voltage drop and resistance calculations · 2019-04-29 · voltage drop calculations short circuit...
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
Voltage Drop andResistanceCalculations
8/29/2016 Copyright 2016 Dan Dudley & Associates Page 1
Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
Voltage Drop Diagram
SOURCE Sub-Panel LOAD
3 %
5 %
Total System Branch Circuit
2 %
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
Voltage Drop Formulas
• Voltage Drop:ØVD = 2 X K X D X I
CM
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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 Page 4
Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
Voltage drop formula
• Find ‘K’– K = R X CM
1000K = 1.24 x 10380
1000
K = 12.87 (OHMS PER CM FOOT)
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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 ?
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
VD PERMITTED (in volts)
• VD permitted = source X .05 or .03• .05 = 5% .03 = 3%
• VD = 230 X .05 = 11.5 volts
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
Calculating ‘K’
• K = R X CM1000
* From Table 8
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
UseUNCOATEDunless toldotherwise
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
1 – Phase VD Example
VD = 2 X 12.87 X 500’ X 10A
10380
= 128,700
10380
= 12.39884 or 12.4 volts
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
3 – Phase Example
VD = 1.732 X 12.87 X 500’ X 10A
10380
= 111,454.2
10380
= 10.7373 or 11 volts
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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Copyright 1999-2015 Dan Dudley
Voltage Drop Calculations
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