NEC Voltage Drop
A worked scenario applying the NEC’s voltage drop guidance to a genuinely long residential circuit.
Here's a worked NEC voltage drop scenario on a residential feeder long enough that voltage drop genuinely becomes the binding constraint.
| Parameter | Value |
|---|---|
| Load | 30A, 120/240V single-phase |
| Run length | 100 feet, one-way |
| Cable under test | 8 AWG copper |
At 100 feet, resistive voltage loss accumulates enough that even an ampacity-adequate conductor can approach or exceed the NEC's 3% branch circuit guidance. Since that guidance is an Informational Note rather than a numbered rule (see the dedicated article on this distinction), it's still worth checking rigorously — many AHJs and utility specs enforce it as a hard limit regardless of its technical status in the code text.
Long residential feeders — to a detached garage, a well pump, or a remote sub-panel — are exactly where "the wire is rated for the current" stops being sufficient reasoning on its own.
Run your own feeder length and load through the NEC Voltage Drop calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks