NEC Voltage Drop Calculator: Estimate Conductor Drop

An NEC voltage drop calculator estimates the voltage lost along a conductor from current, one-way circuit length, conductor resistance, and circuit type. Entering those values gives a calculated drop in volts and as a percentage of nominal voltage, but the result is an engineering estimate—not a substitute for checking the applicable code, conductor data, installation conditions, or electrical safety requirements.

Formula

For a two-wire DC or single-phase circuit, the approximate resistive voltage drop is:

V_drop = I × L × R × 2 ÷ 1000

For a balanced three-phase circuit, the calculator uses:

V_drop = I × L × R × 1.732 ÷ 1000

Here, I is load current in amperes (A), L is one-way route length in feet (ft), and R is conductor resistance in ohms per 1,000 feet (Ω/1,000 ft). The factor 2 accounts for outgoing and return conductors in a two-wire circuit; 1.732 is an approximation of the square root of three used for a balanced three-phase circuit. The calculated percentage is V_drop ÷ system voltage × 100. Estimated load voltage is system voltage minus the calculated drop.

This is a simplified resistance calculation. It does not apply temperature correction, conductor reactance, power factor, or every installation detail. The resistance value must match the conductor material and size, and should come from an appropriate conductor table or manufacturer data. Enter a resistance in Ω/1,000 ft—not Ω/ft.

Worked Example

Using the calculator defaults: 20 A, 100 ft one-way length, 1.2 Ω/1,000 ft, and 120 V, with the selected DC or single-phase factor of 2:

1. Multiply current, length, resistance, and the circuit factor: 20 × 100 × 1.2 × 2 = 4,800.
2. Convert the resistance basis from 1,000 ft: 4,800 ÷ 1,000 = 4.80 V drop.
3. Calculate the percentage: 4.80 ÷ 120 × 100 = 4.00%.
4. Estimate voltage at the load: 120 − 4.80 = 115.20 V.

What “NEC voltage drop” means

NEC commonly refers to the National Electrical Code, a model electrical code published by the National Fire Protection Association. Its adopted edition and local amendments depend on the jurisdiction. Voltage drop is the reduction in voltage between a source and a load as current flows through circuit impedance. Longer runs and higher currents generally produce greater drop; larger conductors generally have lower resistance and reduce it.

A calculator can help compare proposed circuit lengths or conductor resistance values. It does not determine whether a design complies with every NEC requirement. Voltage-drop recommendations and requirements can depend on the relevant code edition, circuit type, occupancy, and local authority. Consult the adopted code and a qualified electrical professional for design or installation decisions.

When to use the estimate

Use this calculation for an initial check when you know the load current, one-way route length, voltage, and conductor resistance. It is useful for spotting a potentially significant drop before investigating conductor choices or obtaining a more complete calculation. For a three-phase circuit, select the three-phase option only when the system and load assumptions are appropriate; the simple factor assumes a balanced load.

• Current: Use the relevant load current for the calculation. A motor or other changing load may draw different current under different operating conditions.
• Length: Measure the route one way, from source to load. Do not double the length yourself for a two-wire circuit; the calculator already applies the return-path factor.
• Resistance: Use matching conductor data, including material and size. Resistance changes with temperature, and AC impedance can differ from simple DC resistance.
• Voltage: Enter nominal circuit voltage, such as 120 V, rather than a drop target or a percentage.

Limits, units, and common mistakes

The calculator uses feet, amperes, volts, and resistance in Ω/1,000 ft. For metric work, convert the route length and resistance consistently before using this particular input format; mixing metres with a resistance value expressed per 1,000 feet gives a wrong answer. A metric resistance value in Ω/km can be converted to Ω/1,000 ft by multiplying by 0.3048, because 1,000 ft is 0.3048 km.

Do not mistake one-way distance for total conductor length. For example, entering 100 ft in a two-wire circuit means a 100-ft route; the formula accounts for the 200-ft outgoing-and-return path through its factor of 2. Conversely, do not use that factor for three-phase wiring: the calculator uses the three-phase factor instead.

The result is approximate because real circuits involve impedance and operating conditions. For AC circuits, conductor reactance and power factor can matter, especially on longer runs, larger conductors, or particular load types. Resistance also rises with conductor temperature. The calculator does not identify an allowable conductor size, ampacity, overcurrent protection, or installation method, and a lower voltage drop alone does not establish a safe or code-compliant design.

Reading the result and improving accuracy

The headline result is voltage drop in volts. The percentage output makes it easier to compare drops on different nominal voltages: the same 4 V drop is a different share of a 120 V circuit than of a 240 V circuit. Estimated load voltage is a simple subtraction from nominal voltage; actual measured voltage can vary with supply voltage and load.

To improve an estimate, verify the route length, confirm the conductor’s resistance at the relevant conditions, and use a method that accounts for AC impedance and power factor when the application requires it. Check whether a code table or calculation method applies to the installation. Treat calculator output as a screening aid, not as a wiring instruction or permission to work on energized equipment.

Frequently Asked Questions

Does the NEC require a specific voltage-drop limit?
Code language and recommendations depend on the adopted edition and application. Do not assume a calculator’s percentage is itself a universal NEC pass/fail threshold; check the locally adopted code and ask the authority having jurisdiction or a qualified professional.

Should I enter one-way or round-trip wire length?
Enter one-way route length. For DC or single-phase circuits, this calculator includes the return path with a factor of 2.

Why is the three-phase factor 1.732?
It is approximately the square root of three, used in a simplified voltage-drop formula for a balanced three-phase circuit. The estimate may need refinement for unbalanced loads or significant impedance and power-factor effects.

Can I use this for aluminum conductors?
Yes, if the resistance input is appropriate for the aluminum conductor’s size and conditions. Do not reuse a resistance value for a different material or conductor size.

Does voltage drop tell me what wire size to install?
No. The calculation does not check ampacity, temperature ratings, wiring method, overcurrent protection, or all code rules. Conductor selection and installation should be confirmed against applicable requirements by a qualified electrical professional.

Related

• Honkai Star Rail Geniuses Greeting Light Cone: Stats & Best Users
• Asus Recovery Partition: What It Is & How to Use It
• How to Choose a Desk Lamp for a Small Study Desk

in cur | Load current | A | 20 | 0
in len | One-way conductor length | ft | 100 | 0
in res | Conductor resistance | ohm/1000 ft | 1.2 | 0
in volt | System voltage | V | 120 | 0
sel cir | Circuit type | DC or single-phase=2, Three-phase=1.732
out drop | Voltage drop | V | cur * len * res / 1000 * cir | 2
out pct | Voltage drop | % | drop / volt * 100 | 2
out end | Estimated voltage at load | V | volt - drop | 2