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Voltage Drop Calculation #129897
11/09/05 12:32 PM
11/09/05 12:32 PM
M
mhulbert  Offline OP
Member
Joined: Oct 2004
Posts: 132
Anaheim, CA USA
I have an instructor telling me that to do a VD calc, you use the following formula:
VD=(Amps x 1 way Length x R)/1000'

Where R = ohm/kFt from NEC Ch 9, Table 8

It seems to me like you would have to double the length since we are only counting the resistance in one direction...is this correct? I have always used the online calculators, and I'm trying to learn how to do it on paper.

Thanks, Mike

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Re: Voltage Drop Calculation #129898
11/09/05 08:10 PM
11/09/05 08:10 PM
W
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Yup. One way or the other you need to incorporate the round trip distance.

Some equations do this by fudging the resistance values (essentially using the resistance of 2000 feet of conductor, and reporting it as the resistance for 1000 feet 'there and back again')

Others look at the voltage drop on one side only because there is assumed to be no voltage drop in the neutral.

But in all cases there needs to be some consideration of the full circuit.

-Jon

Re: Voltage Drop Calculation #129899
11/09/05 08:49 PM
11/09/05 08:49 PM
B
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
VD=(Amps x 1 way Length x R)/1000'
This would be the correct formula if you are
using a 240 volt 2 wire circuit. There is no return distance required.
If you have a 2 wire 120 volt circuit then you need to count the return distance in the caculation.

Re: Voltage Drop Calculation #129900
11/09/05 10:03 PM
11/09/05 10:03 PM
D
DiverDan  Offline
Member
Joined: Mar 2005
Posts: 38
Sacramento, CA
Also, you need to use a return distance of the square of three (1.73) for a three phase circuit.


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