300 kcmil THHN Copper Ampacity
300 kcmil copper THHN: 320A base at 90°C, 285A usable after 110.14(C) termination limit.
Base320A
Usable285A
NEC Table310.16
Full derating calculation
Applications
300 kcmil copper is commonly used for commercial main feeders (240-285A), hospital and data center power distribution.
Voltage drop by distance
| Distance (ft) | 240V Drop | % Drop | Verdict |
|---|---|---|---|
| 50 ft | 1.2V | 0.5% | OK |
| 75 ft | 1.8V | 0.8% | OK |
| 100 ft | 2.5V | 1.0% | OK |
| 150 ft | 3.7V | 1.5% | OK |
| 200 ft | 4.9V | 2.0% | OK |
| 250 ft | 6.1V | 2.5% | OK |
| 300 ft | 7.3V | 3.1% | Caution |
Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
Important: For reference only. Not a substitute for a licensed electrician. Electrical work can cause serious injury, death, fire, or property damage if performed incorrectly. Always hire a licensed electrician for electrical work, especially for service upgrades, panel work, and 240V circuits. Values are derived from NFPA 70 (NEC) for educational purposes. Always verify against your locally adopted NEC edition and amendments - local jurisdictions may enforce stricter requirements. WireRef provides reference information only and is not responsible for work performed based on this content. NEC® is a registered trademark of the National Fire Protection Association. Free NEC access via NFPA · OSHA Electrical Safety · Terms of use.
NEC 2023 references verified April 2026