- 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.
Current in Amps = Power in Watts/Voltage in Volts
Current in Amps = Voltage in Volts/Resistance in Ohms
Current in Amps = The Square Root of Power in
Watts/Resistance in Ohms
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. Actually, it's somewhat less than
that because the rated output of a lamp is figured at 13.5 volts, not 12 volts - 13.5
volts is typical when your engine in running and the Alternator is working correctly.
We'll use the 12 volt figure anyhow - Let’s assume you have to run a wire 6 feet from
a relay to the lamp - check out the chart below. Using the
10 Amp column you’ll find that you can run 10 Amps on 15 feet of 18 AWG with only 1/2
Volt drop. Go to the next size larger for a safety margin and you’re at 16 AWG (See
the note below on output losses with 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.
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. When you are installing auxiliary equipment in your car or truck, it's always
a good idea to keep track of how much current you're drawing total so you can make sure
your alternator is up to the job!
Note that wire sizes for lighting is more critical than for
other applications — The rated output of a lamp is figured at 13.5 volts, not 12. So
with a 1/2 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.
