Conductor Size & Voltage Drop Calculators for Single and Triple Phase AC


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Three Phase Voltage Drop (V_{D})
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About the Voltage Drop (V_{D}) Formula
In the following formula, we will use the letter "K" to represent Specific Resistance, which has a value of 10.8 when using copper conductors. "I" represents Amperage. "L" represents the Length of the run."CMA" is the Circular Mil Area, or the cross section of the conductor measured in mils (.001 inch). We multiply K by the square root of 3 in three phase. For single phase, we would multiply K by 2.
You can click on the different parts of the formula below for an explanation of each variable.
Please note that this formula is for threephase (3ø) rigs only. Click here for the singlephase formula.
In a power distribution system, the Voltage Drop (VD) is the portion of the voltage lost during a particular run. It is important to know the VD of a run in order to ensure that we stay inside the Allowable Voltage Drop (AVD), as defined by the National Electric Code (NEC).
The AVD is 3% for a Main Circuit (aka: Feeder Circuit) and 2% for a Branch Circuit for a total of 5%. If your VD is greater than the AVD for the circuit type in question, you are not in compliance with the NEC standard. You will need to lower your VD, until you are compliant.
The NEC defines the following AVDs:
Voltage:  120v  208v  240v  480v 

Feeder Circuit AV_{D}  3.6  6.24  7.2  14.4 
Branch Circuit AV_{D}  2.4  4.16  4.8  9.6 


Please note: Calculations from this page are set up as guidelines. You still have to make the decision as to how much cable to lay down in any rig. If you have any questions contact us.
Voltage Drop Calculator






