Extension Cords

We all probably have extension cords around that we use from time to time.  You may not give any consideration the type of cord you use but you should.  Extension cords come in different sizes and lengths and are meant for different applications.  Sure, for plugging in light that has a current draw of only 2 amps you don’t really have to pay attention to the cord you use but on more powerful items you should.


So, here are a few extension cords I’ll talk about in the post.


The most basic and obvious feature of an extension cord is the number of prongs it has.  For regular 120V extension cords you have the choice of wither two or three prongs.  Two pronged cords are meant for very light duty applications as we’ll see below.  The three pronged varieties include a ground prong.


The jacket of  extension cords have information molded into them that tell you about the conductors in the cord, the jacket, and the extension cord itself.  Here’s an example of the writing.


The “UL” on the cord tells you that this cord has been approved by the Underwriters Laboratory.  The UL certifies and tests products to determine if they meet required standards and are safe.

The next block of letters tells you about the cord.  The meaning of these letters is outlined by the National Electric Code (NEC) in the US. Some of these letters are:

  • S – Hard Service Flexible Cord
  • SJ – Junior Hard Service Flexible Cord
  • T – Thermoplastic Insulation
  • O – Oil Resistant Jacket
  • W – Moisture Resistant (aka for outdoors)
  • P – Parallel wire construction

The definition of other letters can be found at buyextensioncord.com and HomeDepot.com.

The blue cord above has the letters ‘SJTW’ meaning this is a Junior Hard Service Flexible cord with Thermoplastic Insulation that is moisture resistant.

The next block of text starts with an ‘E’ and I’m unsure of its meaning.

Next up, is the temperature rating of the jacket which is 60 degrees Centigrade.  This is followed by ‘300V’ which is the voltage rating of the jacket.  Last, is the information detailing the type of conductors (wires) in the extension cord.  For this cord ’12AWGX3C’  tells us that the conductors are 12 gauge as defined by the American Wire Gauge and that there are three wires.  For AWG, the diameter of the wire gets smaller as the numbers increase.  So, a 10 gauge wire is larger than an 18 gauge wire.


Here’s the info off of the short yellow cord.  The writing tells us that this is a heavier duty cord with a higher insulation voltage rating.  The VW-1 code means that is is Flame Retardant.  It still has three 12 gauge conductors though.

The gauge of the wire and the length of the cord both affect where and how it should be used.  The larger the diameter of the wire the more current it can carry.  The larger the diameter of the wire, the less voltage drop there is along its length.  The longer the cord, the more voltage drop their is along its length.  Voltage drop also increases with an increase in current.  As you might have also guessed, the larger and longer the cord, the more it cost.  If you open the AWG link from above you’ll find a table that lists the maximum amount of current that a conductor can carry as determined by the NEC.

To pick your extension cord, you need to know the current required and the length needed.  To determine the current required look on the tag of the item you’re needing to power.  You should see the maximum amount of current draw listed.  Unless your custom making an extension cord, you’ll have to buy one in a standard length.  You can then check out a site with a voltage drop calculator or reference the table on the last page of a pdf called “Choosing and Using Extension Cords.”  The author of the pdf suggests not allowing any more than a 5% voltage drop.  (The entire pdf is informative.)

Lets work through an example.  Take a look at the data tag on my Delta dust extractor.  It says that at 120V the maximum current draw will be 10.8 amps.


From the picture at the very top I have a 12AWG x 25 foot, 12AWG x 50 foot, 16AWG x 50 foot, and 16AWG x 100 foot extension cord I can choose from.  What are the effects from each cord?  Lets run this through the voltage drop calculator.  The results are:

  • 12AWG x 25′ – 1V drop (0.8%)
  • 12AWG x 50′ – 2.1V drop (1.8%)
  • 16AWG x 50′ – 5.3V drop (4.4%)
  • 16AWG x 100′ – 10.5V drop (8.8%)

So, it seems that either of the first three extension cords would be OK to use with my dust collector.   From experience though, the dust collector takes longer to get up to speed using the 16AWG x 100′ than it does with the 12AWG x 25′ cord.  If you choose to use the chart in the linked pdf, the results are very similar and lead to the same conclusions.

In conclusion, use the biggest reasonable conductor with the shortest length cord.  I say reasonable because a larger conductor costs more money, will make for a bigger cord, and may make for a stiffer cord as well but it will have less voltage drop.  A shorter cord will loose less voltage, be easier to deal with, also cost less.
An addendum…always store your cords in nicely wound coils.  Throwing them in the corner will just magically tie the cord in knots and will only frustrate you when you go to use it next time.   To keep them coiled you can use Velcro pieces, commercially available plastic or metal hangers, or a simple piece of looped rope.  Taking care of your extension cords will greatly extend the life of the cords and make sure they’re always ready for use.



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