Voltage drop on a long power cord

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W. eWatson 15 years ago

I have had a 100' 16 gauge power cord that I've used around our property for various simple needs. Recently, I bought a Stihl electric weed trimmer, and noticed that it ran less energetically on the 100' cord than a 50' cord. Makes sense, and it may eventually contribute to the demise of the trimmer, so I bought a 100' 14 gauge cord. It seems a better fit.

However, I found a couple of places where I needed a 50' extension. I have several 25 and 50' cords that are all 16 gauge. Attaching a 50' extension seemed OK.

I started wondering about the voltage drop, so measured it at 150' and

100' and right out of the outlet. All agreed at 125v AC. So the idea of a "drop" must apply to current or power (V*I). That is, the load.

Comments?

BTW the 100' 14 gauge container showed a variety of devices that could be used with it. It differs for 2 prong and 3 prong. I have a 3 prong, but the trimmer is 2. The way the info was provided leaves me guessing about what devices are for 2 or 3 prong. Is there some such table on the web that makes it clear?

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Globemaker 15 years ago

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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John Fields 15 years ago

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]---

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Rich Webb 15 years ago

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.

Rich Webb Norfolk, VA

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krw 15 years ago

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

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Rich Webb 15 years ago

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

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krw 15 years ago

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'.

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W. eWatson 15 years ago

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.

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W. eWatson 15 years ago

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.

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krw 15 years ago

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.

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ehsjr 15 years ago

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

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Voltage drop on a long power cord

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