Getting your electrical wire size right is important

Monday, February 20, 2012

For example, for larger electrical loads at longer distances from an outlet, a #12 industrial extension cord is a good investment, especially when powering a large electrical motor

by RALPH WINFIELD

Do any of you still reach for an extension cord that is long enough to go from a remote receptacle to the load that you want to power? That is acceptable when the load is a trouble light with a wattage of 60 watts or less.

You all remember basic electrical formulae from your high school physics classes, I assume? (!) If so, you will remember that power in Watts (W) equals Voltage (V) times Amperage (A).

If we plug the 60-watt trouble lamp into a standard 120-volt receptacle, the light should come on and the amperage draw will be:

W = VA or
A = W/V = 60/120 = 0.5 amperes or amps

With such a low current flow, a very small electrical conductor, such as the #16 or #18 American Wire Group (AWG), will keep the light on without any difficulty or power loss along the cord.

Here's an analogy. Whenever I taught electrical basics, I always used the water pipe or line to highlight the "too small" pipe or wire.

If you need to supply water, do you use the smallest hose you can buy? Not likely. Even if you are washing the car or the tractor using a 50-foot long, one-half inch diameter hose, you will find that the water has lost most of the pressure along that 50-foot hose. In other words, the water just trickles out the end even if you have a 60 pounds per square inch (psi) supply at the water hydrant in the shop.

The same "friction losses" that cause the pressure drop in the water hose can cause a voltage drop in electrical lines. As a result, if you switch the water hose to a three-quarter-inch diameter, you will reduce the pressure drop and increase the flow rate significantly.

Similarly if you increase the electrical wire size, you will reduce the voltage drop and thereby maintain the desired wattage output to the item in use.

In a recent article, I talked about a 1,000-watt engine coolant heater. If it is plugged into a 120-volt outlet by means of an extension cord, the amperage draw to provide full power should be:

W = V x A
A = W/V = 1000/120 = 8.33 amperes

If the cord is short (not more than 10 feet), a #16 AWG extension cord would be acceptable. If the cord is longer, it should be at least a #14.

For larger electrical loads at longer distances from an outlet, a #12 industrial extension cord is a good investment. This is especially important if you are powering a larger electrical motor.

If a smaller cord is used for a motor under load, the voltage available to the motor will be reduced by heat generated along the cord. As voltage drops, the motor will respond by increasing the current draw (demand) in an effort to maintain power output. I can guarantee the result will be a tripped breaker or burned-off fuse.

The reason I offer that guarantee is based on the fact that many of you already own a metal cut-off saw for use in the shop if you do any welding or fabricating. It is rated at 15 amperes. If you operate it, even without an extension cord, I'll bet that you have tripped a 15-amp breaker.

I am sure that you have already installed a 20-amp breaker. I have one in the implement shed and one in the newer shop. And, just as an aside, when I had the electrician put the 20-amp breaker in the shop, the electrical inspector requested/demanded that the receptacle also be 20-amp rated. I have a bright orange receptacle to go along with the bright red breaker.

To date I have never used the cut-off saw with an extension cord. But if and when I ever do, I will be using the one #12 AWG cord that I keep for higher load usage.

Many of you already know that most electric motors under two horsepower are designed to run on 120 or 240 volts. Is there any doubt about which voltage they should be connected to, especially if the motor is at a significant distance from the electrical service? Running on 240 volts, the amperage and thus the line losses are reduced by 50 per cent.

Look for the wire size on electrical extension cords. It will be printed on or impressed into the cord at regular intervals. Do not select based on the outside diameter of the cord.

Cord diameter will vary depending on the type of covering required for various environmental conditions. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.