Voltage Drop and Resistance Calculations · 2019-04-29 · Voltage Drop Calculations Short Circuit...

Copyright 1999-2015 Dan Dudley

Voltage Drop Calculations

Voltage Drop andResistanceCalculations

8/29/2016 Copyright 2016 Dan Dudley & Associates



Voltage Drop Diagram

SOURCE Sub-Panel LOAD

3 %

5 %

Total System Branch Circuit

2 %




Voltage Drop Formulas

• Voltage Drop:ØVD = 2 X K X D X I

CM




VD FORMULA PRACTICE

• Problem:– #10 Str, 435 feet, 20 amp load– What is the Voltage Dropped

– Look at Table 8 Resistance– #10 Str = 1.24 ohms per 1000’

Table 88/29/2016 Copyright 2016 Dan Dudley & Associates



Voltage drop formula

• Find ‘K’– K = R X CM

1000K = 1.24 x 10380

1000

K = 12.87 (OHMS PER CM FOOT)




Plug the values into formula

• VD = 2 X K X D X I10380

VD = 2 X 12.87 X 435 X 2010380

VD = 223938 VD = 21.57 Volts10380




Voltage Drop Formulas

• Wire Size:– CM = 2 X K* X D X I

VD permitted

Note the ‘*’ by the ‘K’.

Use 12.9 for Copper

Use 21.2 for Aluminum




CM example

• First get VD allowed: 5% or 3% ?• Branch Circuit = 3%• Total System or Source = 5%• Example:(from source)

– 230 volt, 325 feet, 15 amp load– What size wire do I need ?




VD PERMITTED (in volts)

• VD permitted = source X .05 or .03• .05 = 5% .03 = 3%

• VD = 230 X .05 = 11.5 volts




CM example continued

• CM = 2 X K* X D X IVD Permitted

CM = 2 X 12.9 X 325 X 1511.5 volts

CM = 125775 Need 10936 CM11.5 #8 cu = 16510 CM




Calculating ‘K’

• K = R X CM1000

* From Table 8




Voltage Drop Rules:

• The ‘2’ is for 1Æ• For 3Æ Change ‘2’ to ‘1.732’• ‘D’ is the distance ‘ONE WAY’• ‘I’ is the total load in Amperes• ‘CM’ is the conductor size (T.8)• ‘K’ is the RESISTANCE of a CM foot

* Note: Ø = Phase




Voltage Drop Notes:

• * when using the formula for ‘CM’ or WireSize always use the approximate ‘K’ of12.9 Cu and 21.2 Al.

• * VD permitted is:– 3% for a BRANCH CIRCUIT– 5% for the TOTAL SYSTEM




UseUNCOATEDunless toldotherwise




1 – Phase VD Example

VD = 2 X 12.87 X 500’ X 10A

10380

= 128,700

10380

= 12.39884 or 12.4 volts




3 – Phase Example

VD = 1.732 X 12.87 X 500’ X 10A

10380

= 111,454.2

10380

= 10.7373 or 11 volts




Short Circuit Calculation• Circuit is 120v, 20A, with Inverse time CB• #12 Cu Circuit conductors, cut both the ungrounded and• grounded conductors 4 feet from circuit breaker.• #12 Cu wire resistance = 1.93 ohms 1000’• #12 Cu = 8’ 1.93 X .008 = .01544 ohms resistance

• Current Flow = I = E/R = 120v / .01544 = 7,772.02 amps

• Cut conductors at 2 feet from circuit breaker:

• #12 Cu = 4’ 1.93 X .004 = .00772 ohms resistance

• Current Flow = E/R = 120v / .00772 = 15,544.04 amps




Ohms Law & Voltage DropPractice Questions

12 Questions

30 minutes


Voltage Drop and Resistance Calculations · 2019-04-29 · Voltage Drop Calculations Short Circuit...

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