Safety First, of Course
Tip #1: Please do not skip over this section unless you have a great deal of experience with electrical wiring and components.
Neutralize the Ultimate Power Source
Your car or truck battery may appear dull and non-threatening, but it is a potentially life-threatening storehouse of electrons looking for any way out at the speed of light.
Before you start, disconnect the vehicle battery. Do not take shortcuts. Disconnect it.
Tip #2: Always remove the ground terminal clamp first followed by removing the positive side clamp. This greatly reduces the risk of shorting the battery, which is both exciting and very dangerous. Removing the positive terminal first risks you or a tool coming into contact with any metal connected to the negative terminal, which is most of the car.
Sizing up Your Wire
Many people make the analogy that electrical current through a wire is like water through a hose. Unlike water hoses, however, pushing excessive electrical current through a wire that is too small will melt the insulation or cause a fire.
Fortunately, you already know how much current your LED light bar requires. So, you can easily choose the size of wire to feed it safely. Below is a list of automotive wire sizes and their safe current ratings.
| Amps | ||||||||
| Length in feet | 5A | 10A | 15A | 20A | 25A | 30A | 40A | 50A |
| 15 | 16 | 12 | 10 | 10 | 8 | 8 | 6 | 6 |
| 20 | 14 | 12 | 10 | 8 | 8 | 6 | 6 | 4 |
| 25 | 14 | 10 | 8 | 8 | 6 | 6 | 4 | 4 |
| 30 | 12 | 10 | 8 | 6 | 6 | 4 | 4 | 2 |
| 40 | 12 | 8 | 6 | 6 | 4 | 4 | 2 | 2 |
| 50 | 10 | 8 | 6 | 4 | 4 | 2 | 2 | 1 |
The left column is the length of a wire, in feet. The top row is the number of amps applied to the wire. Choose a column that is equal to or greater than the maximum current draw of your light bar. The intersection between the selected row and column indicates the proper size of wire, in AWG or American Wire Gauge, you should use for the circuit.
Tip #3: It never hurts to choose one gauge larger, which means a lower AWG. This provides a greater margin of safety.
Tip #4: When wiring LED light bar relays or switches, fuse all positive side wires such as from the positive battery terminal to the light bar switch. The fuse rating should not correspond to the wire size but instead to the maximum current expected on that wire. In this case, that would be the light bar’s maximum current.
Wire Grades
There are three grades of automotive wire: GPT, GXL and TXL. Your local auto parts will have GPT wire but probably not GXL or TXL grades. These grades have a higher temperature range and better resistance to solvents and other chemicals. If you can afford them, use them.
Connectors
Your LED light bar probably has an IP67 or higher rating. Why compromise that with leaky connectors? Look for quality watertight wire connectors in a local marine store. When wiring LED light bar connectors, use crimpers that are specified for them.
Good watertight connectors do not need a coating of silicone or electrical tape. If you are so compelled, however, use marine grade silicone and professional quality electrical tape.
Tip #5: Wrap electrical tape snugly over connectors without stretching or it will slowly unravel later.
To Relay or Not to Relay?
Automotive relays are inexpensive and most handle 40A of current, which is more than enough for even large LED light bars. The relays are switched via a cab switch whose circuit carries less than 1A. Therefore, for a large light bar you can use a smaller cab switch and smaller wire on the switch circuit.
