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Voltage Drop Calculator (NEC 3% Limit)

Quick answer

This calculator computes voltage drop with VD = 2 x L x I x R/1000 for copper or aluminum.

  • NEC 210.19 recommends max 3 percent branch and 5 percent total - check any circuit.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How the Voltage Drop Calculator Works

Vdrop = 2 × I × R × L (single-phase), Vdrop = √3 × I × R × L (three-phase)

Calculates voltage loss in electrical cables due to wire resistance. Excessive voltage drop causes equipment malfunction, reduced efficiency, and energy waste. Worked example: 15 A through 100 ft of 14 AWG copper (2.53 ohm/1000 ft) drops 2 x 100 x 15 x 0.00253 = 7.6 V - 6.3 percent at 120 V, double the 3 percent limit. Stepping up to 10 AWG cuts it to 3.0 V (2.5 percent). Three-phase runs drop less: multiply the single-phase result by 0.866 because the balanced phase currents partially cancel, so a 24 V drop becomes 20.8 V on the same conductors.

  1. 1

    Step 1

    Enter the system voltage and load current

  2. 2

    Step 2

    Enter the cable run distance (one way)

  3. 3

    Step 3

    Select wire size and material

  4. 4

    Step 4

    Choose single-phase or three-phase

Use Cases

Sizing cables for long-distance runs

Solar panel array wiring design

Industrial power distribution planning

Ensuring code compliance for electrical installations

Tips

  • 1

    Keep voltage drop under 3% for lighting circuits

  • 2

    Allow up to 5% for motor and heating loads

  • 3

    Use copper for lower voltage drop vs. aluminum

  • 4

    Consider larger wire sizes for long distances

Common Mistakes

  • Forgetting to double the distance (round-trip)

  • Using single-phase formula for three-phase systems

  • Ignoring temperature effects on wire resistance

  • Not accounting for connection resistance

FAQs

Why is voltage drop important?

Excessive voltage drop causes dim lights, slow motors, and equipment damage. It also wastes energy as heat in the wires. Codes limit drop to 3-5%.

How can I reduce voltage drop?

Use larger wire gauge, shorter cable runs, copper instead of aluminum, or higher system voltage. For DC systems, consider running at higher voltage then converting down.

Does voltage drop affect power loss?

Yes, power loss = I² × R. The lost power becomes heat in the cable. Higher currents have disproportionately higher losses due to the squared relationship.

What is the maximum allowed voltage drop?

NEC 210.19(A) FPN recommends 3 percent on branch circuits and 5 percent total including feeders. For sensitive 12 V LED strips even 1 V is 8 percent - size the wire generously for low-voltage runs.

How do I calculate voltage drop?

VD = 2 x L(ft) x I x R(per 1000 ft) for single-phase. 20 A, 50 ft, 12 AWG (1.59 ohm/kft): VD = 2 x 50 x 20 x 0.00159 = 3.2 V. For three-phase multiply by 0.866 instead of 2.

Which has more voltage drop, copper or aluminum?

Aluminum resistivity is 1.64x copper, so the same size drops 64 percent more: 100 ft of 10 AWG Al at 20 A drops 4.0 V versus 2.4 V for copper. Use one size larger for aluminum.

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