The ampacity of 4/0 copper wire is not a single fixed number; it depends entirely on the temperature rating of your terminations and the insulation type. For standard installations, the allowable ampacity is 230 amps (based on the 75°C column). However, if you are calculating derating factors for conduit fill, you start with the 90°C column value of 260 amps. If you are dealing with older equipment or NM-B cable limitations, the value drops to 195 amps (60°C column).

Below is the complete decision framework, data tables, and derating math you need to size 4/0 AWG copper correctly without failing inspection or creating a fire hazard.

Quick Reference: Ampacity of 4/0 Copper Wire

This data is pulled directly from NFPA 70: National Electrical Code (NEC) Table 310.16. Bookmark this row for your most common bench and jobsite lookups.

NEC Table 310.16 Allowable Ampacities for Copper Conductors (Ambient 30°C / 86°F)
Conductor Size (AWG/kcmil) 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
3/0 AWG 165 A 200 A 225 A
4/0 AWG 195 A 230 A 260 A
250 kcmil 215 A 255 A 290 A
Bench Note: 4/0 AWG copper has a bare diameter of 0.46 inches (11.68 mm) and a cross-sectional area of 107.2 mm². It is exceptionally stiff. When pulling it through conduit, use a high-quality wire pulling lubricant to avoid damaging the THHN/THWN-2 insulation jacket.

How to Read the NEC Table 310.16 Data

Before you cut and strip your wire, you must understand what the three temperature columns actually represent. The NEC does not allow you to simply pick the highest number. According to Cerrowire's official ampacity guidelines and NEC 110.14(C), the lowest temperature rating of any connected component, termination, or conductor dictates your baseline ampacity.

  • The 60°C Column (195A): Use this only if your wire is NM-B (Romex), if the equipment is rated strictly for 60°C, or if the equipment is 100A or less and the temperature rating is unmarked. (Note: 4/0 NM-B is rare and highly impractical; you will almost always be using THHN/THWN-2 in conduit).
  • The 75°C Column (230A): This is the industry standard for terminations. Almost all modern breakers, panelboard lugs, and disconnect switches rated 100A and above are tested and listed for 75°C terminations. This is your termination limit.
  • The 90°C Column (260A): THHN/THWN-2 and XHHW-2 wire insulation is rated for 90°C. However, you cannot use 260A as your final breaker size because the lugs will overheat. The 90°C column is used exclusively as the starting point for derating calculations.

Decision Path: Selecting the Correct Temperature Column

Use this decision tree to determine your final allowable ampacity before applying any environmental adjustments.

Installation Scenario Column to Use Base Ampacity Action / Next Step
Standard panel-to-panel feeder using THHN in PVC conduit, modern 200A breaker. 75°C 230 A Verify voltage drop. If under 3%, terminate and torque to spec.
More than 3 current-carrying conductors in a single raceway, or ambient temp > 86°F. 90°C (for derating math) 260 A Apply derating multiplier (see next section), then compare result to 75°C termination limit. Use the lower number.
Connecting to an older, unmarked piece of equipment or using NM-B cable. 60°C 195 A Do not exceed 195A. Upgrade equipment if 200A capacity is required.

Derating Factors: Adjusting for Conduit Fill and Ambient Heat

The base ampacity of 4/0 copper assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a raceway. When you violate either condition, you must derate the wire using the 90°C column as your starting point.

Worked Example: Conduit Fill Derating
Imagine you are pulling two sets of 120/240V single-phase feeders through a single 2-inch PVC conduit. That gives you 4 current-carrying conductors (the two hots from each feeder; neutrals carrying only unbalanced load do not count as CCCs in this specific single-phase scenario, but let's assume 4 CCCs for the math).

  1. Identify CCC count: 4 conductors.
  2. Find the multiplier: NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 CCCs.
  3. Apply to 90°C base: 260A × 0.80 = 208 Amps.
  4. Check termination limit: Your panel lugs are rated 75°C (230A).
  5. Final Ampacity: The NEC requires you to use the lower of the derated value or the termination value. Therefore, your final allowable ampacity is 208 Amps.
Safety Caveat: Because 208A is less than the standard 200A breaker continuous load threshold (which requires 125% sizing, meaning a 200A continuous load needs 250A of wire capacity), 4/0 copper is not sufficient for a 200A continuous load in a 4-CCC conduit. You must step up to 250 kcmil or 300 kcmil copper, or separate the runs into different conduits.

Ambient Temperature Derating
If your conduit runs across a rooftop or through a boiler room where the ambient temperature is 40°C (104°F), you apply the correction factor from NEC Table 310.15(B)(1). For 90°C wire at 40°C ambient, the factor is 0.91.
Math: 260A × 0.91 = 236.6A. Since 236.6A is still higher than the 75°C termination limit of 230A, the termination limit governs, and your final ampacity remains 230A.

Limitations: What the Ampacity Chart Cannot Tell You

Knowing that 4/0 copper is rated for 230A at 75°C does not guarantee a successful installation. The ampacity table ignores three critical physical and electrical realities:

1. Voltage Drop Over Distance

The NEC recommends a maximum 3% voltage drop for feeders. 4/0 copper has a resistance of roughly 0.0608 ohms per 1,000 feet at 75°C. If you are running a 200A load over 150 feet, your voltage drop will be approximately 5.4 volts on a 240V system (about 2.2%). This is acceptable. However, if the run extends to 250 feet, the drop hits 3.7%, exceeding the 3% guideline. In that scenario, you must upsize to 250 kcmil or 300 kcmil, regardless of the wire's thermal ampacity.

2. Physical Lug Fit and Torque

4/0 AWG wire is massive. Many older 100A or 150A breakers and lugs are not physically sized to accept a 0.46-inch diameter conductor. Forcing it can strip the lug threads or deform the wire strands, creating a high-resistance hotspot. Furthermore, NEC 110.14(D) requires you to torque terminations to the manufacturer's specified value. For a standard 200A panel lug accepting 4/0 copper, this is typically between 250 and 300 inch-pounds. Use a calibrated torque wrench; guessing the tightness is a leading cause of thermal failure at the panel.

3. Short-Circuit Withstand Rating

Ampacity measures continuous thermal capacity, not fault survival. Under a 10,000A short-circuit event, the breaker must clear the fault before the 4/0 copper melts or vaporizes. Ensure your overcurrent protective device (OCPD) has an Adequate Interrupting Rating (AIR) for your specific service transformer's available fault current.

The Default Pick for 200A Services

If you are wiring a standard 200-amp residential service entrance or a subpanel feeder under 100 feet, buy 4/0 AWG copper THHN/THWN-2. Use the 75°C column (230A) for your termination check. Torque your panel lugs to the manufacturer's exact spec (usually 250-300 in-lbs). If your run exceeds 100 feet, calculate voltage drop; if it drops below 234V at the load under full draw, step up to 250 kcmil copper.