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