AMO's Cable Calculator

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

 Available Cable Calculators
1ø VDSingle Phase Voltage Drop
1ø CSSingle Phase Conductor Size
3ø VDThree Phase Voltage Drop
3ø CSThree Phase Conductor Size
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Three Phase Voltage Drop (VD)

About the formula | Use the calculator | Use the walkthrough

About the Voltage Drop (VD) 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.

Three-Phase Voltage Drop (VD):
VD = √3 × K × I × L

Conductors per leg × CMA

Please note that this formula is for three-phase (3ø) rigs only. Click here for the single-phase 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:
Feeder Circuit AVD3.
Branch Circuit AVD2.

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

Circuit Type:
VD = 18.706 × Amps (I) per Leg:
× Length (L) of run in feet:

Conductors per leg
Circular Mil Area
(UseQuickInsert at right)
Line Size:CMA:

Calculator Walkthrough

This section will walk you through the calculator while filling in the values for the calculator above. For a detailed explanation of this formula, please see the Formula Explanation.

Note that you can calculate the Total CMA by skipping directly to step 5 below.
 1. Are we looking at a Main Run (Feeder) or Branch circuit?

 2. How many Amps (i) does the circuit draw (per leg)?

 3. What Voltage are we using?

 4. What is the Length (L) of the circuit in feet?

 5. How many conductors are we using (per leg)?

 6. What type of conductor are we using?

 7. What is the Circular Mil Area (CMA) of our Conductor?

  Our total CMA (Number of Conductors × CMA) is:

 8. Answer: press "Calculate" below.

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