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Voltage Drop Calculator

Calculates voltage drop across a conductor run. Ensures voltage at load meets NEC recommendations of 3% for feeders and 5% for branch circuits.

Results

Voltage Drop
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% Drop
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Voltage at Load
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NEC Compliance
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Based on NEC Chapter 9 Tables 8 & 9. Resistance values at 75°F. For higher ambient temperatures, actual voltage drop may be slightly higher.

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How to Use This Calculator

1. Enter the load current in amps. Use the actual expected load, not the breaker rating. For motors, use the nameplate FLA.

2. Set the system voltage and circuit type. Single-phase uses a factor of 2 in the formula. 3-phase uses 1.732 (square root of 3).

3. Select your wire gauge and material. The calculator uses NEC Chapter 9 Table 8 resistance values for the selected conductor.

4. Enter the one-way distance. Measure from the panel or source to the load. The calculator accounts for the round-trip automatically.

How the Voltage Drop Formula Works

The calculator uses conductor resistance values from NEC Chapter 9 Table 8 (DC resistance) to compute the voltage loss over the wire run length.

Single-Phase: VD = (2 x I x D x R) / 1000

3-Phase: VD = (1.732 x I x D x R) / 1000

DC: VD = (2 x I x D x R) / 1000

VD% = (VD / System_Voltage) x 100

Where I is load current in amps, D is one-way distance in feet, and R is conductor resistance in ohms per 1000 feet. If the drop exceeds 3%, the calculator suggests the next wire size up that would bring the drop within limits.

Frequently Asked Questions

What is an acceptable voltage drop per NEC?
NEC recommends a maximum of 3% voltage drop for feeders or branch circuits individually, and 5% total for the combination of feeder plus branch circuit. These are recommendations in the NEC informational notes, not hard code requirements, but most inspectors enforce them.
How do I calculate voltage drop for a 3-phase circuit?
For 3-phase circuits, use VD = (1.732 x I x D x R) / 1000, where I is the line current in amps, D is one-way distance in feet, and R is the conductor resistance per 1000 feet from NEC Chapter 9 Table 8. The 1.732 factor replaces the factor of 2 used in single-phase calculations.
Does wire temperature affect voltage drop?
Yes. Conductor resistance increases with temperature. NEC Chapter 9 Table 8 values are at 75 degrees F. For higher ambient temperatures, apply a correction factor. Copper resistance increases about 0.4% per degree F above 75 degrees F.

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