For a 225 amp panel, use 4/0 AWG copper or 300 kcmil aluminum wire, protected by a 225A breaker. This assumes copper conductors, 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the conduit.
The Core Sizing Data: NEC Ampacity Tables
When sizing feeders, you cannot simply match the wire's 90°C ampacity to the breaker size. According to NFPA 70 (NEC) Article 110.14(C), the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device. Almost all standard 225A panelboard lugs and molded-case breakers are rated for 75°C.
| Material | Wire Size | Insulation Type | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Derating Base) |
|---|---|---|---|---|
| Copper | 4/0 AWG | THHN/THWN-2 | 230A | 260A |
| Aluminum | 300 kcmil | XHHW-2 | 230A | 265A |
| Copper (Too Small) | 3/0 AWG | THHN/THWN-2 | 200A (Fails 225A req) | 225A |
Why 4/0 Copper? (And Why 3/0 Fails the Test)
A common mistake on the jobsite is pulling 3/0 AWG copper for a 225A service. Looking strictly at the 90°C column of NEC Table 310.16, 3/0 copper is rated for exactly 225 amps. However, as Schneider Electric termination guidelines and the NEC explicitly state, you cannot use the 90°C column for final ampacity unless the breaker lugs and panel terminations are explicitly stamped for 90°C. They almost never are in this amperage class.
At the mandatory 75°C termination rating, 3/0 copper drops to 200A. Feeding a 225A breaker with a 200A-rated termination creates a thermal bottleneck. The lug will overheat before the breaker's thermal-magnetic trip curve engages. Stepping up to 4/0 AWG copper gives you a 75°C ampacity of 230A, safely clearing the 225A requirement with a 5A margin.
Variables That Force a Size Increase
The 4/0 Cu / 300 kcmil Al baseline assumes perfect conditions. Real-world installs rarely comply. Use this decision tree to determine if you need to upsize your feeder.
| Site Condition | NEC Rule | Effect on 4/0 Cu Wire | Required Action |
|---|---|---|---|
| Continuous Load (On for 3+ hours) | 210.20(A) / 215.3 | Must size at 125% (281.25A minimum) | Upsize to 350 kcmil Cu (310A at 75°C) |
| High Ambient Temp (e.g., 46-50°C attic) | 310.15(B)(1) | Derating factor of 0.82 applies to 90°C base (260A * 0.82 = 213.2A) | Upsize to 250 kcmil Cu |
| Conduit Bundling (4-6 current-carrying conductors) | 310.15(C)(1) | 80% derating factor (260A * 0.80 = 208A) | Upsize to 250 kcmil Cu |
Voltage Drop Verification at 150 Feet
Ampacity keeps the wire from melting; voltage drop ensures your equipment actually runs. The NEC recommends a maximum of 3% voltage drop for feeders (Informational Note to 210.19). Let's run the math for a 225A load on 4/0 copper at a distance of 150 feet on a 240V single-phase system.
- Formula: VD = (2 × K × I × D) / CM
- K (Copper): 12.9 ohms-cmil/ft
- I (Current): 225A
- D (Distance): 150 ft
- CM (Circular Mils for 4/0): 211,600
Calculation: (2 × 12.9 × 225 × 150) / 211,600 = 4.11 Volts.
Percentage: (4.11 / 240) × 100 = 1.71%.
At 150 feet, 4/0 copper performs beautifully, staying well under the 3% threshold. However, if your run exceeds 250 feet, the voltage drop will cross 2.85%, and you should strongly consider stepping up to 250 kcmil or 300 kcmil copper to maintain optimal power quality, especially if the panel feeds sensitive electronics or large HVAC compressors.
When the AHJ or an Engineer Must Step In
- Parallel Conductors: If you are paralleling two sets of 3/0 copper to achieve 225A (NEC 310.10(G)), the physical spacing, exact length matching, and phase grouping must be inspected to prevent inductive heating and unbalanced impedance.
- High Fault Current: If the utility transformer can deliver >22,000 AIC (Ampere Interrupting Capacity) at the service point, standard 10kAIC breakers will violently fail. An engineer must calculate the available fault current to specify the correct series-rated or fully-rated breaker and wire bracing.
- Specialized Industrial Loads: Welding receptacles or large motor feeders have specific duty-cycle derating rules (NEC Article 630 and 430) that override standard Table 310.16 ampacities.
Frequently Asked Questions
Can I use 250 kcmil copper instead of 4/0 AWG for my 225 amp panel?
Yes. 250 kcmil copper is rated for 255A at 75°C. While 4/0 AWG (230A) is the minimum code-compliant size, using 250 kcmil is a common and smart practice if the price difference is negligible at your local electrical supply house. The thicker wire provides a lower voltage drop over long distances and makes future upgrades to a 250A panel trivial. Just ensure your panel lugs are physically rated to accept the larger conductor diameter.
What size equipment ground wire do I need for a 225A feeder?
According to NEC Table 250.122, a 225A breaker requires a minimum 4 AWG copper or 2 AWG aluminum equipment grounding conductor (EGC). If you upsized your ungrounded (hot) conductors for voltage drop reasons (e.g., using 250 kcmil instead of 4/0), NEC 250.122(B) requires you to proportionally increase the size of your ground wire to maintain the same fault-clearing impedance ratio.
Does a 225A panel mean I can pull 225 amps continuously?
No. Standard thermal-magnetic breakers and panelbus bars are rated for 100% of their nameplate value only for non-continuous loads (loads that cycle off within 3 hours). If your calculated continuous load is exactly 225A, the NEC requires the feeder and overcurrent device to be sized at 125% of that load (281.25A). You would need a 300A breaker and 350 kcmil copper wire, unless you purchase a specialized (and highly expensive) breaker explicitly listed for 100% continuous duty.
How does using aluminum wire change my conduit fill requirements?
Aluminum wire requires a larger physical cross-section than copper for the same ampacity (300 kcmil Al vs 4/0 Cu). According to the Copper Development Association and standard conduit fill tables, 300 kcmil XHHW-2 aluminum has a larger outside diameter than 4/0 THHN copper. If you are pulling three phases, a neutral, and a ground, you will likely need to step up from a 1.5-inch PVC or EMT conduit to a 2-inch conduit to stay under the NEC 40% fill limit and avoid damaging the insulation during the pull.
