Voltage Drop

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guggemos

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It's been a few years since I've done a manual VD calc. Years ago I had a DOS program that had the wire impedance values from the Westinghouse Cat. # 55000 and their VD calc method along with the NEC Table 9 values. I also remember using the 460V value (13.8V @ 3%) for the utilzation voltage for a VD calc on a motor circuit VS a 480V value (14.4V @ 3%) for feeder circuit. I've used WireGuide99 but I need to do a manual check. (Later it will be checked with ETAP.) Anybody have an opinion or tips about Voltage Drop manual calcs?
 
voltage drop calcs

voltage drop calcs

Thanks, Roger, but I was hoping for something a little bit heavier on the impedance values and less on the calc tables. If anybody has a Westinghouse Cat. #55000 it is in the calc section.
 
For AC, the IEEE std 141 formula is :
Vd = V + IRcos@ + IXsin@ - sqrt[V^2-(IXcos@-IRsin@)^2]

where:
Vd = L-N volt drop
V = source voltage
I = current
R = AC Resistance from NEC Table 9 (Ohms to Neutral)
X = AC Reactance from NEC Table 9 (Ohms to Neutral)
@ = phase angle
 
Most field calculators for voltage drop are based on DC resistance values found in NEC Chapter 9, Table 8, including the hand-held Construction Master ElectricCalc.

Few calculators use NEC Table 9, much less consider power factor or conduit reactances. But even Table 9 formulas don't consider impedance variables that NEC 310.15(B)(6)(C) requires engineering supervision; such as skin effects, ferrous-raceway size, derated temperature rise, or underground ambient models.

Those who need to model all these impedance factors in the field, and can't program the formulas themselves, will probably need a PC with a wireless link to an ETAP server.
 
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guggemos said:
Thanks. You all have been helpful.
Click to expand...
From the NEC Art. 215:

1-Phase VD = (2 * Length * Z * Current) / 1000,

where the impedance Z = SQRT(R^2 + X^2).

R = conductor resistance per 1000 ft., & X = conductor reactance (inductive) per 1000 ft. as taken from NEC Chap. 9, Table 9.
The table also includes precalculated values for Z @ 0.85 PF, so you don't have to worry about the square root. Although, be careful only using this value if your circuit's PF strays much from 0.85.

3-Phase = 1-Phase VD * SQRT(3) / 2, (where SQRT(3) / 2 is approx. 0.866).
 
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Your link takes me to a video download sight w/ some racey videos. You might want to check the URL on that.:confused:

Moderators note; Edited to remove link and give Marc time to check it out.
 
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hdpeng said:
Your link takes me to a video download sight w/ some racey videos. You might want to check the URL on that.:confused:

Moderators note; Edited to remove link and give Marc time to check it out.
Click to expand...

Yea, I just got mooned. :confused:
 
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