Extension Cord Load Capacity

Old 02-20-19, 07:09 PM
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Extension Cord Load Capacity

I have some working knowledge of permanent electrical wiring and capacities, but am having trouble understanding something with temporary extension cords though . I see various warnings to not connect extension cords to each other, but what I'm trying to figure out is why. My thinking is that it is more of a generic warning just to ensure that people are not exceeding the capacity of the wire by mixing different gauge extension cords.

If you connect, a 12 gauge extension cord to a 12 gauge extension cord, as an example, is there any reason why the full load capacity of that wire would then be diminished? Is there something in the plugging together of the wires that reduces the overall load capacity? If I were to plug in a splitter/wire tap or a surge protector at the end of a single extension cord and rated at the same max load of that extension cord I wouldn't think that would reduce the capacity of the extension cord (correct me if i'm wrong), but maybe it's the connection plus the added distance that impacts? If so, is there some sort of rule of thumb/calculation to figure out the new capacity of two extension cords connected together?
Old 02-20-19, 07:20 PM
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OSHA has a page that gives a nice answer. (It's on page 2)

The advice to never plug two cords together is general advice due to the fact that this rule makes extension cord safety stupid proof. All sorts of things could go wrong by plugging 2 cords together because not everyone has common sense or knows what "could go wrong". You don't want to exceed 100 ft with an extension cord.

The longer the cord, the less capacity it can safely deliver. The smaller the cord, the less capacity it can deliver. Using a long cord or multiple cords, then demanding 15+ amps from the cord can result in a fried cord, damage to the tool, or both. The longer the cord gets, the more electrical resistance it has. Therefore the gauge of the wire needs to be larger the longer a cord is, if you intend to use a tool that draws a lot of amps.

I still recall the day my dad tried starting the tractor while it was on the battery charger while using a 100 ft 16 gauge cord that was on a circuit on the pole with no breaker. The fire speeding through the dry grass looked like primer cord headed for the dynamite. As a kid I thought it was the coolest thing I'd ever seen. LOL

If you would like to understand this better by looking at a chart, Google: Extension Cord Rating table.
Old 02-21-19, 05:37 AM
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Theoretically an extension cord has the same capabilities as an equivalent length of wire (same gauge, same material, etc.).

But things like quality of construction enter into the picture. The plugs and receptacles at the ends might not provide good solid connections resulting in higher resistance at those points which in turn lead to overheating. Just the length of an extension cord carrying its rated power (amperes, watts) will not result in overheating of a stretched out cord because the heat given off by the cord is distributed over its entire length with the amount of heat per foot being the same for a short cord as for a long cord under the same conditions. The voltage drop could, however, result in overheating or poor performance or a tool or other load.

The warning about use of extension cards are mainly for stupid proofing or idiot proofing. There are many permutations of conditions that are safe and conditions that are not safe and many conditions that need math to size up. It is therefore easier to publish a simple 25 word or less warning that steers people away from practically all of the unsafe situations at the expense of scaring people away from a lot of safe situations..

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