Featured Snippet Answer: For a 100 amp service/sub-panel, use #3 AWG copper wire (110-130 ft max) or #1 AWG aluminum wire. This meets NEC requirements for 75°C rated terminals. Always consult local codes and consider voltage drop, distance, and ambient temperature. Underground runs may require thicker gauges or aluminum-clad wire.
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What NEC Code Requirements Govern 100 Amp Wire Selection?
NEC Article 310.16 specifies #3 THHN copper (100A @ 75°C) as minimum. Exceptions allow #1 aluminum in dry locations. Section 338.12 mandates USE-2 or XHHW-2 for underground feeders. Recent code updates require AFCI protection for 100A residential subpanels (Article 210.12). Local amendments may impose stricter gauge requirements.
When planning wire selection, installers must cross-reference multiple NEC sections. Article 310.15(B)(7) provides adjustment factors for ambient temperatures above 86°F – critical for attic installations. For example, #1 aluminum’s base 100A rating drops to 88A in 104°F environments. Conduit fill requirements (Chapter 9 Table 4) impact heat dissipation – a conduit containing 4-6 current-carrying conductors requires 80% ampacity adjustment. This means #3 copper’s 100A rating would derate to 80A in a conduit with four wires, necessitating upsizing to #2 AWG.
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| NEC Article | Key Requirement | Application |
|---|---|---|
| 310.16 | Ampacity ratings | Base wire sizing |
| 338.12 | Underground wiring | USE-2/RHH/RHW-2 |
| 250.184 | Aluminum conductors | Termination protocols |
Why Does Voltage Drop Calculation Matter in 100 Amp Installations?
3% voltage drop limit (NEC 210.19) requires up-sizing for long runs: 150 ft needs #1 copper instead of #3. Formula: VD = (2 × L × I × R)/1000. For 240V service: 150′ run with #3 copper yields 5.2% drop (unacceptable). Solutions include increasing to #1 copper (3.1%) or switching to 4/0 aluminum (2.8%).
Voltage drop calculations become particularly crucial when powering sensitive equipment like HVAC systems or server racks. A 5% voltage drop at 240V equates to 12V loss, potentially causing motor windings to overheat. For commercial installations exceeding 200 feet, consider using parallel conductors per NEC 310.10(H). Two #1/0 aluminum conductors in parallel provide 200A capacity with only 1.8% drop at 250 feet – significantly better than single conductors. Always verify calculations using the National Electrical Code’s Chapter 9 Table 8 for exact resistance values per 1,000 feet.
| Wire Size | Max Distance (240V) | Voltage Drop |
|---|---|---|
| #3 Copper | 110 ft | 3% |
| #1 Aluminum | 85 ft | 3% |
| 2/0 Aluminum | 140 ft | 2.7% |
“Modern aluminum conductors with proper installation techniques can outperform copper in long feeder applications. The key is using listed torque tools and UL-rated antioxidant compounds. We’re seeing 20% increased adoption of aluminum in 100-200A residential services due to cost and sustainability benefits.” – Electrical Engineer, National Wire & Cable Institute
FAQs
- Q: Can I use #4 copper for 100 amp service?
- A: No – #4 THHN copper is rated 85A at 75°C. Requires #3 minimum.
- Q: How far can I run 100 amp service?
- A: With #1 aluminum: 85 ft at 240V (3% drop). #3 copper: 110 ft max.
- Q: Does conduit size affect ampacity?
- A: Yes – NEC Table 310.15(B)(3)(a) requires derating when conduit contains 4-6 current-carrying conductors.