Wiring your car for Safety and Reliability

Dec. 01, 2005 By Ken Beard

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

 

1. Use the smallest reasonable wire size 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 - up to a pratical limit - 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.

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.

formula.gif 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. Now in reality, you have to balance the mathematical results with mechanical reliability and efficiency. For lighting, the rated output is figured at 13.5 volts, not 12 volts. With the 0.5 volt drop shown in the chart, you have 13.0 volts available at the lamp - and at that 95% rated voltage, you are only going to get 80% of the rated output - or the equivalent of 80 watts from a 100 watt lamp. In our example, I'd go to 14 AWG as the wire and connectors are physically stronger, easier to work with, and there's no voltage drop - 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.

Is your Alternator big enough for all your electrical equipment? Each 100 watt lamp is going to draw about 9 amps so six of them is going to suck up about 55 Amps. The other accessories on your car - cooling fan, heater fan, ignition, fuel pump, running lights, etc. - are going to draw roughly another 30-40 Amps - your total power requirement will reach about 90-100 Amps.

It's impossible to compensate for a small alternator by throwing in a bigger battery as the battery will just be drained and the voltage will suffer, affecting your light output and overall performance. Your best solution is to go to a modern, high output alternator of at least 100 Amps or more. If you are really worried about weight, you're better off with a smaller battery. All it really has to do is start the engine if the alternator is large enough to carry the rest of the load after the car is running.

 Wire   Maximum                                        
 Gauge   length                                        
        in feet                                        
        for car                                        
         wiring                                        
        Current                                        
        load in                                        
         Amps @                                        
           12                                          
         Volts                                         
           DC                                          
           1        2        4        6      8   10  
20        106       53       26      17     13         
18        150       75       37      25     18   15    
16        224      112       56      37     28   22    
14        362      181       90      60     45   36    
12        572      286      143      95     71   57    
10        908      454      227      151   113   90    
8         1452     726      363      241   181  145    
6         2342     1171     585      390   292  234    
4         3702     1851     925      616   462  370    
2         6060     3030     1515    1009   757  606    
1         7692     3846     1923    1280   961  769    
0         9708     4854     2427    1616   1213 970    
 Wire   Maximum                                        
 Gauge   length                                        
        in feet                                        
        for car                                        
         wiring                                        
        Current                                        
        load in                                        
         Amps @                                        
           12                                          
         Volts                                         
           DC                                          
           12       15       20      50    100   200   
20                                                     
18         12                                          
16         18       14                                 
14         30       24       18                        
12         47       38       28                        
10         75       60       45                        
8         120       96       72      29                
6         194      155      117      46       23       
4         307      246      185      74       37       
2         503      403      303      121      60  30   
1         638      511      384      153      76  38   
0         805      645      485      194      97  48   

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.

The maximum lengths are based on a ½ volt drop over the indicated length.

To be safe, always choose one or two wire sizes larger than you need for the indicated current carrying capacity and length. For example: You've calculated a 10 amp load over a length of 15 feet. The chart shows that 16 AWG is suitable (12A column). Choose 14 AWG to allow an adequate margin for safety.

Ken Beard, Susquehanna MotorSports
[email protected]
http://www.susquehanna.com/susq


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