Extension Cord Capacity for Dummies

Typical extension cord amperage chart

One of the staffers remembers a lesson she learned about extension cords: she and Daddy were building a fence using a framing nailer, and had set the compressor in the back yard. She'd connected a couple of extension cords and dragged them out in the back yard, but the compressor kept stalling. Daddy explained that she had almost 100 feet of light-gauge extension cord, and it just wasn't supplying enough amperage to power the compressor. Too bad eHow.com's Mary Lougee hadn't learned the same lesson before penning "How to Determine If an Extension Cord Can Handle a Load"¹ for Homesteady.com.

In case you (like Lougee) weren't aware, two variables control the capacity of an extension cord: the gauge of the wire and the length of the cord. Lower gauge wires have greater capacity – a 12-ga cord can handle greater amperage than a 16-ga cord. The longer an extension cord, the lower its capacity – a 100-foot cord has more "power loss" than a 12-foot cord. There are tables that will tell you specific capacity in terms of amperage or wattage.

Lougee's solution? This:

"Check the paper safety tag on the extension cord for the maximum amperage that is safe for the cord."

Wow: now that is useful information... except for one thing: we looked at the extension cords in the AN shop, and out of about ten, only one has a "paper safety tag." That suggestion came after Mary's explaining to you (and we use the word 'explaining' loosely) how to calculate the amperage of your tool:

"Determine the amperage of tools that list the power specification in watts and volts by writing down the wattage shown on the tool plate. Divide the wattage by the voltage to obtain the amperage. For example, if the plate indicates that a tool uses 360 watts and is 18 volt, the amperage of the tool is 360 divided by 18, or 20 amps."

Well, yeah, Mary's right that that's how you calculate amperage... except for one thing: you don't need an extension cord for an 18-volt tool; since the only tools that run on 18 volts are cordless, battery-driven tools. Idiot.

No, a tool like the shop's DeWalt DW717 Double-Bevel Miter Saw runs on 110-volt power, meaning that its 15-amp motor draws about 1800 watts (that's probably a startup draw). As for Lougee's suggestions? Well, it's pretty obvious that she's never given the topic much (if any) thought. Those of us who have to run that miter saw will reach for a 12-gauge cord (10-gauge if it's longer than 50 feet).
Oh, yeah, and our Dumbass of the Day probably didn't know that, like chains, the weakest extension cord in a string of cords will define the maximum amperage. Just in case you didn't know...

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how_6682094_determine-cord-can-handle-load.html/

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