The 4/0 copper amp rating is the maximum continuous current a 4/0 AWG (0.46-inch diameter) copper conductor can safely carry without exceeding its insulation temperature limits, which is 230 amps in the standard 75°C NEC column.

What this number changes in a real installation is whether your heavy feeder runs—like a 200A main service or a large detached garage subpanel—will pass inspection, avoid tripping from thermal buildup, and prevent melting the breaker lugs. Undersizing this wire doesn't just risk a fire; it guarantees a failed inspection and a costly re-pull. Before we look at the math, remember that any work on main service panels involves lethal utility-side voltage. Always de-energize, verify dead with a tested meter, and consult your local Authority Having Jurisdiction (AHJ), as local code always has final authority over general NEC-style guidance.

The 4/0 Copper Amp Rating: Definition and Core Numbers

When you buy 4/0 AWG (pronounced "four-aught") copper wire, you are buying a conductor with a cross-sectional area of 211,600 circular mils. The ampacity is not a single fixed number; it shifts based on the thermal rating of the insulation and the environment. According to NEC Table 310.16, the allowable ampacities for 4/0 copper are:

Temperature Column Common Insulation Types Ampacity (Copper)
60°C (140°F) TW, UF 195 Amps
75°C (167°F) THHW, THWN, XHHW 230 Amps
90°C (194°F) THHN, THWN-2, XHHW-2 260 Amps

For almost all modern residential and commercial feeder applications using THHN/THWN-2 in conduit, you will be looking at the 75°C or 90°C columns. However, as we will cover next, the 90°C column is rarely used for the final breaker sizing.

The Temperature Column Trap (And What People Confuse It With)

The most common mistake DIYers and junior apprentices make is looking at the 90°C column (260A for 4/0 copper) and assuming they can protect the wire with a 250A breaker. This violates NEC 110.14(C) regarding termination temperature limits.

The Termination Rule: Most standard breakers and panel lugs are only rated for 75°C terminations. Even if your wire insulation is rated for 90°C (like THHN), the ampacity for overcurrent protection sizing is capped at the 75°C column (230A) because the heat will transfer from the wire into the breaker lug and degrade the mechanical connection.

You can use the 90°C column for derating. If you have multiple current-carrying conductors in a single conduit, you start your derating math at 260A (90°C), apply your adjustment factors, and then verify that the final derated number doesn't drop below your 75°C termination requirement.

What people commonly confuse it with: People frequently confuse 4/0 AWG with 4 AWG. 4 AWG copper is a vastly smaller wire (rated for only 85A at 75°C). Another massive confusion is assuming 4/0 Aluminum has the same rating. 4/0 Aluminum is only rated for 205A at 75°C, which is why it is technically undersized for a standard 200A residential service unless specific high-temp terminations are used.

Worked Numeric Example: Sizing a 200A Service Feeder

Let's run the actual math for a classic scenario: pulling a 200A residential service feeder from the meter base to the main panel, with a one-way run distance of 150 feet. We are using 4/0 AWG Copper THHN in PVC conduit.

Step 1: Verify Ampacity
A 200A main breaker requires wire rated for at least 200A at 75°C. 4/0 Copper at 75°C is rated for 230A. (Passes).

Step 2: Calculate Voltage Drop
NEC recommends a maximum 3% voltage drop for feeders. We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM

  • K (Copper resistivity at 75°C) = 12.9 ohms per mil-foot
  • I (Current) = 200 Amps
  • D (Distance) = 150 feet
  • CM (Circular Mils for 4/0) = 211,600

VD = (2 × 12.9 × 200 × 150) / 211,600
VD = 774,000 / 211,600 = 3.65 Volts

On a 240V system, a 3.65V drop is 1.52%. This is well under the 3% threshold. 4/0 Copper is the perfect, code-compliant choice for this run without needing to upsize to 250 kcmil.

Where You Meet 4/0 Copper in Practice

You won't find 4/0 wire in standard branch circuits. This is heavy infrastructure wire. You will encounter it in:

  • 200A Residential Main Services: The standard feed from the utility meter to the main breaker panel.
  • 400A Service Splits: Modern large homes often use a 400A meter socket that splits into two 200A panels. Each leg uses 4/0 copper.
  • Large Subpanels: Feeding a 200A subpanel in a detached workshop or an EV charging hub.
  • Solar Inverter Outputs: High-capacity solar arrays (e.g., 20kW+ systems with battery backups) pushing continuous high current to the main busbar.
  • Heavy Machinery: Feeders for large welders, CNC plasma tables, or commercial HVAC compressors.
Pro-Tip for Pulling: 4/0 copper is incredibly stiff. When pulling three or four strands through 2-inch PVC conduit, use a high-quality wire pulling lubricant (like Polywater) and a pulling grip. Never bend 4/0 copper to a radius tighter than the manufacturer's minimum (usually 8x the cable diameter), or you will stress-fracture the copper strands inside the insulation.

Decision Tree: Should You Actually Use 4/0 Copper?

Copper is expensive. In 2026, a 500-foot spool of 4/0 copper THHN can easily exceed $1,500, whereas aluminum is a fraction of the cost. Use this decision path to make your final material call.

Your Scenario Constraint / Requirement Material & Size Pick
200A Main Service, short run (<100 ft) Must fit in standard 2" conduit, standard 75°C lugs 4/0 Copper
200A Subpanel, long run (>150 ft) Voltage drop is a concern, budget is tight 250 kcmil Aluminum (Upsized for VD and cost)
225A or 250A Feeder Exceeds 200A, standard 75°C terminations 250 kcmil Copper (Rated 255A at 75°C)
200A Service, tight budget, large conduit OK Need 200A, willing to pull larger wire 250 kcmil Aluminum (4/0 Al is only 205A, too close to the 200A limit for continuous loads)
The Default Pick: If you are wiring a standard 200A residential service or subpanel under 150 feet, buy 4/0 AWG Copper THHN. It fits easily in 2-inch conduit, terminates perfectly in standard 75°C lugs, provides a 30A safety buffer over the 200A breaker, and keeps voltage drop under 2%. Do not mess with aluminum unless the run exceeds 150 feet and you are trying to save money on the wire cost (factoring in the larger conduit required for Al).

Frequently Asked Questions

Can I use 4/0 copper on a 225A breaker?

No. At 75°C, 4/0 copper is rated for 230A. While 230A is technically higher than 225A, NEC 240.4(B) allows the "next size up" rule for overcurrent protection only if the calculated load does not exceed the wire ampacity. However, standard breaker sizes jump from 200A to 225A to 250A. If your calculated continuous load requires a 225A breaker, you should step up to 250 kcmil copper (rated 255A at 75°C) to maintain a safe thermal margin and avoid inspector pushback.

Does the ground wire need to be 4/0 as well?

Absolutely not. According to NEC Table 250.122, the minimum equipment grounding conductor for a 200A overcurrent device is 6 AWG copper or 4 AWG aluminum. Sizing your ground to match your current-carrying conductors is a massive waste of money and makes pulling the wire through conduit unnecessarily difficult.

How do I strip 4/0 copper without nicking it?

Do not use standard wire strippers or a utility knife, which will score the copper and create a hot spot. Use a dedicated heavy-duty cable stripper (like the Knipex or Greenlee stepped blade strippers) or carefully score the THHN insulation longitudinally with a specialized stripping tool, then peel it back. Always leave the proper insulation setback (usually 1/16" to 1/8" from the lug barrel) to prevent arcing.