The right size extension cord (Supplying power to your tailed devils #2)

In March 2009 I wrote a little bit about electricity - this post is really a continuation of that one.  If you've ever finished with a tool and been surprised at how warm the cord was when you unplugged it, this post is for you.
Basically, the longer the extension cord the more resistance / friction there will be and the more likely that there will be a significant voltage drop - sometimes dropping it below the threshold where your electric motor tools will operate well.  Plus (added bonus!) if the extension cord is too anemic / wimpy, it can heat up or even melt.  Can you say fire hazard?  What to do about it?  The answer is to use an extension cord that is big enough for the tool.  Your tools may even last longer and perform better if you do.
This is a table that I derived originally from one posted on this site (well worth your time) but checked using other tables.  Length by Amps, results in wire gauge:

Remember that with wire gauge, like shotgun bore gauge, the smaller the number, the bigger it is.
To use this table for 110/120v circuits you'll need to know the amps used by your tools.  Mostly motors will be clearly (if somewhat microscopically) labeled with the amps used.  If not, you can derive amps from Watts by using the formula: Amps = Watts/Volts.  So, a 1400 watt blow dryer will draw nearly 13amps on the "high" setting.  No wonder the cord gets warm!  In the motor label picture Amperage is indicated "AMPS" but it is often abbr'ed to "A":

But wait!  This motor lists VOLTS as "208-240/115" and AMPS as "6.1-6.0/12.0" - be careful when reading such.  This is just indicating that the motor can be wired to use 220v instead of 110v.  Note that the 115 at the end of the volts corresponds with the 12.0 at the end of the amps.  This (12 amps) is the number you will use if you plug the tool into regular house wiring.  If the motor is only 110v, it will only have one A/Amp number to worry about.  So, using the above table you could using the following extension cords with the pictured motor safely: a 25' 14 gauge and a 50' 12 gauge extension cord... ...nothing longer than that!  And this is only a 1 Horse Power (HP) motor tool!  Here are some common tools and the amps they draw:  Circular Saw 12 to 15 amps.  Corded drill 3 to 7 amps.  Contractor's Saw / Table Saw 15 to 20 amps.  Router 4 to 6 amps.  Remember, your mileage may vary.  When it doubt, check the label.

As you can see, it pays to have a few different sizes and lengths of extension cords.  But how to pick out the right one for the job?  When you are buying an extension cord, the Gauge label will generally read 14/3 Gauge or 12/3, etc.  The first number (14 or 12 in our examples) is the gauge used in the above table.  the /3 means that there are 3 separate wires in the in the jacket of the cord - one for each prong on the plug: Green is  ground, Black is hot, White is neutral.  (Just like house wiring, except the Green ground wire is often just bare copper.  Switches and outlets often label the right terminal/bolt to attach that bare copper wire to by coloring it green.) Older 2 wire cords are missing the Green/Ground prong and are not as safe as 3 wire cords.  On new cords the gauge is often stamped repeatedly down the extension cord, but not marked at all on older ones.  Eventually you will be able to tell by hefting the cord - there is quite a big difference between the various gauges, also a large size difference.

When in doubt, the shortest thickest extension cord is the one to use.


For more information on extension cords, especially about voltage drop, this PDF is great.