The base ampacity of 4/0 AWG copper wire is 230 Amps in the 75°C column and 260 Amps in the 90°C column, per NEC Table 310.16. However, for almost all standard residential and commercial terminations—including main service panels, subpanels, and standard lugs—you must use the 75°C column (230A) as your final allowable ampacity due to equipment termination limits.
While 4/0 copper is a heavyweight conductor used for 200A and 225A feeders, reading the ampacity table is only the first step. Real-world installations require adjusting for ambient heat, conduit bundling, and voltage drop. Below is the complete reference data, derating procedures, and answers to the most common field questions.
How to Read the 4/0 AWG Copper Ampacity Table
Before pulling wire, you need to know how to navigate the National Electrical Code (NEC) ampacity tables. The table is divided by conductor material (copper vs. aluminum) and temperature rating (60°C, 75°C, and 90°C). The temperature rating corresponds to the insulation type stamped on the wire jacket, such as THHN, XHHW-2, or UF-B.
Bookmark this quick-reference rule: Always calculate derating adjustments using the 90°C column (if your wire is rated for it), but never exceed the 75°C column value for your final termination limit unless the breaker or lug is explicitly marked for 90°C. Standard copper building wire like THHN/THWN-2 is dual-rated 75°C/90°C, making it the most versatile choice for heavy feeders.
| Temperature Rating | Common Insulation Types | 4/0 Copper Ampacity | Application Notes |
|---|---|---|---|
| 60°C (140°F) | TW, UF-B | 195 Amps | Used for older installations or specific underground direct-burial cables. |
| 75°C (167°F) | RHW, THHW, THW, THWN, XHHW, USE | 230 Amps | The standard termination limit. Use this value for sizing breakers and lugs. |
| 90°C (194°F) | THHN, THWN-2, XHHW-2 | 260 Amps | Used exclusively as the starting baseline for derating calculations. |
Derating 4/0 Copper: When the Base Value Drops
The 230A and 260A figures assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a raceway. When conditions change, the NEC requires you to apply correction and adjustment factors.
How derating modifies the base value: Derating is applied to the 90°C column value. If you pull four 4/0 AWG THHN copper wires through a single conduit (e.g., two hots, one neutral, one ground for a 120/240V split-phase feeder), you have three current-carrying conductors. If you add a second circuit, you hit four current-carrying conductors, triggering an 80% adjustment factor per NEC 310.15(C)(1).
- Step 1 (Base 90°C): 260 Amps
- Step 2 (Adjustment): 260A × 0.80 = 208 Amps
- Step 3 (Termination Check): Compare 208A to the 75°C termination limit (230A). Since 208A is lower, your final allowable ampacity is 208 Amps. You can no longer use this wire on a 225A breaker; it must be protected at 200A.
NEC Table 310.16 only covers thermal limits. It does not account for:
- Voltage Drop: Pushing 200A through 4/0 copper over a 150-foot run will result in a voltage drop exceeding the recommended 3% limit for feeders. You must upsize to 250 kcmil or 350 kcmil for long runs.
- Conduit Fill Limits: 4/0 wire is physically massive. Pulling three 4/0 conductors plus a #4 AWG ground requires a minimum of 1.5-inch PVC or EMT conduit per NEC Chapter 9 fill tables. Forcing them into a 1.25-inch pipe will damage the insulation.
- The Weakest Link Rule: Per NEC 110.14(C), your circuit ampacity is limited by the lowest temperature rating of any connected component. If your wire is 90°C but the breaker lug is rated 60°C, you are legally bound to the 60°C column (195A).
4/0 Copper Wire Ampacity FAQ
Can I use 4/0 copper wire for a 200-amp residential service?
Yes, 4/0 copper is more than adequate for a 200A service, as its 75°C rating is 230A. However, it is often overkill for standard residential service entrance conductors. Under the residential service exception in NEC 310.12(A), the minimum wire size for a 200A residential service is actually 2/0 AWG copper (rated 175A, but legally permitted for 200A residential feeds). Electricians typically only upgrade to 4/0 copper for a 200A service if the run from the meter to the panel is exceptionally long (to mitigate voltage drop) or if the installation is commercial, where the 310.12 exception does not apply.
What size breaker or lug can I terminate 4/0 copper into?
Because standard breakers and panelboard lugs are rated for 75°C, you must use the 75°C ampacity of 230A. This means 4/0 copper can be safely terminated on a 200A or 225A breaker. If you are landing the wire on a 250A breaker, 4/0 copper is insufficient (you would need 250 kcmil copper, rated 255A at 75°C). Always torque the lug to the manufacturer's specified inch-pound rating using a calibrated torque wrench; 4/0 copper requires significant mechanical pressure to ensure a low-resistance connection and prevent thermal runaway.
How does high ambient temperature affect 4/0 copper in an attic?
If you are routing a 4/0 feeder through an attic where the ambient temperature regularly hits 110°F (43°C), you must apply an ambient temperature correction factor. For 90°C THHN wire at 110°F, the correction factor is 0.87.
Math: 260A (90°C base) × 0.87 = 226A.
Because 226A is still below the 230A termination limit, the wire remains valid for a 200A or 225A circuit. However, if the attic reaches 122°F (50°C), the factor drops to 0.82 (260 × 0.82 = 213A), which still passes for a 200A breaker but leaves very little margin for error.
Should I use 4/0 copper or 4/0 aluminum for a 200A feeder?
This depends on your budget and termination hardware. 4/0 AWG aluminum (XHHW-2) is rated for only 180 Amps at 75°C. Therefore, 4/0 aluminum cannot be used for a standard 200A feeder (you would need to step up to 250 kcmil aluminum, rated 205A).
If you specifically want to use 4/0 AWG wire, you must use copper to achieve the 230A rating required for a 200A or 225A breaker. While 4/0 copper is significantly more expensive per foot than aluminum, it is easier to bend, requires smaller conduit, and does not require anti-oxidant paste or specialized aluminum-rated lugs.
