V
Common: 120V, 240V, 208V, 480V
A
Rated current draw of the load
ft
Distance from panel to load (one way)
Select the wire gauge you plan to use
Voltage Drop
—
⚠️ NEC Guideline: The National Electrical Code recommends voltage drop not exceed 3% for branch circuits and 5% total from the service to the outlet. Always verify with a licensed electrician and local code.
Was this calculator helpful?
Sources & Methodology
Voltage drop calculations follow standard electrical engineering formulas based on Ohm's Law and wire resistance tables from the National Electrical Code and manufacturer data.
How Voltage Drop Is Calculated
Voltage drop occurs when electrical current flows through the resistance of a wire. Every wire has resistance — the longer and thinner the wire, the more resistance, and the more voltage is "lost" before reaching the load.
The Formula
Single-Phase: VD = 2 × K × I × L ÷ CM
Where K = 12.9 (copper) or 21.2 (aluminum), I = current in amps, L = one-way length in feet, CM = wire circular mil area. The factor of 2 accounts for the round-trip conductor path (hot + neutral). Three-phase uses a factor of 1.732 instead of 2.
Wire Resistance by Gauge (Copper)
Wire gauge (AWG) determines resistance. Larger gauge numbers = thinner wire = more resistance per foot. Here's a reference for copper wire:
- 14 AWG: 3.14 Ω/1000 ft — Max 15A circuits
- 12 AWG: 1.98 Ω/1000 ft — Max 20A circuits
- 10 AWG: 1.24 Ω/1000 ft — Max 30A circuits
- 8 AWG: 0.778 Ω/1000 ft — Max 40A circuits
- 6 AWG: 0.491 Ω/1000 ft — Max 55A circuits
- 4 AWG: 0.308 Ω/1000 ft — Max 70A circuits
💡 Rule of Thumb: For runs over 100 feet, upsize your wire by one gauge. For runs over 200 feet, upsize by two gauges. This keeps voltage drop within NEC's recommended 3% for branch circuits.
Frequently Asked Questions
What is the maximum allowable voltage drop per NEC?
NEC Article 210.19(A)(1) recommends branch circuit conductors be sized so voltage drop doesn't exceed 3%. Including feeder conductors, total voltage drop should not exceed 5%. These are recommendations, not mandatory requirements in all jurisdictions — but they represent best practice for equipment performance and longevity.
Does copper or aluminum wire have less voltage drop?
Copper has significantly lower resistivity than aluminum — about 37% less resistance per foot for the same gauge. This means a copper wire will have 37% less voltage drop than the same gauge aluminum wire under identical conditions. However, aluminum is lighter and cheaper, making it common for large feeder runs when upsized appropriately (typically 2 gauges).
How do I reduce voltage drop without changing wire gauge?
Options include: (1) Increase source voltage — running 240V instead of 120V halves the current for the same wattage, cutting voltage drop significantly. (2) Move the panel closer to the load. (3) Use multiple smaller circuits instead of one long run. (4) Use higher voltage for the long run and step down at the destination with a transformer.
Why does voltage drop matter for motors and equipment?
Motors are particularly sensitive to low voltage. A motor running at 10% below rated voltage draws significantly higher current to maintain torque, leading to overheating and premature failure. LED drivers, variable frequency drives (VFDs), and sensitive electronic equipment also perform poorly with low voltage. The NEC 5% maximum is a conservative safe limit for most equipment.
Do I use one-way or round-trip distance for voltage drop?
The formula already accounts for round-trip distance by multiplying one-way length by 2 (single-phase) or 1.732 (three-phase). Enter the one-way distance — from your panel to the load — and this calculator handles the math. Do not double it yourself or you'll get an inflated result.
Related Calculators