Voltage drop on a long power cord

Measure the voltage drop at the 3 locations while the appliance is running. The voltage drop v=3Dir =3D current times resistance, so with no current flowing to the appliance, the resistance causes no voltage drop.

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It applies to the voltage dropped across the resistance of the
conductors in the extension cord(s) when current is passing through
them.

For example, 16AWG wire has a resistance of about 4.02 ohms per
thousand feet, so your 100' 16 gauge extension cord would have a
resistance of about 0.8 ohms.

With 15 amps through the cord, then, the voltage drop would be:

     E = IR = 15A * 0.8R = 12 volts

Now, assuming that you had a resistive load which drew 15A from the
mains, right at the mains, means that its resistance would be:

          E     125V
     R = --- = ------ ~ 8.3 ohms
          I      15A

and, with the load connected to the end of the extension cord, the
circuit would look like this: (View using a fixed-pitch font) 
 

125AC>---[0.4R]---

If there's no current flowing then there's no voltage drop.

With AWG 16 being about 0.004 ohms/ft, you should see a drop of about

0.004 * 100 * 2 * 10 = 8 VAC, assuming a 10 A draw from the trimmer. The AWG 14 cord should be about 0.0025 ohms/ft, so your drop would be reduced to about 5 VAC.

But, don't forget that the receptacle you're using isn't a perfect voltage source; there will be additional drops all the way back, plus small additional resistances at every connection. Swag an additional 5 volts from the receptacle to the service panel.

Could you rephrase that? A listed, double-insulated tool could have two prongs, one probably wider than the other (that's the neutral). Something with exposed metal may add an earth/frame ground for a third wire but that should never carry any current.

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Rich Webb     Norfolk, VA

Also don't forget that power drops with the square of voltage.

And that the load may pull more current in response to the voltage drop, leading to additional losses. Moral: use big wire.

--
Rich Webb     Norfolk, VA

Induction motors will. Universal, not so much. The moral of the story is right on. I wouldn't use less than 14AWG, no matter how short and at least

12AWG above 50'.

Very good answers above. Thanks. It all make sense now that I ponder it.

I missed an important observation. The trimmer kept tripping the GFI about every 10 min or less when I was using the 150' line. What's the cause there? Awhile ago, I ran it with the 100' 14 gauge cord and it popped the GFI once after maybe 20-30 min of off/on usage.

It is a little weirdly expressed. I'll pass though given all the other explanations, and finally looking at the trimmer manual. It has rating for various cord configurations.

Likely capacitive leakage, but it could be a "brown-out" issue too. Old GFCIs had such "issues". Replace it and that problem should go away.

If the trimmer isn't double insulated, replace it. If it is, find & fix the nicks in the insulation of both the 100' and 150' extension cords. A GFCI tripping is telling you something - it is not normal for it to trip, regardless of whether the trimmer runs for 10 minutes, 20 minutes,

30 minutes - or *any* amount of time. The most common cause of GFCI tripping when using long extension cords is open insulation in the cord itself. Plugging one cord into another and laying the connection on the ground, or a nick in the cord insulation is the root cause. Nicks in the extension cord insulation can be the devil to find - but if the cord is obviously worn, just replace it. Expensive, but your safety is worth every penny.

If the trimmer isn't double insulated, it's old an may be ready for replacement anyway. If it's shorting line to metal case, it will trip the GFCI and it needs to be replaced.

As in your previous question about GFCI tripping - if this GFCI is located outdoors or in an unheated outbuilding, replace the thing.

To emphasize a point: in the absence of any defect, a GFCI receptacle will not trip even if you draw its full rated current (~15 amps) continuously forever.

Ed