Voltage drop on a long power cord

[W. eWatson]

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?

 

[Rich Webb]

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?

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.

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?

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.

 

[[email protected]]

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.

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

 

[Rich Webb]

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.

 

[[email protected]]

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

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'.

 

[W. eWatson]

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.

 

[W. eWatson]

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.

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.

 

[[email protected]]

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.

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.

 

[ehsjr]

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.

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

 

[shrtrnd]

What's happening, is that you're trimmer at 150', is trying to draw extra current, through the extension cords. Feel the cord, and you'll be able to feel the heat in the
wires. You could reduce the problem by using larger AWG wire (12 or 10 gauge), but
that'd run you some money. Besides possible damage to the trimmer from drawing
too much current at 150', you're overheating the extension cords themselves.
Keep it up, and something besides your GFCI is going to go.