Table of Contents
What Is Voltage Drop?
Voltage drop is the reduction in voltage across a conductor as electrical current flows through it due to the conductor's resistance. All conductors have some resistance, and by Ohm's law (V=IR), current through this resistance creates a voltage drop. Excessive voltage drop causes equipment malfunction, reduced lighting output, and motor overheating.
The National Electrical Code (NEC) recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder plus branch circuit). Many engineers design for even lower drop to ensure reliable operation.
Voltage Drop Formula
Where L is one-way length in feet, R is resistance in ohms per 1000 feet, I is current in amps, and the factor of 2 accounts for the round-trip (supply and return conductors).
Copper Wire Resistance (ohms/1000ft at 75°C)
| AWG | Ω/1000ft | Max Amps (NEC) |
|---|---|---|
| 14 | 3.14 | 15 |
| 12 | 1.98 | 20 |
| 10 | 1.24 | 30 |
| 8 | 0.778 | 40 |
| 6 | 0.491 | 55 |
Frequently Asked Questions
What is an acceptable voltage drop?
NEC recommends no more than 3% for branch circuits and 5% total. For sensitive electronics, aim for under 2%. For motor circuits, excessive drop can prevent starting.
How do I reduce voltage drop?
Use larger gauge wire, shorten the run, increase the voltage (240V vs 120V halves the current), or split loads across multiple circuits. Each increase in wire gauge roughly halves the resistance.
Does AC or DC matter?
For DC and single-phase AC at power frequencies, the calculation is the same. For three-phase AC, multiply the single-phase drop by 0.866. At high frequencies, skin effect increases effective resistance.