Voltage Drop Calculation Guide: NEC 2026 Standards
Voltage drop is the silent killer of electrical motors and sensitive electronics. For runs over 100 feet, standard wire sizing often fails to meet code recommendations.
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and a total of 5% including the feeder. For a standard 120V circuit, a 3% drop is 3.6 Volts. If your calculated drop exceeds this, you must increase the wire gauge (upsize) to maintain equipment performance and safety.
The Physics of Voltage Drop
Every foot of wire adds resistance to an electrical circuit. As current (Amps) flows through that resistance, voltage is lost as heat. This is known as Ohm's Law in action ($V = I \times R$). For short runs inside a house, this loss is negligible. But for a detached garage, a well pump, or a long driveway gate, the loss can be enough to prevent a motor from starting or cause a computer to reboot randomly.
The "Rule of Thumb" vs. The Code
Many old-school electricians use the "100-foot rule" — if the run is over 100 feet, just go up one wire size. While often correct, it's not scientific. The NEC (National Electrical Code) doesn't technically enforce voltage drop as a safety violation for branch circuits (it's in the Informational Notes, like NEC 210.19(A)), but most local inspectors treat it as a requirement for "good workmanship."
Why 3%?
Sensitive electronics and electric motors are designed to operate within a specific voltage tolerance (typically +/- 10%). If you start with 120V at the panel but only have 110V at the outlet, a large motor (like a table saw or AC compressor) will draw more current to compensate for the lower voltage, causing it to run hotter and fail sooner.
AWG Circular Mils Reference Table
| Wire Size (AWG) | Circular Mils (CM) | Resistance (Ω/1k ft Cu) |
|---|---|---|
| #14 | 4,110 | 3.07 |
| #12 | 6,530 | 1.93 |
| #10 | 10,380 | 1.21 |
| #8 | 16,510 | 0.764 |
| #6 | 26,240 | 0.491 |
| #4 | 41,740 | 0.308 |
| #2 | 66,360 | 0.194 |