What Size Wire for a 225 Amp Breaker? The Complete 225A Wire Size Guide

The minimum 225A wire size for a standard residential or commercial feeder is 4/0 AWG copper or 300 kcmil aluminum, assuming a 75°C termination rating and no more than three current-carrying conductors in a raceway. This heavy-duty feeder size bridges the gap between standard 200A residential services and 300A+ light commercial panels, requiring precise attention to NFPA 70 (NEC) ampacity tables, physical pulling limits, and voltage drop calculations. Getting the 225a wire size wrong doesn't just mean a failed inspection; it means risking melted lugs, excessive voltage drop on startup loads, or conduit jamming during the pull.

The Core NEC Rules for 225A Wire Sizing

When sizing conductors for a 225-amp breaker, you cannot simply look at the highest number in the ampacity tables. The National Electrical Code dictates that conductor sizing must be based on the temperature rating of the equipment terminations, not just the insulation on the wire. Per NEC 110.14(C), equipment rated 100A or greater is generally assumed to have 75°C terminations unless explicitly marked otherwise.

Even if you buy 90°C rated THHN or XHHW-2 wire, you must use the 75°C column of NEC Table 310.16 to determine your baseline ampacity for overcurrent protection sizing.

As shown in the table, 3/0 copper and 250 kcmil aluminum fall short of the 225A requirement when bound by the 75°C termination rule. You must step up to 4/0 copper or 300 kcmil aluminum to safely land on a 225A breaker.

Worked Numeric Example: Sizing a 225A Subpanel Feeder

Ampacity tables only tell half the story. If your feeder run is long, voltage drop will force you to upsize the wire beyond the minimum NEC ampacity requirements. The NEC recommends (and many local AHJs mandate) a maximum voltage drop of 3% for feeders.

The Scenario: You are running a 240V, 225A non-continuous feeder to a detached workshop subpanel. The one-way distance is 350 feet. You plan to use copper THHN in PVC conduit.

Step 1: Check minimum ampacity.
4/0 AWG Copper is rated 230A at 75°C. It clears the 225A breaker requirement.

Step 2: Calculate Voltage Drop (VD).
Using the standard single-phase VD formula: VD = (2 × K × I × D) / CM

• K (Copper resistivity at 75°C) = 12.9
• I (Current) = 225A
• D (Distance) = 350 ft
• CM (Circular Mils for 4/0 AWG) = 211,600

VD = (2 × 12.9 × 225 × 350) / 211,600
VD = 2,031,750 / 211,600 = 9.60 Volts

Step 3: Check the percentage.
(9.60V / 240V) × 100 = 4.0%

Step 4: Upsize and recalculate.
Let's jump to 300 kcmil Copper (CM = 300,000).
VD = 2,031,750 / 300,000 = 6.77 Volts
(6.77V / 240V) × 100 = 2.82%

Final Verdict: For a 350-foot run, the correct 225a wire size is 300 kcmil Copper, not the minimum 4/0 AWG. Always run your specific dimensions through a voltage drop calculator before buying wire.

Where You Meet This in Practice

Theory is clean; the jobsite is not. When you move from a 200A service to a 225A or 250A feeder, the physical realities of the wire change the installation drastically.

• Conduit Fill and Jamming: Three 4/0 AWG THHN conductors technically fit inside a 2-inch Schedule 40 PVC conduit based on the 40% fill rule in NEC Chapter 9. However, pulling three 350-foot sticks of 4/0 copper through 2-inch pipe with bends is a nightmare that often results in jamming or damaged insulation. In practice, bump the conduit to 2.5-inch or 3-inch PVC to reduce pulling tension and friction.
• Termination Torque: 4/0 and 300 kcmil wires require significant mechanical force to seat properly. You must use a calibrated torque wrench or torque screwdriver set to the manufacturer's specified inch-pound rating on the panel lugs. A loose lug on a 225A feeder will arc and melt the busbar under heavy load.
• Aluminum Prep: If you choose 300 kcmil aluminum to save on material costs, you must use an anti-oxidant compound (like Noalox) on the stripped conductor ends before termination. Aluminum oxidizes rapidly in air, creating a high-resistance layer that causes thermal runaway at the breaker lug.
• Bending Radius: Thick wire doesn't bend on a dime. NEC 300.34 and Chapter 9 dictate minimum bending radii. You will need large-sweep conduit bends and plenty of physical clearance inside the panel gutter to route the wire without stressing the termination point.

Common Confusions and Mistakes

The most frequent mistake DIYers and junior electricians make with heavy feeders is falling into the 90°C trap. Wire manufacturers print the 90°C rating on the jacket of THHN/THWN-2 wire. Looking at Table 310.16, 4/0 AWG copper is rated for 260A in the 90°C column. A builder might incorrectly assume they can use 4/0 copper on a 250A breaker because 260A > 250A.

This is a code violation. The 90°C column is only used as a starting point for ampacity derating (e.g., when you have more than three current-carrying conductors in a conduit or high ambient temperatures). The final derated ampacity must still be equal to or greater than the load, but the breaker size is strictly limited by the 75°C termination rating of the panel. For a 225A breaker, you are locked to the 75°C column.

Another common error is ignoring the continuous load multiplier. If your 225A load is considered 'continuous' (expected to run for 3 hours or more, like commercial lighting or HVAC), NEC 210.19 requires you to multiply the load by 125%. A 225A continuous load becomes 281.25A, meaning 4/0 copper (230A) is now vastly undersized, and you would need 350 kcmil copper (310A at 75°C).