The Basics of Electricity and Vehicle Lighting

Trailer Wiring Data

How to select the size of electric cable

Wire is used to carry current from the battery through the vehi- cle to various electrical components: lights, instruments, etc.

In automotive use, the grounded return system is used almost exclusively. In this system, a single wire is run from the battery through switches to the load and the vehicle itself provides the ground or return to the battery.

It is important to note that all electric components have a good ground. In a combination of vehicles (tractor-trailer) it is important that a good and adequate size ground wire be pro- vided between the two vehicles to carry the current.

Wire has certain resistance to current flow. The smaller the wire, the greater the resistance.

Also, as the amount of current (amps) increases, the resis- tance (ohms) increases.

For a 12-volt system there are two factors to be considered:

• 1) Voltage drop is the primary consideration in selecting the proper electrical wire size. The length of wire in a circuit is the major contributing factor to voltage drop.
• 2) The current carrying capacity of the wire. The heavy load in an undersize wire may increase the temperature of the PVC insulation above a safe level of 180° F.

The Wire Size charts shown below are provided for easy selection of wire sizes. These charts, however, neglect losses due to connections or switches. To use either of these charts:

• Determine the total wire length from the power source to the most distant point in the circuit.
• Determine the total electrical load on the circuit.
• Find the load along one edge of the chart. (6v or 12v) depend- ing on the vehicle voltage in question, taking the next higher load if the indicated load falls between two points on the chart.
• Find the cable length required along the edge of the chart, taking the next higher length (if the exact length is not cited).
• At the intersection of the length and load column, read the minimum size wire required for that particular circuit.

Add 2 feet for connections and slack to reach around obstacles.
Wire Length Calculation:
3+45+4+8+8+2 = 70 feet

Example

On a 12-volt system, operating at an ambient temperature of 125° F, determine the wire size required for a clearance light circuit having eleven lights using No. 97 bulbs

• Determine that each bulb is rated at 0.69 amps. The total load is 0.69 amps x 11 = 7.59 amps.
• Find the distance from the power source to the most distant light. Total distance = 3 + 45 + 4 + 8 + 8 + 2 = 70 ft.
• In the “Wire Size Chart” below, find the column with the “Length of Wire” that matches the example. Your most distant light is 70 ft. Next, find the row with the ampere load that matches the example. Hint: The next highest value above 7.59 amps is 8.0 amps. At the intersection of this row and column, you’ll find a 12 gauge wire indicated. This is the correct wire size to use.
• Check to see if it’s safe to use a 12-gauge wire at an ambient temperature of 125° F. In the “Temperature Chart” below, find the temperature column that matches the example. Your ambient temperature is 125° F. Next, find the row with the wire gauge that matches the example. Your wire is 12 gauge. At the intersection of this row and column, you’ll find that the current carrying capacity of a 12-gauge wire at 125° F is 32 amps. That’s well in excess of your 8-amp load, so you’re okay.

WIRE SIZE CHART
For A 12-Volt System	For A 6-Volt System	Length of Wire (Most Distant Light)
		10’	20’	30’	40’	50’	60’	70’	80’	90’	100’
Load in Amps	Load in Amps	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge	Wire Gauge
1.0	0.5	18	18	18	18	18	18	18	18	18	18
1.5	0.75	18	18	18	18	18	18	18	18	18	18
2.0	1.0	18	18	18	18	18	18	18	16	16	16
3.0	1.5	18	18	18	18	18	16	16	16	14	14
4.0	2.0	18	18	18	16	16	16	14	14	14	12
5.0	2.5	18	18	18	16	14	14	14	12	12	12
6.0	3.0	18	18	16	16	14	14	12	12	12	12
7.0	3.5	18	18	16	14	14	12	12	12	12	10
8.0	4.0	18	16	16	14	12	12	12	10	10	10
10.0	5.0	18	16	14	12	12	12	10	10	10	10
11.0	5.5	18	16	14	12	12	10	10	10	10	8
12.0	6.0	18	16	14	12	12	10	10	10	8	8
15.0	7.5	18	14	12	12	10	10	10	8	8	8
18.0	9.0	16	14	12	10	10	8	8	8	8	8
20.0	10.0	16	16	12	12	10	10	8	8	8	8

TEMPERATURE CHART
Cable Size	Maximum Current Carrying Capacity (For 12 Volts at Listed Temperatures)
	120° F	125° F	150° F
20 Gauge	15 Amps	13 Amps	9 Amps
18 Gauge	18 Amps	15 Amps	11 Amps
16 Gauge	22 Amps	19 Amps	14 Amps
14 Gauge	27 Amps	23 Amps	17 Amps
12 Gauge	40 Amps	32 Amps	24 Amps
10 Gauge	50 Amps	42 Amps	31 Amps