by Lila Langford

Decreasing the wire size of a circuit will result in increased resistance in the circuit. This may or may not be a problem. If the wire size had been larger than required by the National Electrical Code (NEC) standards, going to a wire size smaller that still meets those standards will result in the system continuing to operate safely and as expected. However, if the new wire size is smaller than the NEC requirements, then component life and system performance may be lower than expected.

The NEC standards are established to assure that electrical products operate efficiently and safely. They were developed so that we have guidelines when selecting our conductors to prevent a restriction in current flow that would result in component failure or a safety hazard. If the restriction or resistance is too high, not only can we have a reduction of power to the device resulting in failure, but also high levels of heat that could result in a breakdown of the insulation on the conductor, which could result in a direct short in the system.

The wires that we use as conductors are rated by their ability to carry current through their copper or aluminum component. We want to make certain that the conductor will be able to provide the current flow needed. The conductor is then covered with an insulator. The insulator is rated by its ability to withstand temperature. The insulator may also be rated according to its ability to withstand oil or other chemicals.

The resistance of the wire is directly impacted by the cross section and length of the wire (see Figure 1 below): 1) resistance increases with an increase in length, and 2) resistance increases with a decrease in cross section. The higher the gauge (ga) number of the wire, the smaller the cross section. For example, 18ga wire is smaller in cross section than 16ga wire.

Figure 1

Let’s say that we have two wires that are both 18ga, but one wire is 2 feet long and the other is 4 feet long. The wire that is 4 feet long would measure higher resistance than the wire that is 2 feet long.

If we have two wires that are both 4 feet long, but one wire is 18ga and the other wire is 16ga, the 18ga wire would measure more resistance because it would have a smaller cross section.

Let’s look at this a little closer. The NEC indicates that for a particular motor size (based upon amps), we must use 12ga wire. If instead we use a wire size of 14ga in this instance, the resistance will be higher, resulting in higher temperatures through the wire and at the power source. The resulting temperatures could be high enough to cause a fire.

What if we chose to use 10ga wire in this example? That would not be a problem in performance or safety. In doing so, the cost of the wire will be higher in that it would be more conductor material. We must also make certain that the larger wire size will fit the connections/terminals. Using 10ga wire when the standards require 12ga will result in less resistance and less heat through the wire and connections.

I remember an incident where a homeowner chose to connect a swimming pool pump motor using two 25-foot light duty extension cords plugged into an outdoor outlet. Not only were the extension cords not suitable for the amp rating of the motor, but the 50-foot distance also resulted in additional resistance. The resulting high resistance was enough to start a fire at the outlet.

Wire size is an important characteristic when making our circuits and connections. Be certain to always adhere to local, state, and federal codes. They are established to achieve proper pe

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