What is an acceptable voltage drop?

The NEC recommends no more than 3% for branch circuits and 5% total for feeders plus branch circuits combined.

Why does the formula multiply by 2?

Electricity must travel from the panel to the load and back, so the total wire length is twice the one-way distance.

What happens if voltage drop is too high?

Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction or fail prematurely.

Does wire temperature affect resistance?

Yes — resistance increases with temperature. The values used here are for copper wire at 75°C, which is standard for NEC calculations.

Should I upsize the wire gauge?

If voltage drop exceeds 3%, the calculator recommends a larger gauge. Going up one or two sizes is often the easiest solution for long runs.

Everyday

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Voltage Drop Calculator - Wire Size Amperage Length

Calculate voltage drop and percentage drop for electrical wire runs based on gauge, length, and current.

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How to use this calculator

👉 Fill in the boxes below and your answer appears instantly — no maths needed, we do it all for you! 🎉

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In plain English — what does this do?

📏 Some countries say ‘30 degrees’ and others say ‘86 degrees’ for the same weather! Some say ‘5 km’, others say ‘3 miles’. This tool swaps between them instantly so you always know what it means.

📌 Direct Answer & Summary

The Voltage Drop Calculator determines how much voltage is lost along a wire run due to the wire's resistance. It helps electricians and DIYers select the correct wire gauge to keep voltage drop within acceptable limits (typically under 3% for branch circuits).

Wire Gauge (AWG)

System Voltage

One-Way Length (ft)

Current (Amps)

What is Voltage Drop Calculator - Wire Size Amperage Length?

How to use it

1️⃣ Select the wire gauge from the dropdown (AWG sizes #14 through #1/0).
2️⃣ Enter the one-way length of the wire run in feet.
3️⃣ Enter the load current in amps.
4️⃣ Select the system voltage (120 V or 240 V).
5️⃣ Click Calculate to see the voltage drop and percentage. If drop exceeds 3%, a larger gauge recommendation is shown.

Formula

V_drop = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current in amps, and R is the resistance in ohms per 1000 ft for the selected gauge. Percentage drop = (V_drop / V_system) × 100.

💡 See it in action — a real example

240 V circuit, #12 AWG (1.98 Ω/1000 ft), 50 ft run, 15 A: V_drop = (2 × 50 × 15 × 1.98) / 1000 = 2.97 V, which is 1.24% — well within the 3% limit.

❓ Common questions

What is an acceptable voltage drop?: The NEC recommends no more than 3% for branch circuits and 5% total for feeders plus branch circuits combined.
Why does the formula multiply by 2?: Electricity must travel from the panel to the load and back, so the total wire length is twice the one-way distance.
What happens if voltage drop is too high?: Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction or fail prematurely.
Does wire temperature affect resistance?: Yes — resistance increases with temperature. The values used here are for copper wire at 75°C, which is standard for NEC calculations.
Should I upsize the wire gauge?: If voltage drop exceeds 3%, the calculator recommends a larger gauge. Going up one or two sizes is often the easiest solution for long runs.

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