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Q: electrical extension cord usage ( Answered,   1 Comment )
Subject: electrical extension cord usage
Category: Miscellaneous
Asked by: cwanda-ga
List Price: $10.00
Posted: 14 Apr 2006 12:19 PDT
Expires: 14 May 2006 12:19 PDT
Question ID: 718931
Why not plug one extension cord into another?  I bought a 40' cord to
plug into a 50' cord to use for an electric mower.  Both cords have
the same rating (3-wire, 16 guage, 13 amps).  Mower draws 9 AMP's, 120
VAC.  New cord says Do Not Plug One Extension Cord Into Another.  Is
this a safety issue?  Fire hazard?  Just aimed at people who might use
a lower-rated cord with a higher-rated cord and expect higher-rated
performance?  I am looking for something from electricians or
electrical engineers, underwriters labs, or the like.
Subject: Re: electrical extension cord usage
Answered By: redhoss-ga on 14 Apr 2006 14:28 PDT
Hello cwanda, I am not an electrical engineer. However, I did start in
EE before changing to mechanical. I do feel like I am capable of
answering your question. The fact that your 16 Ga. cords have 13 amp
ratings is based on the resistance of 16 Ga. wire. The whole thing is
related to "Ohm's Law" which states:

E = I x R or Voltage = Current x Resistance

In your case the resistance of 16 Ga. wire is 4.016 ohms per 1,000 feet:

So, the longer the wire (extension cord) the more voltage drop you
get. This does two bad things. The loss in voltage generates heat and
causes a reduction in performance of what ever is plugged into the
extension cords. Too much voltage drop will damage the equipment you
are using. The amperage rating of each of the extension cords you have
is based on the particular length of the individual cord. When you
plug one cord into the other you have doubled the voltage drop and cut
the effective amperage capacity in half.

Here is a good article on extension cords:

You will notice that for a 100 foot 16 Ga. cord (which is about what
you have when you plug the 50 and 40 foot cords together) they rate
the cord at a maximum of 5 amps. According to this chart you should
have 12 Ga. wire for your cord. Now, from personal experience, you may
be able to use your two existing cords without any problem. However,
it will very likely reduce the life of what ever tool you have plugged
in because it will be operating at a reduced voltage. This will cause
more heat to build up in the tool and could cause a premature failure.
Of course everything fails eventually and whether you would notice the
difference or not is debatable. If you operate the tool for extended
periods, you can get enough heat in the extension cords to actually
melt the insulation. I suppose after enough time a fire could result.

As to the safety issue. The junction of the two cords can be a source
for problems. If the connection is partially pulled apart, it can
either cause a very high resistance, start a fire, or cause a shock. I
was using an extension cord with a screw gun on a hot humid day and
sweating profusely. My sweat ran into the connection and I got a
pretty good shock. So, the fewer connections you have the better.

If you should use a higher rated cord with a lesser rated cord, you
would want to use the larger gauge cord closest to the receptacle. To
be safe you should probably invest in a 100 foot 12 Ga. cord.

I think I have covered everything, but in the event that I haven't,
please ask for a clarification.

Enjoy your mowing, Redhoss
Subject: Re: electrical extension cord usage
From: mikedesigns-ga on 02 Jun 2006 14:12 PDT
You may want to have a look at voltage drop and other calculation
software and freeware at

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