Voltage Drop Calculator

Voltage Drop Calculator: Check Wire Loss Before You Pull Cable

A 120-volt air conditioner at the end of a 250-foot 14 AWG extension cord may receive only 105 volts. The unit struggles to start, overheats, and fails within weeks. This happens because every foot of copper or aluminum conductor resists current flow, converting a portion of electrical energy into heat. That lost potential is called voltage drop.

The voltage drop calculator above simplifies the math. It uses conductor material, wire size, circuit length, load current, and operating temperature to apply the standard impedance formula and return the exact voltage loss and the percentage drop for the circuit.

Circuit Parameters
Conductor Material
Circuit Type
One-way distance from source to load
Standard rating: 75°C. Adjust for hot attics or cold environments.
How this calculation works

Single-phase / DC: VD = 2 × I × R × (L ÷ 1000)

Three-phase AC: VD = 1.732 × I × R × (L ÷ 1000)

Where R is the DC resistance per 1000 feet at the operating temperature. Resistance values are sourced from NEC Chapter 9, Table 8. Temperature correction applies approximately 4% per 10°C deviation from 75°C.

For AC circuits, actual impedance may be slightly higher due to reactance in steel conduit. This calculator uses DC resistance for a conservative estimate.

Disclaimer: This calculator provides estimates for informational purposes only. Always consult a licensed electrician and follow your local electrical code. Wire sizing must also satisfy ampacity requirements per NEC Table 310.16 regardless of voltage drop calculations. The 3% and 5% thresholds referenced are NEC Informational Notes and may be enforced as local amendments.

What Is Voltage Drop and Why Does It Matter?

Voltage drop is the reduction in voltage that occurs as current moves through a conductor. It is not a flaw in the power source; it is a physical consequence of conductor resistance. For direct-current (DC) circuits, drop depends solely on resistance. For alternating-current (AC) circuits, the total opposition to current–called impedance–includes both resistance and reactance.

The National Electrical Code (NEC) provides guidance in Informational Notes: branch circuit conductors should limit voltage drop to 3%, while the combined feeder and branch circuit should not exceed 5%. Running beyond these percentages wastes energy, reduces torque in motors, shortens lamp life, and can cause electronic power supplies to shut down unpredictably. Some jurisdictions enforce these notes as amendments, so inspectors may require documentation.

How Do You Calculate Voltage Drop by Hand?

Electricians rely on two common formulas.

Single-phase AC or DC circuits: Voltage Drop = 2 × I × R × L

Three-phase AC circuits: Voltage Drop = 1.732 × I × R × L

Where:

  • I = load current in amperes
  • R = AC or DC resistance per 1,000 feet from NEC Chapter 9, Table 8 or Table 9
  • L = one-way circuit length in feet (the factor of 2 accounts for the outgoing and return path in single-phase systems)

For copper and aluminum wire, resistance values change with temperature. At 75°C, 12 AWG solid copper has approximately 1.6 ohms per 1,000 feet of DC resistance. Aluminum of the same gauge has roughly 2.6 ohms per 1,000 feet. If your installation runs at a different temperature, adjust resistance using the temperature correction factor from NEC Table 310.15(B)(1).

Example: A single-phase 120-volt circuit feeds a 16-amp load through 100 feet of 12 AWG copper in steel conduit at 75°C.

  • R = 1.6 ohms / 1,000 ft
  • Voltage Drop = 2 × 16 A × 1.6 ohms × (100 / 1,000) = 5.12 volts
  • Percentage drop = 5.12 V / 120 V × 100 = 4.27%

That result exceeds the recommended 3% for branch circuits, signaling that the installer should upsize to 10 AWG or relocate the load.

Which Factors Increase Voltage Drop the Most?

Several variables push voltage drop higher or lower:

  • Conductor material. Aluminum has roughly 60% higher resistance than copper for the same gauge, producing greater drop.
  • Wire size. Each three-gauge step roughly halves resistance. Moving from 14 AWG to 12 AWG reduces resistance by approximately 37%.
  • Circuit length. Drop scales linearly with distance. Doubling the run doubles the loss.
  • Load current. Drop scales linearly with amperage. A 20-amp load loses twice as much voltage as a 10-amp load on the same wire.
  • Temperature. Conductor resistance climbs approximately 4% for each 10°C increase above 75°C. A hot attic can raise conductor temperature significantly.
  • Power factor (AC only). Low power-factor loads, such as induction motors, increase reactive voltage drop in long runs.

When Should You Size Wire for Voltage Drop?

Not every circuit demands a formal calculation. A 15-amp general lighting circuit in a small room rarely justifies concern. You should calculate voltage drop when:

  • The one-way run exceeds 100 feet (30 meters)
  • The load is sensitive to undervoltage (motors, medical equipment, large LED drivers)
  • The conductor is aluminum rather than copper
  • The installation operates in ambient temperatures above 40°C
  • Compliance with local amendments to the NEC is required

How to Reduce Voltage Drop in Existing Circuits

Four practical methods cut voltage loss without replacing the entire distribution panel:

  1. Increase wire gauge. Upsizing from 12 AWG to 10 AWG on a long branch circuit often brings drop below 3%.
  2. Shorten the route. Relocating a load closer to the panel or adding a sub-panel midway reduces total conductor length.
  3. Lower the load. Splitting a heavily loaded circuit into two separate homeruns halves the current on each path.
  4. Raise system voltage. Where permitted, switching from 120 volts to 240 volts for the same load halves the current and cuts drop by half.

Frequently Asked Questions

What is the maximum allowable voltage drop in a circuit?
The National Electrical Code recommends a maximum voltage drop of 3% for branch circuits and 5% overall from the service entrance to the outlet. Exceeding these values can cause equipment to overheat, reduce motor torque, and shorten the life of lamps and electronic components. Local amendments may impose stricter limits.
Does temperature affect voltage drop calculations?
Yes, conductor resistance increases with temperature. For every 10 degrees Celsius above standard 75°C ratings, copper and aluminum resistance rises approximately 4%, which slightly increases the voltage drop under heavy loads. Hot attics or bundled cables in conduit can push conductors well above ambient temperature.
Why does AC voltage drop differ from DC voltage drop?
AC circuits experience additional reactance from inductance and capacitance in the cable, especially in large wire sizes and steel conduit. This reactive component, called impedance, is added to resistance to calculate total AC voltage drop. DC circuits rely on pure resistance alone, making their math slightly simpler.
Can I use a smaller wire if the run is short?
Short runs reduce voltage drop, but wire size must still satisfy the National Electrical Code ampacity tables to prevent overheating. You cannot install a gauge smaller than the minimum required for the circuit breaker protecting that conductor. Temperature and bundling requirements also apply regardless of run length.
How does aluminum compare to copper for voltage drop?
Aluminum has roughly 60% higher resistance than copper for the same gauge, so it produces a larger voltage drop at equal current and length. To match copper performance, aluminum conductors are typically upsized by two gauge sizes, such as replacing 12 AWG copper with 10 AWG aluminum.
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