voltage drop and resistance calculations · copyright 1999-2019 dan dudley ohms law & voltage...

Copyright 1999-2019 Dan Dudley

Voltage Drop andResistance

Calculations


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 8


Voltage drop formula

• Find ‘K’

• K = R X CM

1000

K = 1.24 x 10380

1000

K = 12.87 (OHMS PER CM FOOT)


Plug the values into formula

• VD = 2 X K X D X I

10380

VD = 2 X 12.87 X 435 X 20

10380

VD = 223938 VD = 21.57 Volts

10380



• 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 I

VD Permitted

CM = 2 X 12.9 X 325 X 15

11.5 volts

CM = 125775 = 10936 CM

11.5

#8 cu = 16510 CM


Voltage Drop Formulas• Distance:

• D = CM X VD permitted

2 X K X I

• D = 10380 X 11.5 volts

2 X 12.87 X 20

• D = 119,370

514.80

• D = 231.876 or 232 feet



• Load:

• I = CM X VD permitted

2 X K X D

§ I = 10380 X 11.5

2 X 12.87 X 325

§ I = 119370

8365.5

§ I = 14.269 amps


Calculating ‘K’

• K = R X CM

1000

* 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 Wire Size always use the approximate‘K’ of 12.9 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

30 minutes

voltage drop and resistance calculations · copyright 1999-2019 dan dudley ohms law & voltage...

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