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# Figuring Out Voltage Drop

#1
07-27-04, 06:24 AM
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Join Date: Jun 2004
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Figuring Out Voltage Drop

Is there an voltage drop calculator available on the web?

#2
07-27-04, 06:26 AM
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Many, many, many. Use your favorite search engine (if it's up today) and search for "voltage drop calculator". Or try this one.

#3
07-27-04, 06:27 AM
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Join Date: Jun 2004
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Thanks. That works perfect. What is an acceptable percentage drop?

#4
07-27-04, 06:37 AM
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Depends on the load. Pure resistive loads such as light bulbs can tolerate quite a bit of voltage drop. Motor loads can tolerate very little. Some people use 2%, some 3%, some 5%. The higher numbers can be used for main panel to load calculations. The lower numbers for main panel to subpanel, or subpanel to load.

#5
07-27-04, 06:44 AM
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Join Date: Jun 2004
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So am I right to assume that a 100 foot run from a 200 amp main to a 100 amp sub would use #1 Copper? or should it be 1/0

Thanks

Leslie

#6
07-27-04, 08:13 AM
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I'd use #2 (1.6% voltage drop). Many would use #3 (2% voltage drop). 100 feet isn't very far for a 240-volt feeder. Did you put 120 volts or 240 volts into the calculator?

#7
07-27-04, 08:53 AM
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Join Date: Jun 2004
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Thanks for the quick help, I did put 120 but it would be 240. It will be cheaper to run #2 so that works perfect.

Thanks
Leslie

#8
07-27-04, 08:56 AM
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Join Date: Jul 2004
Posts: 37
Originally Posted by John Nelson
Did you put 120 volts or 240 volts into the calculator?
Can I ask a related question here. A 240 volt circuit is two, 120 volt circuits in opposite phase, right? So in terms of calculating voltage drop (or current carrying capacity), isn't it the same if you entered 120 or 240 into the calculator? You're still calculating voltage drop on a 120 volt conductor.

Another question: dual pole circuit breakers. Is a 30 amp dual (240 volt) breaker actually two, electrically isolated 30 amp breakers mechanically ganged? That is to say, will it trip on an overload (> 30 amp) on either of the two hots that you are feeding?

Thanks, John, for your knowledge and patience.

#9
07-27-04, 09:03 AM
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The absolute voltage drop is the same, but the percentage voltage drop is different. Absolute voltage drop is dependend only on current. It is independent of voltage.

An example might help. Suppose you have 15 amps running on #14 wire over 50 feet. That gives you a drop of 4.7 volts. If that's a 120-volt circuit, that's a 3.9% voltage drop, but if that's a 240-volt circuit, it's only a 2% drop. Now suppose you run two 15-amp circuits, each pulling the full 15 amps, on a 240-volt feeder (or multiwire circuit) over that same 50 feet. The total voltage drop is still 4.7 volts, but it is split across the two 120-volt circuits, so each circuit only sees a 2% voltage drop. That is because there is no current flowing on the neutral, and thus no voltage drop on the neutral.

All of this illustrates the longer the distance, the higher the voltage is required to make it feasible over reasonably-sized wire.

Another question: dual pole circuit breakers. Is a 30 amp dual (240 volt) breaker actually two, electrically isolated 30 amp breakers mechanically ganged? That is to say, will it trip on an overload (> 30 amp) on either of the two hots that you are feeding?
Correct.

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