Who This Guide Is For
Use this guide when you need a practical voltage-drop check for a branch circuit, feeder, motor run, low-voltage circuit, or early conductor-size comparison.
Why Voltage Drops
Every real conductor has resistance. When current flows, part of the source voltage appears across that resistance instead of the load.
Vdrop = I × Rpath
Longer conductors have more resistance. Larger conductors generally have less resistance. Higher current produces proportionally more voltage drop and power loss.
Formula Summary
When resistance is given in ohms per 1,000 feet and length is one-way feet:
DC or single-phase: Vdrop = 2 × L × I × R ÷ 1000
Balanced three-phase: Vdrop = √3 × L × I × R ÷ 1000
The factor of 2 represents the outgoing and return conductor path in a two-wire circuit.
Worked Example
A 120 V single-phase circuit carrying 20 A over 100 ft with conductor resistance of 0.5 Ω per 1,000 ft has a 2.0 V drop.
Vdrop = 2 × 100 × 20 × 0.5 ÷ 1000 = 2.0 V
Drop % = 2.0 ÷ 120 × 100 = 1.67%
Voltage-Drop Percentage
Drop % = Vdrop ÷ Vsource × 100
A 2 V drop on a 120 V circuit is 1.67%. The same 2 V drop on a 24 V circuit is 8.33%, so low-voltage systems are especially sensitive to conductor loss.
Resistance Changes with Temperature
Metal resistance increases with temperature. A resistance value taken at room temperature can understate voltage drop when conductors operate hot. Material, conductor construction, terminals, and connections also matter.
For AC systems, reactance may be significant. Conductor arrangement, raceway, spacing, frequency, and power factor can affect the result beyond simple resistance.
Effects at the Load
- Resistive heaters may produce less heat.
- Motors may draw more current, produce less torque, or have starting difficulty.
- Lighting may dim or electronic equipment may shut down.
- Conductor losses increase as I²R heating.
Acceptable operating voltage must be checked against the actual equipment rating and applicable installation requirements.
Common Mistakes
- Entering round-trip length when the formula expects one-way length.
- Using conductor resistance at the wrong temperature.
- Ignoring reactance and power factor on larger AC feeders.
- Treating voltage drop as a substitute for ampacity, protection, or terminal-temperature checks.
Reference Notes and Assumptions
The simplified formulas use resistance-based voltage drop for DC, single-phase, and balanced three-phase circuits. Use conductor resistance from manufacturer data or an applicable electrical table, and verify installation requirements with the adopted electrical code.