How to determine extension cord gauge by measuring ohms?

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tmcdanel

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Rimini, Montana, USA
I have some 50' & 100' extension cords that are not labeled well (the gauge is not printed on the insulation) and I want to know their gauge.


How many ohm's is a 12 gauge 50' cord?
How many ohm's is a 14 gauge 50' cord?
How many ohm's is a 12 gauge 100' cord?
How many ohm's is a 12 gauge 100' cord?
 
If you look up 'copper wire resistance' you will get tables of resistance per unit length for various different wire gauges. Pretty easy to calculate.

For example http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html shows that 12ga wire has a resistance of 1.588 ohms per 1000 feet at 20C.

So for a 100 foot 12 ga extension cord, if you short one end (giving you 200 feet total of 12ga wire as measured from the other end), at 20C you would expect a resistance of 0.317 Ohms.

Measuring such low resistance with a common multimeter will be an exercise in frustration. Normal contact resistance to your probe leads will be higher. If you have an adjustable current bench power supply, then you can flow perhaps 2A down the wire and measure the voltage drop. There are also special meters specifically for measuring low resistance.

-Jon
 
tmcdanel said:
I have some 50' & 100' extension cords that are not labeled well (the gauge is not printed on the insulation) and I want to know their gauge.
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If none of the markings are readable, and there should be marks every couple of feet, perhaps it's time to retire these cords and get new ones. Or if these are shop-built (not factory) cords, remove one end off and take a look.

(Agreed that measuring the resistance with a multimeter isn't going to get the info you want.)
 
winnie said:
Measuring such low resistance with a common multimeter will be an exercise in frustration. Normal contact resistance to your probe leads will be higher. If you have an adjustable current bench power supply, then you can flow perhaps 2A down the wire and measure the voltage drop. There are also special meters specifically for measuring low resistance.

-Jon
Click to expand...

I agree. Basically, digital multimeters drive the test load with a precision current source and then measure the voltage developed to calculate the resistance. However, even on the lowest ohms scale the current is usually less than 100 microamps, and typically around 30 microamps. That limits the accuracy when measuring low resistances as mentioned above.

A 4-wire resistance measurement can be used to virtually eliminate contact resistance as an issue. Unfortunately, meters and instruments with 4-wire capability are mostly in the several thousand dollar range. But you can make one yourself.
A 4-wire measurement applies a test current through 2 leads, but measures the voltage developed on 2 other leads so there is no common resistance that is added to the measurement. Also, by applying the current even a very small distance on the conductor outside of the region where the voltage drop is measured, the contact resistance connecting the current source and voltmeter is no longer a significant issue. That's because the impedance of the current source and voltmeter are so high compared to contact resistance.

A precision current source can be acquired such as the one below that applies up to 200 milliamps.

https://www.analog.com/en/design-ce...aluation-boards-kits/dc1531a.html#eb-overview
 
What about shorting one end of the cord and connecting a car battery to the other end and measuring the current? Problem is would need a DC clamp, which is usually only on higher end meters. Might not want to do that on the 50 foot cord without some extra resistance as you would be around 80 amps. Or plug in a couple electric heaters and measure the VD.
 
Pick up the two 50' cords, one in each hand the heavier one is #12.
Both #12 100' cords will be the same, weight, ohms.
 
Sometimes you don't really need to do the math.....

We provide pole mounted temps, w/ many receptacle outlets

So then the 'effin woodpeckers run a 100' cord into the jobsite, and wonder why it melts it's way down thru the snow w/10 of 'em choppin' wood

:lol:
~RJ~
 
electrofelon said:
What about shorting one end of the cord and connecting a car battery to the other end and measuring the current? Problem is would need a DC clamp, which is usually only on higher end meters. Might not want to do that on the 50 foot cord without some extra resistance as you would be around 80 amps. Or plug in a couple electric heaters and measure the VD.
Click to expand...

I like the way you think.
 
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