The Direct Answer: 3/0 Copper Wire Ampacity at a Glance
For standard installations, 3/0 AWG copper wire has an allowable ampacity of 200A when terminated at standard 75°C equipment. While the 90°C insulation column technically allows 225A, National Electrical Code (NEC) termination rules almost always force you to use the 75°C rating for overcurrent protection and breaker sizing. If you are pulling wire for a 200A residential service entrance or a heavy-duty subpanel feeder, 3/0 copper is your baseline minimum.
Bookmark Quick-Jump: For a standard 200A main breaker, buy 3/0 AWG XHHW-2 or THHN copper and terminate it using the manufacturer's specified torque (typically 250–300 in-lbs). Do not use the 90°C column to size your breaker.
| Insulation Type | 60°C Column | 75°C Column (Standard Termination) | 90°C Column (Derating Only) | Max Standard Breaker |
|---|---|---|---|---|
| THHN / THWN-2 | 165A | 200A | 225A | 200A |
| XHHW-2 | 165A | 200A | 225A | 200A |
| THW / THWN | 165A | 200A | N/A | 200A |
| USE-2 (Underground) | N/A | 200A | 225A | 200A |
How to Read the NEC 310.16 Ampacity Table
The ampacity table in NFPA 70 (National Electrical Code) Table 310.16 is divided into three temperature columns: 60°C, 75°C, and 90°C. The most common mistake DIYers and junior apprentices make is looking at the 90°C column because modern wire insulation (like THHN) is rated for it, and then sizing the breaker for 225A. This will fail inspection.
Which column applies to your installation? Under NEC 110.14(C), the ampacity of the circuit is limited by the lowest temperature rating of any connected component. Standard breakers, lugs, and busbars in residential and light commercial panels are tested and rated for 75°C terminations. Even if your wire insulation can handle 90°C, the breaker lug cannot. Therefore, the 75°C column (200A) dictates your maximum continuous load and overcurrent device rating.
The 90°C column is not useless, however. It exists specifically to give you a higher mathematical baseline when you are forced to apply derating factors for heat or bundling, which we cover next.
Applying Derating Factors to 3/0 Copper
Ampacity tables assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway. When your installation deviates from these assumptions, the base ampacity must be reduced. This is where the 90°C column saves your project from requiring a massive wire upsizing.
Derating modifiers are found in NEC Table 310.15(B)(1) for ambient temperature and Table 310.15(C)(1) for conductor bundling. You multiply the base 90°C ampacity by these percentages. If the final derated number drops below your required load (e.g., 200A), the wire is too small.
Worked Derating Example: You are running a 200A feeder to a detached garage through a hot attic (45°C / 113°F ambient). You are pulling four current-carrying conductors (two hots, one neutral, one ground is not counted, but let's say you have a multi-wire branch circuit sharing the conduit, totaling 4 current-carrying wires).
- Base 90°C Ampacity (3/0 Cu): 225A
- Temperature Correction (45°C for 90°C wire): 0.91 multiplier
- Bundling Adjustment (4 conductors): 0.80 multiplier
- Calculation: 225A × 0.91 × 0.80 = 163.8A
Result: 163.8A is below your 200A requirement. 3/0 copper fails this specific run. You must upsize to 4/0 AWG copper or 250 kcmil to maintain the 200A capacity after derating.
Decision Path: Which 3/0 Copper Insulation and Breaker to Choose
Use this decision matrix to lock in your materials list before heading to the supply house. This path terminates in a concrete pick based on your specific physical environment.
| Installation Scenario | Wire Insulation Pick | Conduit / Raceway Requirement | Breaker & Termination |
|---|---|---|---|
| Standard Indoor Service Entrance (Panel to meter, dry, <30°C, max 3 conductors) | 3/0 AWG THHN/THWN-2 | 2-inch PVC or EMT minimum | 200A Main Breaker, 75°C lug rating |
| Outdoor / Underground Conduit (Wet locations, direct burial transitions) | 3/0 AWG XHHW-2 or USE-2 | Schedule 80 PVC where exposed to physical damage | 200A Breaker, ensure lugs are rated for wet/dry |
| Hot Environment / Bundled (Attics >113°F, or >3 current-carrying wires in one pipe) | REJECT 3/0. Upsize to 4/0 AWG XHHW-2 | Upsize conduit to 2.5-inch to manage heat and pull tension | 200A Breaker (4/0 derates safely above 200A) |
The Default Concrete Pick: If you are wiring a standard 200A residential service entrance in a typical climate, purchase 3/0 AWG XHHW-2 copper. XHHW-2 has a thinner insulation profile than THHN, making it significantly easier to pull through conduit sweeps and bend into tight panel gutters, while offering identical 75°C/90°C ampacity ratings and superior moisture resistance.
What the Ampacity Table Cannot Tell You
Knowing that 3/0 copper handles 200A is only half the battle. The NEC ampacity tables do not account for physical constraints or electrical efficiency over distance. Before you cut your first wire, verify these three hidden variables.
1. Voltage Drop Over Distance
Ampacity measures heat dissipation, not voltage delivery. If your 200A subpanel feeder is more than 100 feet long, 3/0 copper will suffer from excessive voltage drop under heavy load. The industry standard (and NEC informational note recommendation) is to keep voltage drop under 3% for feeders. According to the Southwire Voltage Drop Calculator, a 200A load at 240V on 3/0 copper over 150 feet results in a 4.1% drop. For runs exceeding 120 feet at full load, upsize to 4/0 AWG or 250 kcmil aluminum (which requires upsizing to 350 kcmil for equivalent copper ampacity and drop management).
2. Physical Bend Radius and Gutter Space
3/0 AWG copper is exceptionally stiff. NEC 300.34 mandates specific bending space in enclosures to prevent insulation damage and excessive stress on busbars. A standard 200A load center often has very tight gutter space. If you do not leave enough slack to form a gentle, sweeping bend (minimum bend radius is typically 5 times the wire diameter for shielded cable, but practically requires a wide sweep for unshielded THHN), you will crack the insulation or fail to seat the wire fully into the breaker lug. Always use a mechanical wire bender or a heavy-duty ratcheting bender for 3/0; hand-bending will result in kinks.
3. Termination Torque Requirements
NEC 110.14(D) requires that terminations be tightened to the manufacturer's specified torque using a calibrated instrument. You cannot 'feel' the correct tightness on a 3/0 lug. Under-torquing creates a high-resistance connection that will arc, overheat, and melt the breaker lug under a 150A+ continuous load. Over-torquing can strip the lug threads or crush the copper strands, reducing the effective cross-sectional area. Check the breaker datasheet (e.g., Square D or Eaton); a 200A main breaker lug typically requires between 250 and 300 inch-pounds of torque. Use an insulated torque screwdriver or a beam-style torque wrench calibrated in inch-pounds to secure the connection.
For further reading on termination limits and temperature ratings, reference the Cerrowire Ampacity Charts and always cross-reference with your local Authority Having Jurisdiction (AHJ), as local amendments can occasionally supersede baseline NEC guidance.
