Feeder Wire Size Calculator — NEC 215.2

Calculate feeder conductor size and service entrance wire size per NEC 215.2 and Table 310.15(B)(16). Enter load amps, system voltage, feeder length, and conductor material. The calculator finds the smallest AWG that satisfies both the ampacity requirement and your allowable voltage drop — for copper or aluminum feeders at any voltage used in US residential and commercial installations.

Feeder Details

Results

Recommended Feeder Size —
Conductor Ampacity (75°C) —
Calculated Voltage Drop —

Ampacity: NEC Table 310.15(B)(16), 75°C column. Resistance: NEC Table 9. Always verify against current code and local amendments.

Worked Example — 100A Garage Subpanel, 60 ft from Main Panel
1Given: 100A subpanel in a detached garage, 60 ft from the main panel. Using aluminum conductors in PVC conduit.
2Ampacity (NEC 215.2): Feeder conductors must be rated ≥ 100A at 75°C. #2/0 AWG aluminum = 135A ✅ (next size up from #1/0 = 120A which also works). Per NEC 240.4(B), 120A ≥ 100A so #1/0 Al is acceptable.
3Voltage drop check: VD = (2 × 21.2 × 100 × 60) / 211,600 (for #1/0 Al in DCR terms) ≈ 1.2% ✅ Well within 3%.
4Overcurrent protection (NEC 215.3): 100A breaker in main panel protects the feeder. Subpanel main breaker also 100A.
✅ 100A garage subpanel at 60 ft: #1/0 AWG aluminum conductors + 100A double-pole feeder breaker. Voltage drop ≈ 1.2% — well within NEC recommendation.

Also try:

AWG wire size calculator for branch circuits voltage drop calculator for copper and aluminum wire ground wire size calculator — NEC Table 250.122 EV charger wire size guide — NEC 625.42 voltage drop reference table — color-coded NEC compliance

How to Use This Feeder Size Calculator

Enter the system voltage, load current, one-way feeder length, and conductor material. Select the allowable voltage drop — typically 3% for feeders per NEC 215.3 recommendation, or 5% if considering the combined feeder plus branch circuit budget. The calculator finds the smallest AWG that satisfies both the NEC ampacity requirement and your voltage drop limit.

For service entrance conductors, use the same inputs with the distance from the meter to the main panel. Also size the equipment grounding conductor using the ground wire size calculator based on the service overcurrent device rating.

Why Feeder Size Matters

Undersized feeder conductors restrict available current to all downstream branch circuits. Excessive voltage drop on the feeder means every load on the panel operates at reduced voltage, causing motors to run hot, lights to dim, and sensitive electronics to malfunction. Oversized conductors cost more and add weight but eliminate voltage problems.

Feeder sizing is one of the most consequential decisions in an electrical installation because it cannot easily be changed after the conduit is filled and walls are closed.

What Affects Feeder Wire Size

Load current and feeder length are the dominant factors. A short 100-amp feeder may fit in #1/0 AWG copper, but the same 100-amp feeder over 300 feet may require #3/0 AWG to keep voltage drop under 3%. Conductor material matters because aluminum has higher resistance and lower ampacity per AWG than copper. System voltage also affects the drop percentage — a 480V feeder sees much lower drop percentage than a 120V feeder at the same wattage.

Common Feeder Sizing Scenarios

  • 100-amp subpanel feeder to a detached garage or workshop
  • 200-amp service entrance conductors from the utility meter to the main panel
  • 60-amp feeder for a large HVAC unit, EV charging station, or hot tub
  • Feeder to a second main panel in a multi-building campus or farm
  • Long-run aluminum feeder for a rural property where distance is the limiting factor

Frequently Asked Questions

Per NEC Table 310.15(B)(16), a 100-amp feeder typically uses #1/0 AWG copper or #3/0 AWG aluminum at 75°C. The exact size depends on feeder length and allowable voltage drop.

A 200-amp service typically uses #4/0 AWG copper or 350 kcmil aluminum at 75°C per NEC Table 310.15(B)(16). Verify with voltage drop for the actual service entrance run length.

A feeder runs between the service equipment (or a separately derived system) and the last overcurrent device in a panel. Branch circuits supply loads directly. Feeders are sized per NEC 215.2 and branch circuits per NEC 210.19.

Yes. Aluminum is commonly used for feeder conductors and service entrance wire because it is lighter and less expensive. Aluminum feeders require proper termination methods and use-rated connectors to avoid corrosion issues.

The NEC recommends no more than 3% voltage drop on a feeder and 3% on a branch circuit, with a combined maximum of 5%. These are recommendations (not hard requirements), but are considered engineering best practice.

Per NEC Table 310.15(B)(16), a 60-amp feeder requires minimum #6 AWG copper (65A rated) or #4 AWG aluminum (65A rated) at 75°C. Account for feeder length and voltage drop before finalizing the size.