What Size Wire When Adding Accessories

[ringadingh]

What size wire? Thanks to Bob, Yellow Jacket

Quite often when adding electrical accessories the question of what size wire needs to be used comes up. The quick answer is, “It depends on the current draw of the accessory.”

It’s very easy to be confused about the correct size wire since you can find conflicting information on the web.

Below is a question asked by a member about adding a horn along with my response:

Quote According to the Standard Engineering Handbook at work a 16ga wire can handle a 22 amp load, a 14ga wire can handle 32 amps, and a 12ga can handle 41 amps. There is a note that says these are very conservative figures. Am I missing something? Since a horn works for only seconds at a time why would it require a 12ga or even 14 for that matter.


These listed capacities are “Maximum Chassis Wiring” ratings. These ratings are for a single wire in open air and not in a bundle or wiring harness and at temperatures below 86 degrees Fahrenheit. Granted these are in fact conservative ratings. For instance, a 12 gauge wire will conduct much more than 41 amps. However, you must realize that as the current increases, so does the heat generated. A 12 gauge wire conducting 41 amps of current is going to be hot. You probably would not want to touch it for very long. You can increase the current until the wire actually begins to glow red. The only limit is when the wire gets so hot it melts. Obviously, this would be an unsafe condition.

Let me talk about the heat generated by current for a moment. All conductors have an inherent amount of resistance. Different materials have different amounts of resistance. I’m only going to address copper. Resistance is measured in ohms. Heat is a direct and linear result of the amount of current and the amount of resistance.

12 gauge copper wire has a resistance of 1.588 ohms per 1000 feet of wire.
14 gauge copper wire has a resistance of 2.525 ohms per 1000 feet of wire.
16 gauge copper wire has a resistance of 4.016 ohms per 1000 feet of wire.

Notice that the 14 gauge wire has a resistance 1.59 times higher than the 12 gauge and the resistance of the 16 gauge wire is 2.528 times that of the 12 gauge wire.

Since the heat generated is directly related to the current and the resistance, it follows that if all three wire sizes are conducting the same amount of current, then the 14 gauge wire will be 1.59 times hotter than the 12 gauge and the 16 gauge will be 2.528 times hotter than the 12 gauge.

Understand that heat is the enemy of electrical wiring. Too much heat and the insulation melts or burns. This can result in short circuits, burned wiring and electrical fires.
The National Electrical Code and the National Fire Protection Association has determined the safe ratings of electrical wiring for Commercial, Residential and Automotive applications to be the following:

12 gauge copper wire: Over current protection not to exceed 20 amps.
14 gauge copper wire: Over current protection not to exceed 15 amps.
16 gauge copper wire: Over current protection not to exceed 10 amps.

In other words, the fuses or circuit breakers should be no larger than that listed above for these sizes of wires.

These are long established ratings concerned primarily with safety.

In response to the statement : “Since a horn works for only seconds at a time why would it require a 12ga or even 14 for that matter?” My reply is that you are removing your safety factor if you use a smaller wire.

Sure, it may work and never cause a problem. But what happens if something goes wrong? What if the switch arced shut and wouldn’t turn off? What if the wire to the horn rubbed through the insulation and caused a short circuit sufficient to draw 20 amps on a 16 gauge wire but not enough to blow the fuse? Do you want to take the risk of having an electrical fire?

Here are a few more tips:

The most important thing to remember when working on electrical systems on a motorcycle is this:
Never replace any fuse with a fuse rated at a higher amperage than the one being replaced.

If you need to add wiring when installing an accessory, always use a good quality copper stranded wire. This can be found at most auto supply stores.

To determine amperage from watts, simply divide the wattage by the voltage. For example, a pair of driving lights with 55 watt bulbs would draw 9.16 amps at 12 volts. (110 divided by 12.)

Maximum current draw on a circuit should not exceed 75%-80% of the capacity of the fuse in that circuit. I.E., a circuit with a 10 amp fuse should normally draw no more than 7 to 8 amps.

In the case of two driving lights rated at 55 watts each, and drawing 9.16 amps, you would want to put that on a circuit rated for 15 amps rather than only 10 amps.

In summation:

For a 10 amp fuse use 16 gauge wire.
For a 15 amp fuse use 14 gauge wire.
For a 20 amp fuse use 12 gauge wire.
For a 30 amp fuse use 10 gauge wire.