Voltage Drop Calculator
Calculate voltage drop by wire gauge, distance, and amperage. NEC-compliant results with pass/fail indication and recommended wire sizing.
Voltage drop of 6.60% exceeds the NEC recommended maximum of 3%. Consider upgrading to 8 AWG copper wire to meet the 3% threshold.
How It Works
This calculator determines voltage drop using the standard NEC formula:
Formula: VD = (2 × L × I × R) / 1000, where L is one-way distance in feet, I is current in amps, and R is the wire resistance per 1,000 feet based on gauge and material.
Percentage drop is then calculated as (VD / Source Voltage) × 100. The NEC recommends a maximum of 3% drop on branch circuits. Results are flagged as pass or fail against this threshold.
If the selected wire fails, the calculator recommends the smallest wire gauge that keeps voltage drop at or below 3% while meeting ampacity requirements.
Frequently Asked Questions
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electricity travels through a wire. Excessive voltage drop causes equipment to underperform, motors to overheat, and lights to dim. The NEC recommends keeping voltage drop at or below 3% for branch circuits and 5% total for feeder plus branch circuits.
What is the NEC 3% voltage drop rule?
NEC Article 210.19(A) Informational Note No. 4 and Article 215.2(A) Informational Note No. 2 recommend that voltage drop on branch circuits not exceed 3%, and total voltage drop (feeder plus branch circuit) not exceed 5%. While these are recommendations rather than hard requirements, most inspectors and engineers treat them as the standard.
Does wire material affect voltage drop?
Yes. Aluminum wire has about 1.6 times the resistance of copper wire for the same gauge, resulting in higher voltage drop. To compensate, you typically need to go up one or two wire sizes when using aluminum. Aluminum is cheaper but requires larger conduit and has different termination requirements.
How do I reduce voltage drop on a long run?
You can reduce voltage drop by increasing the wire gauge (using thicker wire), increasing the system voltage (e.g., 240V instead of 120V), or reducing the circuit length. Doubling the voltage cuts the current in half, which cuts voltage drop in half for the same wire size.
Is voltage drop calculated one-way or round-trip?
Voltage drop is calculated using the round-trip distance (out and back), which is why the formula multiplies the one-way distance by 2. This calculator asks for one-way distance and handles the round-trip math automatically.