When to use: Use this calculator when sizing conductors for branch circuits and feeders. NEC 210.19(A) FPN No. 4 recommends keeping branch circuit voltage drop to 3% or less, with a combined feeder + branch circuit maximum of 5%. Excessive voltage drop causes motors to overheat, lights to dim, and equipment to malfunction. Critical for long runs, motors, and sensitive electronic equipment.
This free voltage drop calculator sizes copper and aluminum conductors so branch circuits and feeders stay within the National Electrical Code's recommended limits. Enter the system voltage, conductor material and size, load current, and one-way run length, and it returns the percent voltage drop plus the minimum wire gauge needed to hit a 3% or 5% target. It's built for electricians, electrical engineers, designers, and anyone sizing wire for long runs, motors, or sensitive equipment.
Voltage drop is the loss of voltage as current travels through the resistance of a conductor. The standard NEC approximation uses:
VD = (2 × K × I × L) ÷ CM (single-phase) VD = (1.732 × K × I × L) ÷ CM (three-phase)
where K is the resistivity constant (≈12.9 for copper, ≈21.2 for aluminum), I is the load current in amps, L is the one-way length in feet, and CM is the conductor area in circular mils. Percent drop = (VD ÷ source voltage) × 100.
Worked example: a 120 V, 20 A load on 100 ft of #12 copper (6,530 CM): VD = (2 × 12.9 × 20 × 100) ÷ 6,530 ≈ 7.9 V, or about 6.6% — over the limit, so you'd upsize to #10 or #8. This calculator does that sizing for you.
1. Choose the system voltage (e.g. 120 V, 208 V, 240 V, 480 V). 2. Select conductor material (copper or aluminum) and the wire size you intend to use. 3. Enter the load current in amps and the one-way circuit length in feet. 4. Read the resulting percent voltage drop, and use the “Minimum Gauge” output to pick a conductor that meets the 3% (recommended) or 5% (maximum) target.
The NEC recommends a maximum of 3% voltage drop on a branch circuit (210.19(A), Informational Note 4) and 3% on a feeder (215.2(A), Informational Note 2), with a combined feeder-plus-branch-circuit total not exceeding 5%. Excessive voltage drop makes motors run hot and lose torque, dims lighting, shortens equipment life, and can trip electronics. It becomes critical on long runs, motor circuits, and sensitive loads.
The NEC recommends no more than 3% on a branch circuit and 3% on a feeder, with a combined maximum of 5% (NEC 210.19(A) and 215.2(A) informational notes). These are recommendations in the informational notes, not mandatory rules — though some local jurisdictions adopt them as enforceable.
No. It appears in NEC Informational Notes, which are advisory and not enforceable by themselves. However, many engineers design to it as best practice, and some local amendments make it mandatory — always verify with the AHJ.
Use VD = (2 × K × I × L) ÷ CM for single-phase (or 1.732 × K × I × L ÷ CM for three-phase), where K ≈ 12.9 for copper and 21.2 for aluminum, I is amps, L is one-way feet, and CM is the conductor circular-mil area. Percent drop is VD divided by the source voltage.
Increase the conductor size (more circular mils), shorten the run, use copper instead of aluminum, raise the system voltage, or run parallel conductors. Upsizing wire gauge is the most common fix.
Yes — single-phase uses a factor of 2 (current flows out and back), while balanced three-phase uses 1.732 (√3). The calculator applies the correct factor for the system you select.
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