Voltage Drop Calculator

Related tools: Wire Size Calculator, AWG Wire Resistance Calculator, and Wire Derating Calculator.

Voltage drop is the loss of potential that occurs as current flows through a circuit conductor path under load. The result is used during branch-circuit and feeder design to evaluate whether the voltage available at the load remains suitable for the equipment being served.

Enter Load current, One-way length, System voltage, Phase model, Conductor resistance, and Entered drop limit. The calculator returns the Voltage drop in volts and percent, the Entered limit in volts, the Voltage-drop margin, the Phase multiplier, and the Limit check.

The calculation is useful when the conductor resistance is already known from a verified source for the selected conductor material, AWG or kcmil size, and temperature basis. It evaluates that specific resistance value; it does not choose a conductor size.

Circuit Resistance and Path Length

Voltage drop is produced by current flowing through conductor resistance. For a two-wire single-phase or DC circuit, current travels outward on one conductor and returns on the other. The calculation therefore uses twice the entered One-way length.

For a balanced three-phase circuit, the calculator uses the three-phase multiplier of \sqrt{3}. This represents the line-to-line voltage-drop relationship for a balanced three-phase load.

Phase model	Phase multiplier	Circuit condition evaluated
Single-phase or DC two-wire	2 ×	Outgoing and return conductor loop
Balanced three-phase	\sqrt{3} ×	Balanced line-to-line three-phase circuit

The entered Conductor resistance is expressed in ohms per 1,000 ft. It should match the actual conductor being evaluated, including the intended material, size, conductor operating temperature basis, and any verified resistance data used by the design team.

Voltage-Drop Formula

The calculator first converts the entered one-way conductor length into a resistance basis using the conductor resistance value per 1,000 ft.

For the selected phase model, voltage drop is calculated as:

\(\displaystyle V_D = \frac{I \times L \times R \times M}{1000}\)

Where:

• V_D = voltage drop, in volts
• I = Load current, in amperes
• L = One-way length, in feet
• R = Conductor resistance, in ohms per 1,000 ft
• M = Phase multiplier: 2 for single-phase/DC two-wire or \sqrt{3} for balanced three-phase

Voltage-drop percentage is then calculated from the entered System voltage:

\(\displaystyle \text{Voltage drop \%} = \frac{V_D}{V_{system}} \times 100\)

The calculator converts the Entered drop limit into volts:

\(\displaystyle \text{Entered limit in volts} = V_{system} \times \frac{\text{Entered drop limit}}{100}\)

Its Voltage-drop margin is:

\(\displaystyle \text{Voltage-drop margin} = \text{Entered drop limit} - \text{Voltage drop \%}\)

A positive margin means the calculated drop is below the entered percentage limit. A negative margin means it is above that limit.

Calculation Example

A 120 V, single-phase branch circuit supplies a 20 A load located 100 ft from the source. The verified conductor resistance is 1.588 ohm per 1,000 ft, and the design comparison limit is 3%.

Input	Entered value
Load current	20 A
One-way length	100 ft
System voltage	120 V
Phase model	Single-phase or DC two-wire
Conductor resistance	1.588 ohm/1000 ft
Entered drop limit	3%

Using the two-wire phase multiplier:

\(\displaystyle V_D = \frac{20 \times 100 \times 1.588 \times 2}{1000} = 6.352\text{ V}\)

The resulting voltage drop is:

\(\displaystyle \frac{6.352}{120} \times 100 = 5.2933\%\)

The entered 3% limit corresponds to:

\(\displaystyle 120 \times 0.03 = 3.6\text{ V}\)

The result is therefore:

Result	Value
Voltage drop	6.352 V
Voltage drop	5.2933%
Entered limit in volts	3.6 V
Voltage-drop margin	−2.2933 percentage points
Phase multiplier	2 ×
Limit check	Above entered voltage-drop limit

The circuit drops 6.352 V at the entered load current, leaving approximately 113.648 V at the load if the source is exactly 120 V and other system effects are excluded. The 5.2933% calculated drop exceeds the entered 3% comparison limit.

Design Use and Field Limits

Use the voltage-drop result when comparing conductor alternatives, evaluating a long branch circuit, reviewing a feeder route, or checking whether the available voltage at utilization equipment is adequate for the intended load. A higher-resistance conductor, longer route, larger load current, or lower system voltage increases the percentage voltage drop.

For motors and other voltage-sensitive equipment, voltage drop should be reviewed with the actual operating condition in mind. Starting current, load variation, source impedance, transformer performance, and utility voltage variation can affect voltage at the equipment but are not part of this calculation.

The calculator performs a resistance-based voltage-drop estimate only. It does not determine:

• Conductor ampacity or required AWG/kcmil size
• Adjustment factors or correction factors
• Current-carrying conductor count
• Terminal rating or insulation temperature rating
• Overcurrent protection suitability
• Raceway fill, conduit sizing, conductor pulling conditions, or equipment ratings
• Reactive impedance, power factor, unbalanced three-phase loading, or source impedance
• NEC compliance, local amendments, project specifications, or AHJ acceptance

Verify ampacity, termination limitations, conductor insulation, raceway fill, equipment ratings, overcurrent protection, and applicable code requirements separately. The Entered drop limit is a user-selected comparison threshold; an “Above entered voltage-drop limit” result reports the arithmetic comparison and does not itself establish a code violation or installation approval.