100 Amp Wire Size Calculator
How it Works
01100 Amp Service
Main service panel, subpanel, or detached garage feeder.
02240V Feeder
Standard US residential service at 240/120V split-phase.
03Feeder Length
From utility or main panel to subpanel. Typical 50–200 ft.
04AWG Size
Typical 100A feeder: 3 AWG copper or 1 AWG aluminum.
What is a 100 Amp Wire Size Calculator?
A 100 amp wire size calculator — also called a 100A service conductor sizer or feeder calculator — tells you the exact AWG wire you need for a 100 amp main panel, subpanel, or detached-building feeder. In US residential electrical work, a 100A service is the most common size for single-family homes built before 2000, ADUs (accessory dwelling units), detached garages with workshops, and subpanels serving basement or outdoor areas. Getting the wire size right the first time is critical: oversized wire wastes hundreds of dollars in copper; undersized wire runs hot, trips breakers, and fails inspection.
This tool uses the NEC Table 310.16 ampacity chart — the code standard adopted by every US AHJ (authority having jurisdiction) — combined with the real-world voltage-drop math electricians use daily. For a typical 100A service at 240V split-phase, the answer is 3 AWG copper at the 75°C column or 1 AWG aluminum at the same column. But the story changes when the feeder run gets long: anything over roughly 75 feet from panel to panel starts pushing the copper choice to 2 AWG, and aluminum jumps to 1/0, just to keep voltage drop under the NEC-recommended 3% for feeders.
Inputs are simple: run length (one-way), conductor material (copper or aluminum), and insulation temperature rating (60°C, 75°C, or 90°C column from Table 310.16). Output includes required AWG, calculated voltage drop percentage, ampacity margin, and whether the result is code-minimum or upsized for drop. Great for homeowners planning a service upgrade, electricians estimating a job, or building departments verifying a permit application.
This calculator handles the classic 100A residential scenarios: main panel to main breaker, main panel to detached-building subpanel (shop, garage, ADU), and 100A feeders to commercial tenant spaces. For services larger than 100A (200A, 400A) or three-phase applications, use the main wire-size calculator instead.
"How the 100A Wire Size Calculator Works
100A Wire Size Formula
NEC 310.16 ampacity table at 75°C:
- 3 AWG copper = 100A ampacity (baseline)
- 1 AWG aluminum = 100A ampacity (baseline)
For long runs, voltage drop math: cmils = (2 × K × L × 100) ÷ maxDrop. Keep under 5% for feeders.
Example: 100A Feeder, 150 ft, Aluminum
100A subpanel, 150 ft feeder run, aluminum (cheaper for long runs), 5% max drop = 12V.
- Min cmils = (2 × 21.2 × 150 × 100) ÷ 12 = 53,000 cmils
- 2 AWG Al = 66,360 cmils → passes, but ampacity only 90A (aluminum)
- 1/0 Al = 105,600 cmils → 120A ampacity → passes both
Always upsize one AWG class for long aluminum feeders.
100A use cases
100A Wire Size by Distance
| Distance | Copper (75°C) | Aluminum (75°C) |
|---|---|---|
| Under 50 ft | 3 AWG | 1 AWG |
| 50-100 ft | 3 AWG | 1 AWG |
| 100-150 ft | 2 AWG | 1/0 AWG |
| 150-200 ft | 1 AWG | 2/0 AWG |
| 200+ ft | 1/0 AWG | 3/0 AWG |
Key Takeaways
Sizing a 100A feeder correctly is the single most important electrical decision in a service upgrade or detached-building wire pull. The default answer — 3 AWG copper or 1 AWG aluminum — works for short runs under 75 feet, but any longer pull demands a voltage-drop check. The calculator handles this automatically: enter the actual panel-to-panel distance and it tells you whether you can stay with the baseline or need to upsize to 2 AWG copper / 1/0 aluminum.
Beyond the wire itself, a 100A feeder pull requires a matched neutral (same size as the hots), a ground wire per NEC 250.66 (typically 8 AWG copper for 100A), a properly sized conduit (1.5-inch PVC is standard for 100A feeders), and a 100A-rated disconnect if the feeder supplies a detached structure. Service-entrance wiring is permit-required work in every US jurisdiction — use this calculator to size it right, then hire a licensed electrician to pull the permit, make the connections, and handle the utility coordination.
"100A Wire Size FAQs
What size wire for 100 amp service?
What size wire for 100 amp subpanel?
Can I use 4 AWG for 100 amps?
What size wire for 100 amp 200 feet?
What size conduit for 100A wire?
What is the cheapest wire for 100A service?
What breaker size for 100A wire?
Can I run 100A feeder underground?
Do I need a ground rod for 100A subpanel?
What is 100A service enough for?
Author Spotlight
The ToolsACE Team
Our specialized research and development team at ToolsACE brings together decades of collective experience in financial engineering, data analytics, and high-performance software development.
Disclaimer
Educational reference. Service-entrance and feeder wire sizing is permit-required work. Always hire a licensed electrician and pull the permit.