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EQUIPMENT basics |
Demystifying the common cord
Scott R. Nesbitt
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| The coding on this extension cord shows it can carry a 15-amp load and is made for rugged outdoor use, but is not oil-resistant or made for extremely cold weather. |
Extension cords get no respect. We walk and drive on them. We drag and snag them all over the job site. We overload them. We toss them in a corner when we don’t need them.
Why are we surprised when they bite back?
Extension cords annually kill about 90 people in the United States and cause about 6,900 structure fires, according to the National Fire Protection Association. The Consumer Product Safety Commission has recalled millions of extension cords and power strips for deficient insulation, undersized wires, false labeling and improper plug wiring.
Since you can’t live without them, here are some basics on how to understand them and use them properly.
Anatomy of a cable
The extension cord’s heart and soul is the “cable” that carries the power from one end to the other. From the outside in, the cable includes the outer sheath, antifriction wrap, nonmetallic support cords and the copper wire “conductors” that actually carry the electricity. All these pieces are positioned in a long spiral inside the sheath to give the cord flexibility.
Each conductor is a bundle of small copper wires wrapped in a plastic insulating sheath. This multistrand design is more flexible than a single strand of thicker wire (as found in permanent wiring in homes). The thinner the wires in the bundle, the greater the flexibility.
Look closely at your extension cords. You may see some “curly” ones. The curl results from overheating. The copper wires were overloaded and grew hot. Hot wires grow longer, pushing against the softened plastic insulation and sheathing. The normal long, slow spiral winding of the cable components becomes a tighter, curly spiral.
When the cord cools down, it holds the new curly shape. It’s normal for electric wire to warm up a little when carrying current. And wires wrapped in sheathing can’t dump their heat into the surrounding air, so they run hotter than “open” wires.
Is that curly cord unsafe to use? There’s a risk that some of the thin individual wire strands will have broken if the cord was dragged, pulled or walked on in the overheated condition. That reduces its load capacity. It’s likely the internal insulation will be thinned out in some spots. The cord’s become a candidate for the copper recycling bin.
Cord condition counts
Overheating and related safety problems can be limited by calculating the load you need for the tools you’ll be using, and then choosing the cord or cords needed to supply that power (see tables).
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What reduces delivery capacity? The biggest factor is the thickness of wires in the cord. Next is the condition of the cord; each internal broken strand makes the wire thinner. Next is the condition and number of plugs and sockets in the system; each connection resists some electrical flow.
Finally there’s length. Copper is a good conductor, but not perfect. Each foot of wire “eats” a little power, converting it to heat. More wire, more resistance.
Overheating happens when your cord is too thin, too long or too beaten, and your load is too heavy.
Loading and decoding
Figuring your cord needs starts with looking at the amperage rating plates on your tools and the wattage of your lights. Wattage is voltage times amps. So a 110-volt, 7-amp saw needs 770 watts. A 500-watt halogen lamp needs just under 5 amps at 110 volts.
You don’t need to be perfectly accurate, just in the ballpark when you add up your power needs.
And remember that if you don’t run your saw while your air compressor is running, you don’t need to add the two together. But you’ll need to add the lights to the compressor if you’re working at night.
Your extension cord will tell you what it can deliver. Find the letters and numbers on the cable sheath. The yellow extension cord pictured on page 38 bears the code 12-3 SJTW 60º C.
The 12-3 means 3 conductors of 12-gauge wire. This provides a conservative rating of 15 amps (1,650 watts) at the end of a 100-foot cord. A 14-3 cord has 14-gauge wire, which is thinner than 12-gauge and is rated at 13 amps (1,430 watts). A 16-3 cord has even thinner 16-gauge wire and will give you a reliable and safe 9 or 10 amps. Manufacturers no longer make 18-gauge cords for outdoor use, but if you have an old one, figure it for 5 to 7 amps.
The letter code SJTW breaks down like this: S means “hard service” but J makes it into “junior hard service.” The T means it has a thermoplastic sheath. W means it is water resistant and sunlight resistant, so it’s made for outdoor use. The 60º C is a working temperature rating of 60 degrees Celsius, which converts to 140 degrees Fahrenheit.
Our cord’s SJTW code means it is rated for normal outdoor use and is resistant to moisture and sunlight. If the cord has to be really rugged, look for the S without the J for “junior,” and you’ll have a really heavy, stiff cord that will take a beating.
The letter A is added to indicate a cord that handles extreme heat and cold; look for SJTW-A or SJTW-A3. Cords with oil resistance have an O coding.
Here are some additional common codes and their explanation:
- ST: extra-heavy duty, plastic sheath; not outdoor rated.
- STO: Oil-resistant extra-heavy-duty
- STOW: same as STO, but outdoor-rated
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