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?

 

[John Fields]

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
<wolftracks@invalid.com> wrote:

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?

---
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]---<<--+
|
[8.3R]
|
125AC>---[0.4R]---<<--+


The current in the circuit would be:


E 125V
I = --- = -------------------- 9.1 amperes
R 0.4R + 8.3R + 0.4R


so, instead of drawing the 15A the load was rated for, the maximum it
could get would be 9.1 amps.

In terms of power, without the extension cord it would be dissipating:


P = IE = 125V * 15A = 1875 watts ~ 2.5 horsepower,


while, at the end of cord,


P = I˛R = 9.1A˛ * 8.3R ~ 687 watts = 0.9 horsepower.

Quite a difference!
---

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?

---
Dunno, but the three prongs are there in case the device on the end of
the cord needs an earth ground.

Trimmers and suchlike usually don't since they're double-insulated,
present no shock hazard, and an earth ground isn't needed.


--
JF

 

[Rich Webb]

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
<wolftracks@invalid.com> wrote:

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.

--
Rich Webb Norfolk, VA

 

[Globemaker]

On Jun 11, 12:24 pm, "W. eWatson" <wolftra...@invalid.com> wrote:

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?

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

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 11 Jun 2011 15:06:18 -0400, Rich Webb <bbew.ar@mapson.nozirev.ten>
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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

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.

 

[Rich Webb]

On Sat, 11 Jun 2011 14:40:06 -0500, "krw@att.bizzzzzzzzzzzz"
<krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 11 Jun 2011 15:06:18 -0400, Rich Webb <bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 11 Jun 2011 15:53:00 -0400, Rich Webb <bbew.ar@mapson.nozirev.ten>
wrote:

On Sat, 11 Jun 2011 14:40:06 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 11 Jun 2011 15:06:18 -0400, Rich Webb <bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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.

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]

On 6/11/2011 12:06 PM, Rich Webb wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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.

 

[W. eWatson]

On 6/11/2011 1:25 PM, krw@att.bizzzzzzzzzzzz wrote:

On Sat, 11 Jun 2011 15:53:00 -0400, Rich Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 14:40:06 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 11 Jun 2011 15:06:18 -0400, Rich Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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.

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.

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 11 Jun 2011 17:26:53 -0700, "W. eWatson" <wolftracks@invalid.com>
wrote:

On 6/11/2011 1:25 PM, krw@att.bizzzzzzzzzzzz wrote:
On Sat, 11 Jun 2011 15:53:00 -0400, Rich Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 14:40:06 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 11 Jun 2011 15:06:18 -0400, Rich Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.

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.

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.

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]

W. eWatson wrote:

On 6/11/2011 1:25 PM, krw@att.bizzzzzzzzzzzz wrote:

On Sat, 11 Jun 2011 15:53:00 -0400, Rich Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 14:40:06 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 11 Jun 2011 15:06:18 -0400, Rich
Webb<bbew.ar@mapson.nozirev.ten
wrote:

On Sat, 11 Jun 2011 09:24:57 -0700, "W. eWatson"
wolftracks@invalid.com> wrote:

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.


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.


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.

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