Light Fixture Wire Sizing for Beginners

light bulb with faulty wiring
  • Beginner

Determining the right light fixture wire size is important to ensure the safety of all the home's inhabitants, including the home itself. Currently, the American Gauge Wire system is the standard used in sizing electrical wires.

In general, the higher the amperes, the bigger the wires needed. Utility pole wires carry hundreds of amperes, so naturally, it will require thicker wires, while household appliances and lighting fixtures only need thin wires since they generally carry low current.

Wire Size and Wire Ampicity

Wire ampacity is the recommended amount of current that can run through a wire without it melting, catching fire or getting dangerously hot. You need to match wire size with rated ampacity to ensure safety and efficiency. Wire gauge or size can be found printed on electric cables.

You will normally see number series labels such as 12-3 and 14-2: the first number is the wire gauge.

More Factors That Influence Wire Size

Other factors that affect wire size are the length and wire type. If your wiring circuit length is in excess of 50 feet, you will need a larger gauge wire. There are various types of wires, such as outdoor wires, weatherproof wires, cable wires, to name a few. Light fixture wires generally are standard copper wiring since they are mostly installed indoors.

Light Fixture Wiring Circuits

Light fixture wiring circuits are not as simple as wiring found in household devices, so finding the right wiring size for them can be a bit more tedious. For example, a microwave that only uses one dedicated wire typically has a rated capacity of 20 amps; following the American Gauge Wire system as a guide, you would find that 20 amps require a size 12 wire.

With lighting fixture circuits, you have to take into consideration the voltage capacity of the circuit and the wattage of all the lighting fixtures that run through the circuits to get the recommended wire size. In a 120-volt circuit, you add up the total wattages of the light fixtures on that circuit and divide that by 120.

You then use the resulting quotient to research for the recommended wire size according to a recognized electrical reference chart such as the Audel Mechanical Trades Pocket Manual or Ugly’s Electrical Reference.

As an example, you have eight incandescent bulbs, each with a wattage of 100 watts that run through a 120-volt circuit. First get the sum of the wattage of all the five bulbs, which is 800 watts.

Then divide 800 watts by 120 to get 6.7. Using the Audel Mechanical Trades Pocket Manual as a reference, you will find that the equivalent rated ampacity rating of the resulting quotient is 15, which would require a 14 wire.

As wire sizing involves electricity, it is important that safety precautions are observed to avoid accidents or injuries. If you are unsure about certain procedures, it is highly recommended that you seek the services of a highly trained professional to assist you.

Light Fixture Wire Sizing FAQ

Should I use 12 or 14 gauge wire for light switch?

The size of wire you use depends on the circuit that you are connecting to with that wire. For example, 20 amp circuits must have a 12-gauge wire or larger, but not smaller.

You can always use a bigger wire than what is required, but not smaller. It's also good to choose just one wire size to work with when you're taking on multiple tasks so you don't get them mixed up and create a potential hazard.

What gauge wire for LED house lights?

It is very common to use 14-gauge wire for light fixtures inside the home. Many recommend going up 18-gauge wire for LED lights.

Your best option is to follow manufacturer recommendations for properly and safely connecting light fixtures.

How do I choose wire size?

The size of wire you choose is related directly to the number of amps that will run through the wire. The distance the wire will travel will also impact the gauge size.

What is the most common wire size?

You will mostly find 12- and 14-gauge wires inside of homes for light fixtures and standard outlets. But for powering electrical appliances, 10-gauge and 8-gauge are more common.

What happens if you use too small gauge wire?

If you are using a wire that is too small in gauge for the amount of power running through it, the wire could overheat or melt. It could even catch on fire, which you don't want.