Voltage Drop Calculator
Calculate voltage drop over a wire run based on AWG gauge, wire length, and current. Checks compliance with the NEC 3% recommended maximum.
Voltage Drop
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Voltage Drop
4.710 V (3.92%)
Borderline — consider larger wire
The Formula
Voltage drop is calculated using the wire resistance (from AWG tables) multiplied by the round-trip current path length. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% for feeder plus branch combined.
Variable Definitions
Wire Resistance
Copper and aluminum have different resistivity values. Resistance increases with length and decreases with wire gauge (smaller AWG number = thicker wire = lower resistance).
NEC Recommended Max
The National Electrical Code recommends voltage drop not exceed 3% for branch circuits and 5% for feeder + branch combined.
Conductor Material
Copper has about 40% lower resistance than aluminum for the same gauge, but aluminum is lighter and cheaper. Aluminum requires special termination connectors.
How to Use This Calculator
- 1
Enter the source voltage and load current.
- 2
Enter the one-way wire run length in feet.
- 3
Select the wire gauge and conductor material.
- 4
The calculator checks compliance with the 3% NEC recommended maximum.
- 5
If voltage drop exceeds 3%, try a larger gauge wire or shorter run.
Common Applications
- Sizing electrical wire gauges for residential, commercial, or industrial wiring projects to meet NEC code requirements
- Planning long wire runs to outbuildings, subpanels, outdoor lighting, or well pumps where voltage drop is a concern
- Choosing between copper and aluminum conductors based on resistance, cost, and installation requirements for a given circuit length
Voltage drop occurs along both conductors between source and load
Understanding the Concept
Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction. The NEC recommends a maximum 3% voltage drop for branch circuits. If your calculation shows more than 3% drop, upsize the wire gauge. Each jump in AWG (e.g., 12 to 10) reduces resistance by approximately 20%. Long wire runs — to outbuildings, outdoor lighting, or subpanels — are the most common sources of voltage drop problems. Real-world example: running 120V to a shed 100 feet away with a 15A load on 14 AWG copper. One-way equals 100 ft, round-trip equals 200 ft. Resistance of 14 AWG copper is 3.14 Ω per 1,000 ft, so total resistance equals 0.628 Ω. Voltage drop equals 15A times 0.628 Ω which is 9.42V, or 7.85 percent — well over the 3% NEC recommendation and likely to cause dim lights and slow motor starting. Upsizing to 10 AWG copper (1.24 Ω per 1000ft) drops resistance to 0.248 Ω and voltage drop to 3.72V (3.1 percent) — just within acceptable range. For a 240V well pump 300 feet from the house running at 10A, even 10 AWG copper produces a drop of 7.44V (3.1 percent). 8 AWG drops it to 4.67V (1.9 percent) — a safer choice that also reduces energy waste over the lifetime of the installation.
Frequently Asked Questions
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