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How To Size Outdoor Extension Cord
Old 12-10-2011, 11:18 AM   #1
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How To Size Outdoor Extension Cord

I have a tool that is rated at 115volt (normal power voltage) 5A.
I will be using it outside with a 100 ft extension cord. The manufacturer
recommends 14 gauge minimum. I have a 16 gauge 100 ft extension cord now rated at 10A. If I really need 14 gauge, I will need to buy one. Any references so I can see if I really need 14 gauge as the mfg suggests? The tool has 2 prongs and a ground.

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Old 12-10-2011, 12:19 PM   #2
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You should use 14 gauge for that application. 100 feet is long enough that you will have significant voltage drop, which will impact your tool's performance.

Here are a couple of calculators you can use to estimate voltage drop:
Voltage Drop Calculator - for single and 3 phase ac systems and dc systems
Voltage Drop Calculator JavaScript

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Some Additional Extension Cord Info
Old 12-10-2011, 12:21 PM   #3
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Some Additional Extension Cord Info

Here is what UL's web site has to say about extension cords:

"Before deciding which extension cord to use, first carefully read the manufacturer's instructions for the power tools you will be using. These booklets contain important information about your tools and will provide instructions on their use. The booklets will also indicate whether the tools are suitable for use outdoors. Likewise, the first step in determining which extension cord you will need is to decide whether you will be using the appliance indoors or outdoors. Extension cords that can be used outdoors will be clearly marked "Suitable for Use with Outdoor Appliances." Never use an indoor extension cord outdoors; it could result in an electric shock or fire hazard.

To determine what size, or gauge, cord is needed, you will also have to determine the cord's length. A cord, based on its gauge, can power an appliance of a certain wattage only at specific distances. As the cord gets longer, the current carrying capacity of the cord gets lower. For example, a 16 gauge extension cord less than 50 feet in length can power a 1625 watt (W) appliance. A 16 gauge cord that is longer than 50 feet in length can only power an appliance up to 1250W."

They also offer this advice:

"Quick tip: if your appliance indicates that it uses 5 amps at 125 volts, then its wattage rating is 625W (5x125)."

Hope this helps.
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Old 12-10-2011, 03:36 PM   #4
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What kind of tool, how long and hard will you be using it?

For light, intermittent use, I don't think 16 GA would be a problem. But if you're doing some tough work for many minutes w/o rest, the tool might not like the lower voltage it will get - that can cause it to bog down and draw more current and over-heat.

Based on the info in jdmorton's post, the mfg seems to be being very conservative (why not, they are not buying the cord).

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Old 12-10-2011, 04:00 PM   #5
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I would plug the tool directly into an outlet. Note the sound and speed under working conditions. Then plug your existing extension cord into the outlet and your tool into the extension cord with the cord coiled. Note any differences in sound and speed. Then feel the extension cord to see if it warms up. If the tool does not slow down and the extension cord does not feel warm to the touch, you are good to go.

Just for fun you can place a clamp-on ammeter around one leg of the tool's cord to measure the true ampere load. You will have to rig up an adapter to separate the conductors.
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Old 12-10-2011, 04:43 PM   #6
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Here is the hilbilly method.

Plug in extension cord. Attach tool.
Run tool.
I cable feels too warm to the touch it is too thin.
If it flames out it WAS too thin.
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Old 12-10-2011, 04:46 PM   #7
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Thanks all for the helpful info. I needed to get the extension cord before
all your wisdom was in so settled on 50ft 14G instead. The 50ft help cut
the cost significantly and should help if my China-made cord is really 16G inside. It's length is adequate, tho barely, and helps with the tangled mess that the 100ft 14G seems to do all by itself. I'll need to feel it's temp next time I use it.

I found a calculator this morning that I thought was pretty gave
14G as the answer. Much later, I went back to it and just for giggles started doing what ifs.........ended up at a 1A load and 10 ft and still got the same 14G answer so I guess it was being conservative. It would change if I increased the load a lot but it didn't seem to want to go lower.
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Old 12-10-2011, 05:05 PM   #8
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As a young child, I remember my grandfather shouting "Smoke" after my father plugged in the long cord at the power source. It was some Saturday afternoon project in the garage behind our house. It had to be over 50 years ago.
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Old 12-11-2011, 06:42 AM   #9
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Originally Posted by kaneohe View Post
... and helps with the tangled mess that the 100ft 14G seems to do all by itself.
I have about four cord reels similar to this bought at Home Depot 15+ years ago. Worth every nickel. BAYCO KW130 HD 150' CORD STORAGE REEL: Lamps & Light Fixtures
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Old 12-11-2011, 07:06 AM   #10
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Interesting idea, Walt. Thanks. Wonder if it works for already messed up cords even if you try to untangle them or if you have to start using them on
a brand new untwisted cord..............
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Old 12-11-2011, 09:32 PM   #11
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I've got a couple of those cord reels myself. Makes it much easier storing and cleaning up cords. I've never had a problem reeling in a tangled cord but it takes a little patience if its really tangled.

You shouldn't use the cord on the reel if you're pulling a lot of current though. You have a potential for heat to build up in the coiled cord if its running near capacity because you've drastically reduced the surface area to cool it.


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