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Sensible Wiring

Automotive Wiring

The Way it is Really Done

All of us are concerned about wiring our car properly. So much in fact, that I’ll bet most of you engineer to overkill. What are the issues you need to be concerned about?

  1. Use the smallest possible wire for the required current.               
    • Wire is expensive and the larger you go, the more expensive it is.
    • Wire is heavy and the larger you go, the heavier it gets.
    • Mechanically, smaller wire is easier to route, easier to protect, easier to fit connectors on and therefore, more reliable mechanically (There are limits - see below).
  2. Use a large enough wire so there is no voltage drop. We want whatever it is we are wiring to operate at top efficiency.
  3. Maintain an adequate safety margin. We don’t want to melt any wires do we?

The first thing you have to do is determine the current you have to carry. For DC circuits, that’s relatively easy. Some equipment on a car is rated directly in current draw. Auxiliary fans, fuel pumps and things like that are rated in current draw - Amps. Some equipment is rated in Watts - mostly the lighting equipment. The power requirement in Watts will be printed right on the bulb or stamped in the base. To come up with amps use one of the formulas shown here.

Let’s calculate for a typical 100 Watt Driving Light - the power required is 100 Watts and the voltage is 12 Volts - so the current requirement is 100 Watts/12 Volts = 8.33 Amps. Let’s assume you have to run a wire 6 feet from a relay to the lamp and look at the chart on the next page. Using the 10 Amp column you’ll find that you can run 10 Amps on 15 feet of 18 AWG with only ½ Volt drop. Go to the next size larger for safety margin and you’re at 16 AWG (See the note below on lighting). Now in reality, you have to balance the mathematical results with mechanical reliability. Me, I’d go to 14 AWG as the wire and connectors are physically stronger — plus I only buy three sizes — 14, 12 and 10 AWG. Those three and crimp-on connectors are readily available just about anywhere. And except for primary circuits, those three sizes will cover just about anything you want to wire in a car with an adequate safety margin.

 

 

Wire Gauge AWG

Maximum length in feet for car wiring

 

Current load in Amps @ 12 Volts DC

 

1

2

4

6

8

10

12

15

20

50

100

200

20

106

53

26

17

13

       

18

150

75

37

25

18

15

12

     

16

224

112

56

37

28

22

18

14

    

14

362

181

90

60

45

36

30

24

18

   

12

572

286

143

95

71

57

47

38

28

   

10

908

454

227

151

113

90

75

60

45

   

8

1452

726

363

241

181

145

120

96

72

29

  

6

2342

1171

585

390

292

234

194

155

117

46

23

 

4

3702

1851

925

616

462

370

307

246

185

74

37

 

2

6060

3030

1515

1009

757

606

503

403

303

121

60

30

1

7692

3846

1923

1280

961

769

638

511

384

153

76

38

0

9708

4854

2427

1616

1213

970

805

645

485

194

97

48

  1. Calculate the current load and find the next highest on the top row. Go down that column until you find the length you need to run. The wire gauge required is shown in the far left column.            
  2. The maximum lengths are based on a ½ volt drop over the indicated length.
  3. To be safe, always choose one wire size larger than you need for the required current carrying capacity and length.

For example: You’ve calculated 10 amps load, over a length of 15 feet. The chart shows that 18 AWG is suitable. Choose 16 or 14 AWG to allow an adequate margin for safety.

Wire sizes for lighting is more critical than for other applications — The rated output of any lamp is figured at 13.5 volts, not 12 volts as you might expect. So with a ½ volt drop you are at 13.0 volts. And at 95% of the rated voltage, you are only putting out 80% of the rated luminous intensity - for a 100 watt lamp that’s only 80 watts!! Get what you pay for and figure to the high side when you are sizing wire for lighting.

So how much current are you drawing in your car total? Is your Alternator big enough? Let’s say you have four 100 Watt Driving Lights and you’ve upgraded your OEM Lights to 100 Watts each for another 200 Watts. Now you’re at 600 Watts, or 50 Amps, just in forward lighting! Got an electric cooling fan? Another 8-12 Amps, the same as your heater fan. Dual Horn - 12 to 20 Amps. Stop Lights - 2 Amps each. Marker Lights - 0.5 Amps each. Ignition - 8 to 12 Amps. Fuel Pump - 4 to 8 Amps.

Add 'em all up and see what you've got. If your equipment is sucking up more amps than your alternator is putting out, the balance is being made up by your battery. Eventually your battery will no longer be able to supply that additional current and your lights will start to get dimmer and dimmer. Eventually, your car will roll to a stop as the electronic ignition quits for lack of adequate power.