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Voltage Drop Calculator

Voltage drop along a wire run from current, length and conductor size.

Why voltage drop matters

When current flows through a conductor, resistance causes some electrical energy to be lost as heat. This loss—called voltage drop—reduces the voltage available at the far end of the wire run. For long distances or high currents, even small-diameter wires can lose a significant fraction of your source voltage, dimming lights, reducing motor torque, or causing equipment malfunction. Building codes and electrical standards (like the National Electrical Code) typically limit voltage drop to 3% for branch circuits and 5% for the combination of feeder and branch.

How it works

Voltage drop depends on four factors: how much current flows, how long the wire run is, what material the conductor is made from, and the wire's cross-sectional area. Copper is a better conductor than aluminum, so it has lower resistivity. Larger wires (measured in circular mils, or cmil) have less resistance per unit length. Three-phase systems distribute current across three conductors, so they experience less drop than single-phase systems carrying the same total power.

The calculator uses the standard electrical formula, accounting for the round-trip distance (current flows out and back), the resistivity of your chosen material, and whether you're running a single-phase or three-phase circuit.

The formula

Voltage Drop (V) = (2 × L × I × R) / A for single-phase; (√3 × L × I × R) / A for three-phase

where L is one-way length in feet, I is current in amperes, R is resistivity (copper = 10.37, aluminum = 17.02 ohms·cmil/ft), and A is conductor area in circular mils.

Worked example

Suppose you're running a 120 V, single-phase circuit to a workshop 150 feet away, drawing 20 amps. You choose a copper conductor with a cross-section of 10,380 cmil (roughly #8 AWG).

Step 1: Identify your inputs

  • Source voltage: 120 V
  • Current: 20 A
  • One-way length: 150 ft
  • Conductor area: 10,380 cmil
  • Material: Copper (resistivity = 10.37)
  • System: Single-phase

Step 2: Apply the single-phase formula

Voltage Drop = (2 × 150 × 20 × 10.37) / 10,380

Step 3: Calculate step-by-step

  • Numerator: 2 × 150 = 300; 300 × 20 = 6,000; 6,000 × 10.37 = 62,220
  • Denominator: 10,380
  • Result: 62,220 / 10,380 = 5.99 V (approximately 6 V)

Step 4: Find the percentage

Percentage drop = (5.99 / 120) × 100 = 4.99% (approximately 5%)

This exceeds the recommended 3% for a branch circuit, so you'd need to upgrade to a larger wire (lower cmil number, higher AWG) to reduce resistance and drop.

Common mistakes

Forgetting the round-trip: Many people mistakenly use the one-way distance directly. Electricity must travel out to the load and return, so the effective distance is doubled—that's why the formula includes the 2× factor for single-phase circuits.

Confusing circular mils with wire gauge: Circular mils (cmil) and AWG (American Wire Gauge) are inversely related—larger AWG numbers mean smaller wires and higher cmil values. Always verify which unit your wire specification uses.

Ignoring system type: A three-phase 480 V circuit carrying 100 amps over 200 feet will drop far less voltage than a single-phase 120 V circuit with the same current and distance, even with identical conductor size. Three-phase systems are inherently more efficient for long runs.

Neglecting temperature and insulation: This calculator assumes standard 75°C copper or aluminum. Real-world resistance varies slightly with temperature and insulation type; consult a wire resistance table if precision is critical for code compliance.

Frequently asked questions

What's the difference between copper and aluminum conductors?

Copper has lower resistivity (10.37 vs. 17.02 ohms·cmil/ft), meaning it loses less voltage over the same distance and wire size. Aluminum is cheaper but requires a larger cross-section to achieve the same voltage drop, making it heavier and bulkier for equivalent performance.

Why does three-phase drop less voltage than single-phase?

Three-phase circuits distribute current across three conductors instead of one, and the mathematical relationship (√3 ≈ 1.73 instead of 2) means less voltage is lost. For the same power delivery, three-phase is more efficient over long distances.

What percentage voltage drop is acceptable?

The National Electrical Code recommends a maximum of 3% for branch circuits and 5% for the combination of feeder and branch circuits. Exceeding these limits can cause equipment to underperform or fail, especially motors and heating elements.

How do I convert AWG to circular mils?

Use the formula cmil = 10,383.688 / (0.005·AWG)². For example, #10 AWG ≈ 10,380 cmil. Most wire charts include both measurements; refer to your supplier's table for exact values.

Can I use this calculator for DC circuits?

Yes. This calculator uses resistive voltage drop, which applies to both AC and DC. For AC circuits with significant inductance, you may need to account for impedance, but for typical building wiring, resistance dominates.

What happens if voltage drop is too high?

Lights dim, motors run slowly and overheat, and sensitive electronics may malfunction or shut down. The solution is to use a larger wire (smaller AWG, larger cmil), run a separate circuit closer to the load, or increase the source voltage.

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