Specifications
UBS Wiring Harness Handbook
– 54 –
Wire is used to carry current from the battery through the vehicle 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 provided 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 resistance
(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:
a) Determine the total wire length from the power source to the
most distant point in the circuit.
b) Determine the total electrical load on the circuit.
c) Find the load along one edge of the chart. (6v or 12v) depending on
the vehicle voltage in question, taking the next higher load if the
indicated load falls between two points on the chart.
d) Find the cable length required along the edge of the chart, taking
the next higher length (if the exact length is not cited).
e) At the intersection of the length and load column, read the
minimum size wire required for that particular circuit.
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.
a) Find the total ampere
load in the circuit. There
are 11 No. 97 bulbs.
From the bulb chart on
pages 50 and 51, you
determine that each bulb
is rated at 0.69 amps.
The total load is 0.69
amps x 11 = 7.59 amps.
b) Find the distance from the power source to the most distant
light. Total distance = 3 + 45 + 4 + 8 + 8 + 2 = 70 ft.
c) 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.
d) 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.
HOW TO SELECT SIZE OF ELECTRICAL CABLE
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
POWER
SOURCE
Add 2 feet for connections
and slack to reach around
obstacles.
Wire
Length
Calculation:
3
45
4
8
8
+
2
70 Feet
Most Distant
Light
3’
45’
4’
8’
8’
Trailer Wiring Data