# 220v conversion question

Chris Friesen wrote:

Depending on the total wire resistance the voltage drop to the motor would vary. The power loss formula P = IČ R shows that if you double the voltage you reduce the power loss by a factor of 4. On motors above 2 HP the difference between 120V and 240V is quite noticable.
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Jack Novak
Buffalo, NY - USA
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"Chris Friesen" wrote:

The above is known as "Chris' Therom" which is false.
The total voltage drop across a motor consists of the voltage drop across the motor itself plus the voltage drop across the cable feeding the motor.
The voltage drop across the cable is as follows:
VC = I^2*R
For purposes of explanation, assume:
Case 1: (120V service) R = 1 Ohm I = 10 Amps
VC = 10^2*1 = 100 volts
Case 2: (240V service) R = 1 Ohm I = 5 Amps
VC = 5^2*1 = 25 volts
Increasing the supply voltage from 120V to 240V, all other things being equal, reduces the line losses by a factor of 4:1 (100/25) which results in a higher voltage being delivered to the motor.
All other things being equal , the higher the voltage, the more efficient the connected motor will be.
Lew
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Lew Hodgett wrote:

No. The voltage drop in the cable is VC = I * R
The power lost in the cable is Pc = I^2 * R

No. The voltage drop is 10 * 1 = 10 volts. This means that each motor winding will see 120 - 10 = 110 volts.

No. VC = 5 * 1 = 5 volts. This means that each motor winding will see (240 - 5)/2 = 117.5 volts.
This means that the 240 volt motor will have 6.8% more available power due to the lower cable losses (assuming the same 10 amp current).

The differences are relatively small (6.8%).
A few more things to consider:
To get your 1 ohm resistance, you would need 314 feet of 12 awg wire. That is longer than the typical wiring run in home shops. A shorter run would also make the differences smaller.
The motor starting current may be several times the running current that makes the voltage drop in the wire during start up much worse. This will favor the 240 volt system with if the wire size is the same as the 120 volt system.
The above calculations also have a built in assumption that the 240 volt system was wired with the same gauge as the 120 volt. That may or may not be true. If the 240 volt system was wired with smaller wire then it may have worse voltage drops.
Dan
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"Dan Coby" wrote:

"Duh, Brain Fart Dummy", he says to self.
Thank you for the correction.
Below is the corrected version.
The total voltage drop across a motor consists of the voltage drop across the motor itself plus the voltage drop across the cable feeding the motor.
The voltage drop across the cable is as follows:
VC = I*R
For purposes of explanation, assume:
Case 1: (120V service) R = 1 Ohm I = 10 Amps
VC = 10*1 = 10 volts
Case 2: (240V service) R = 1 Ohm I = 5 Amps
VC = 5*1 = 5 volts
Increasing the supply voltage from 120V to 240V, all other things being equal, reduces the line losses by a factor of 2:1 (10/5) which results in a higher voltage being delivered to the motor.
All other things being equal , the higher the voltage, the more efficient the connected motor will be.
Note:
The resistance value used is totally immaterial since mathematically, it cancels out in the calculations.

Absolutely.
Being able to handle inrush with out blinking is a great advantage.

I long ago standardized on #10AWG for motor conductors.
Simplifies the distribution and any increased costs go away if you buy 15-25 ft, molded cord sets and chop off the receptacle end, then wire to motor for 240V service.
Lew
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On 09/01/2009 08:28 PM, Lew Hodgett wrote:

Absolutely. And I actually did know this before the whole discussion started.
I chose to ignore the line losses because they're supposed to be small to start with on a properly designed circuit, so reducing them further should make very little difference--certainly not one that is noticeable to the user.

The resistance is needed if you want to look at absolute values of line loss rather than ratios.

Surely #10 molded cord sets cost more than #12 or #14?
Chris
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That's where you make your mistake. Line losses are *not* small, particlarly when the machine is starting. The power lost due to the line resistance is the square of the voltage lost. It is *not* insignificant.

Expensive, it's a good idea if you're going to stick to 120V. Note that the difference between #10 and #12 is less than the difference between 120V and 240V. Going from 120V to 240V takes no more copper; #12 to #10 does. Of course, there is nothing stopping your from doing both.

...harder to find, stiffer, harder to work with...
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Think molded cord set.
Definitely flexible.
Definitely available at all the big box stores.
Lew
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On Wed, 02 Sep 2009 19:24:27 GMT, "Lew Hodgett"

#10 is not as flexible as #12.

I've never seen one at the BORG.
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"krw" wrote:

Time for an eye exam maybe?
Lew
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On Thu, 03 Sep 2009 01:13:34 GMT, "Lew Hodgett"

Well, the 25th. 8-)
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stranded, they're about the same. think 10-2 SJ vs. 12-2 SJ.
scott
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krw wrote:

I bought a 12 gage extension cord at HD and that was hard to find. Well, they had like 50 14 gage and just ONE #12. Harder for the next guy to find:-) Also, the extension cord was cheaper than buying the wire, let alone the wire and plugs. I needed a new cable for my table saw. Pretty awesome that a better quality extension cord with molded plugs is cheaper than buying just the wire.
--
Jack
Using FREE News Server: http://www.eternal-september.org /
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That is what I did. HD had about 4 9ft 12gauge ext cords. Now they have 2.
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Just to be clear, I've seen #12 extension cords but no #10.
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On 09/03/2009 10:04 AM, Jack Stein wrote:

Just be aware that it may not be the same cable. Often the spooled stuff is good to 90 degrees C, while the extension cords are only good to 60C.
For most stuff it shouldn't be a problem.
Chris
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Just like it is possible for a 10ga wire to be more flexible than a 14ga wire. Depends on alloys, strand count, and insulation materials.
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Right -
I've used 1.25" diameter of copper in a plastic cover for very high current power supplies. It was so flexible that you could wrap it about your arm. It carried hundreds of amps at -2V so the drop had to be low. The sensing lines were coiled around the length to drop the inductance and respond faster. Time can kill you and have the power supply oscillate in pulsing current.
Oh - the wire - more than 1000 strands! - and the strands were electrical copper - very pure and soft.
Martin
Robatoy wrote:

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On Thu, 3 Sep 2009 13:20:19 -0700 (PDT), Robatoy

A piece of #10 wire will not be more flexible than a #12 piece of the same type. That's the point.
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Plastic is likely different as well and thus the temp spec. Martin
Chris Friesen wrote:

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wrote:

The miracle of Chinese Labour.
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