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Voltage drop calculation #127800 01/04/02 09:44 AM
Joined: Dec 2000
Posts: 84
cinkerf Offline OP
In today's code question of day two methods for performing voltage drop calculations were given for an A.C. circuit. Is one method better than other? Is one method more popular than the other?

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Re: Voltage drop calculation #127801 01/04/02 09:57 AM
Joined: Oct 2000
Posts: 3,961
Bill Addiss Offline

I've always tried to use formulas that are simple (for me) to remember. In this case Ohm's Law; (I = E/R)

E = I x R where E is the Voltage


Re: Voltage drop calculation #127802 01/04/02 11:18 AM
Joined: Jul 2001
Posts: 311
Frank Cinker Offline
Thanks Bill. By the way, Happy New Year....

Re: Voltage drop calculation #127803 01/04/02 11:21 AM
Joined: Jul 2001
Posts: 599
JBD Offline
For short distances (less than 150 ft), I use simple formulas. I like the Square D Motor Data Calculator voltage drop feature.

For longer distances I like to consider the impedance of the system rather than only the resistance. This means worrying about the raceway, Power Factor, and phase configuration. I use the tables in the Ferraz-Shawmut Book of Electrical Information.

Re: Voltage drop calculation #127804 01/04/02 02:09 PM
Joined: Nov 2000
Posts: 2,148
resqcapt19 Offline
I just found this site with some downloadable elecrical software. It has a series voltage drop calculator for multiple loads on long runs. It is free and I downloaded it this morning. Looks like it will work nice, but I haven't really played with it yet.

Re: Voltage drop calculation #127805 01/04/02 06:16 PM
Joined: Jul 2001
Posts: 311
Frank Cinker Offline
Over the years I've take several certification examinations. Only small calculators were permitted to be used. I've had luck using the following calculation for voltage drop for single phase circuits: VD=2KLI/CM. x.866 if three phase circuit. For these particular exam questions power factor, raceway etc were not a factor. I was thankful for that...

For those of you who took exams, what formula did you use?

[This message has been edited by Frank Cinker (edited 01-06-2002).]

Re: Voltage drop calculation #127806 01/09/02 07:49 AM
Joined: May 2001
Posts: 717
George Corron Offline
Ugly's has always been the standard around here. Don, Uglys was issued to me by the IBEW in my apprenticeship, since we came through about the same time didn't you use it ? If they quit publishing Uglys I've always said I'm leaving the trade. It is also the standard used locally for most every test.

Single phase, 2 wire = 2K X L X I / Cm

3 phase, 3 wire = 1.73K X L X I/ Cm

K = 12.9 for copper, 21.2 for aluminum
L = length, I = amps, and Cm = Circular mil area from Chap 9, Table 8

The only thing you have to check occasionally is the "K" rating, that has moved around since I came in the trade because the cma table was revised.

I've been around long enough that #10 wire was 10,000 cm, it has been revised to 10,380, but the formula has pretty much remained the same. [Linked Image]

Re: Voltage drop calculation #127807 01/10/02 11:33 PM
Joined: Nov 2000
Posts: 2,148
resqcapt19 Offline
They didn't give us "Ulglys", but those are the formulas that we were taught.

Re: Voltage drop calculation #127808 01/11/02 03:41 AM
Joined: Dec 2001
Posts: 270
Elzappr Offline
The K factor has changed through the years because the Ohms per 1000ft figure in Table 8 of Ch 9 depends on the assumed temperature. In the '87 code they stopped assuming 25C and started assuming 75C.
K is just shorthand for: (dc resistance value) X (circular mil area) divided by 1000.
You have to watch out for the assumed temperature if you are going to use the formula with a K factor...not to be confused with K factors of transformers!

[This message has been edited by Elzappr (edited 01-11-2002).]

Re: Voltage drop calculation #127809 01/12/02 12:48 AM
Joined: Nov 2000
Posts: 2,236
sparky66wv Offline
I would not have passed my journeyman's without an UGLY's... Is the 2002 out yet?

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