NEC 215 — Feeder Conductor Sizing and the Voltage Drop Recommendation
NEC 215 governs feeders — the conductors between service equipment and a sub-panel's main breaker. Continuous-load multipliers, voltage drop recommendation, and how feeder sizing differs from branch-circuit sizing.
Article 215 covers feeders — the conductors that run from the main service equipment to a sub-panel. Feeders show up on every panel-upgrade-with-detached-garage job, every basement sub-panel, every EV-load-shed sub-panel install.
Feeder ampacity (215.2): feeder conductors must have an ampacity not less than the noncontinuous load + 125% of the continuous load they serve. This is the same continuous-load multiplier you apply to branch circuits — long-running loads get the 125% bump.
Standard residential sub-panel feeder sizes:
- 100A sub-panel: #3 CU or #1 AL THHN/THWN-2 (75°C column)
- 125A sub-panel: #1 CU or #1/0 AL
- 150A sub-panel: #1/0 CU or #2/0 AL
- 200A sub-panel: #2/0 CU or #4/0 AL
Voltage drop recommendation (215.2(A) Informational Note 2): the NEC recommends keeping feeder voltage drop at or below 3%. For runs over ~75 ft, voltage drop usually drives sizing past the ampacity-required minimum. A 100A sub-panel feeding a 100-ft detached garage typically needs upsizing to #1 AL or #2 CU.
Equipment grounding conductor (250.122 — see our EGC sizing guide): the EGC for the feeder is sized by the breaker protecting the feeder, not by the feeder ampacity. A 100A sub-panel feed needs an #8 CU EGC; a 200A feed needs #6 CU.
Neutral sizing for sub-panels: the neutral conductor in a sub-panel feeder must be sized for the maximum unbalanced load on the feeder. For most residential sub-panels, that's the same size as the phase conductors.
The "four-wire to sub-panel" rule: the feeder to a sub-panel is FOUR conductors — two hots, one neutral, one ground (EGC). The neutral and ground bars at the sub-panel are KEPT ISOLATED FROM EACH OTHER. Neutral floats; ground bonds to the panel can. Bonding the neutral at a sub-panel creates a parallel path and is the most-cited Hudson County rough-in failure.
For voltage drop checks on long feeder runs, use the free Malfettone Voltage Drop Calculator at /tools/voltage-drop.
This guide is an educational summary written by a licensed NJ master electrician. It is not a substitute for the National Electrical Code or for the judgment of your local AHJ. For real permit work, verify every code interpretation with your authority having jurisdiction and a licensed electrician of record.