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Subject - Volt Drop Formula
Electricman What formula do you fellas use?

I use 2 X K X L X I / CMIL

K= Conductor resistance in ohms per thousand feet(chapt 9 table 8)X circular mils/ 1000
L= length (one way)
I= load current

substitute 1.73 for 2 on three phase
DiverDan I use the IEEE Std 141 exact formulae:

Vd = V + IRcos(theta) + IXsin(theta) - sqrt(V^2 - (IXcos(theta) - IRsin(theta))^2)
where:
Vd = Voltage drop (Line to Line)
V = Voltage (source)
I = Current in amperes (A)
R = AC Resistance from NEC® Chapter 9 Table 9
X = AC Reactance from NEC® Chapter 9 Table 9
distance (L) is considered from the Resistance & Reactance Tables
where Ohms per unit / 1000 * L in same unit = R or X
theta = Arccos(device or circuit Power Factor) = angle of phase offset

Line to Neutral is computed by Line to Line VD / Sqrt(3) for 3 phase circuits.

You'll also have to get into some power factor averaging when computing the Vd across a panel's bus bar. Total(kW) / Total(kVA) should be close enough for the bus bar PF average.

I forgot to mention that since resistance(impedance) is very sensative to temperature you must multiply the resistance value by the following factor:

R2 = R1[1 + a(T2 - 75)]
where:
R2 = Adjusted Conductor Resistance
R1 = Table Conductor Resistance @ 75ºC
a = Conductor Material Resistivity
a(cu) = 0.00323 and a(AL) = 0.00330
T2 = Ambient Temperature in Celsius (TA)

convert temperature in ºF to ºC with:
TºC = (TºF - 32) / 1.8
Ryan_J I=E/R
rabbitgun I just use the ugly's formula:

V=2K x L x I / CM For single phase

Same formula for 3 phase but multiply the answer by .886

K = 12 for CU loaded to more than 50%

K = 11 for CU loaded to less than 50%

Or, You can always use E = MC2
DiverDan I have a question.
By defination 2KID is only an approximation of Voltage Drop. How are you compensating for temperature? I have seen many instances where a 4 degree temperature difference required a larger conductor. Also, how are you computing the voltage drop from the utility to panel or panel to panel without taking into consideration the bus bar's power factor? Knowing that resistance is cumulative, using 2KID alone without these considerations is just plain wrong.
Ryan_J
quote:
Originally posted by DiverDan

I have a question.
I have seen many instances where a 4 degree temperature difference required a larger conductor.


How does voltage drop ever require a larger conductor? There is nothing in the code that addresses voltage drop, other than for a fire pump.
DiverDan Hi Ryan_J,

Aside from the performace device being greatly hinderd and current draw being increased of any inductive or resistive electrical device in a reduced voltage situation to the point where feeding conductors become very, and sometimes dangerously hot:

NEC® Sections 210.19(a) (FPN No.4) and 215.2(b) (FPN No.2) recommend sizing both feeders and branch circuits to prevent a voltage drop exceeding 3 percent at the farthest outlet, where the maximum total voltage drop of the feeders and branch circuits does not exceed 5 percent. NEC® section 110.3(b) requires equipment to be installed in accordance with the equipment instructions. Therefore, electrical equipment must be installed so that it operates within its voltage rating as specified by the manufacturer. Additionally, NEC® Section 310.15(a) (FPN No.1) states that the ampacities provided in it's Tables do not take voltage drop into consideration.
Ryan_J I agree, but FPN's are not enforcible code, as per 90.5(C). Regarding 110.3(B), I disagree that it requires compliance with the equipment instructions. It requires compliance with the listing instructions, which are often different than the manufacturer's instructions.

I'm not saying that voltage drop isn't a problem...it is. I just don't think the NEC addresses it, other than for fire pumps.
DiverDan
quote:
FPN's are not enforcible code


Myself and all the design and electrical engineers I know have been hit by FPN's that we overlooked....but only once.
electroman 210.19A FPN#4
310.15A1 FPN#1
215.2A3 FPN#2
647.4D
695.7
Thank you Ryan for pointing that out. You learn something new or remember something you forgot everyday. Check out the FPN of Art. 110.12 in the new 05 NEC. Where is that heading?
Ryan_J
quote:
Originally posted by DiverDan

quote:
FPN's are not enforcible code


Myself and all the design and electrical engineers I know have been hit by FPN's that we overlooked....but only once.


That really doesn't change the rule of 90.5(C) though.
Ryan_J
quote:
Originally posted by electroman

Thank you Ryan for pointing that out.

You're welcome.
quote:
Check out the FPN of Art. 110.12 in the new 05 NEC. Where is that heading?
The issue with neat and workmanlike is that it is not defined in the NEC. The intent of the FPN here is to inform the AHJ that if the AHJ chooses he/she can consider looking at the NECA document for more information on what "neat and workmanlike" is. This may then be used to define the term.

It still doesn't make the FPN enforcible, it simply points the AHJ in the direction of obtaining a definition for an undefined term.