Wire size for 225 amps is the specific conductor cross-section—typically 4/0 AWG copper or 300 kcmil aluminum—required to safely carry a 225-ampere load without exceeding the thermal limits of the wire insulation or the panel terminations.
In a real installation, your wire size choice dictates three physical realities: the minimum conduit diameter required to meet NEC fill capacity rules, whether the stripped conductor physically fits into the mechanical lugs of your breaker or subpanel, and the voltage drop experienced at the far end of the run. While 225 amps is a standard breaker size per NEC 240.6(A), sizing the wire to feed it requires navigating the intersection of conductor material, insulation temperature ratings, and termination limits.
Where You Meet 225-Amp Feeder Requirements in Practice
You will rarely see a 225-amp feeder in a standard single-family home's main service, but it is a highly common specification in heavy-residential and light-commercial applications. Typical scenarios include:
- Large Detached Subpanels: Feeding a 225A subpanel to a detached garage or workshop that houses a 100A EV charger, a 50A plasma cutter, and a 30A air compressor.
- Commercial HVAC Systems: Supplying large rooftop units or chiller packages that require dedicated 225A fused disconnects.
- Multi-Family Dwellings: Serving as a feeder to a localized meter bank or a cluster of tenant distribution panels.
The 75°C Termination Rule: Why 3/0 Copper Fails
The most common confusion when sizing wire for 225 amps is relying on the 90°C column of NEC Table 310.16. If you look at the 90°C column for copper, 3/0 AWG is rated for exactly 225 amps. Many builders buy 3/0 AWG THHN, assuming it is perfectly sized.
This is a code violation and a fire hazard.
Under NEC 110.14(C), the ampacity of a conductor is limited by the lowest temperature rating of any connected component, which is almost always the termination lug. Panelboards, breakers, and disconnects rated between 100A and 400A are virtually universally tested and listed for 75°C terminations. Therefore, you must size your wire using the 75°C column.
- 3/0 AWG Copper at 75°C: Rated for 200A. (Undersized for a 225A load).
- 4/0 AWG Copper at 75°C: Rated for 230A. (Correctly sized, as 230A > 225A).
You can use 90°C rated wire (like THHN or XHHW-2) to take advantage of its higher thermal headroom for ambient temperature derating or conduit fill adjustments, but the baseline ampacity for the load must still clear the 225A mark in the 75°C column.
Worked Example: Voltage Drop on a 200-Foot 225A Run
Ampacity tells you the wire won't melt; voltage drop tells you your equipment will actually run. The NEC recommends a maximum 3% voltage drop on feeders. Let's calculate the drop for a 200-foot run of 4/0 AWG copper carrying a full 225A load on a 240V single-phase system.
K (Copper): 12.9
I (Current): 225A
L (Length): 200 ft
CM (Circular Mils for 4/0): 211,600
The Math:
VD = (2 × 12.9 × 225 × 200) / 211,600
VD = 1,161,000 / 211,600 = 5.48 Volts
The Result: 5.48V on a 240V system is a 2.28% drop. This is well under the 3% recommended limit, meaning 4/0 AWG copper is electrically sound for a 200-foot run. If your run exceeds 250 feet, you must step up to 250 kcmil copper to keep the drop below 3%.
Decision Tree: Selecting Your Exact 225A Conductor
Use this decision matrix to lock in your exact wire size, material, and conduit requirements. Do not guess; match your scenario to the row below.
| Scenario & Run Length | Material | Exact Wire Size | Min. Conduit (EMT) | Verdict |
|---|---|---|---|---|
| Standard run, under 150 ft | Copper (THHN/THWN-2) | 4/0 AWG | 2-inch | DEFAULT PICK (Best balance of ease of termination and performance) |
| Budget run, under 100 ft | Aluminum (XHHW-2) | 300 kcmil | 2.5-inch | Choose for cost savings on short commercial runs (requires anti-oxidant paste) |
| Long run, 150 ft to 250 ft | Copper (THHN/THWN-2) | 250 kcmil | 2.5-inch | Mandatory upgrade to prevent >3% voltage drop at full load |
| Long run, over 250 ft | Copper (THHN/THWN-2) | 350 kcmil | 3-inch | Required for extreme distances or high continuous loads |
Common Confusions and Code Traps
1. 'I can just use a 250A breaker since 225A isn't standard.'
False. 225A is explicitly listed as a standard ampere rating in NEC 240.6(A). You do not need to round up to 250A unless your calculated load (after applying 125% for continuous loads) exceeds 225A. Using a 250A breaker on a 225A-rated panel busbar is a direct code violation.
2. 'I'll use aluminum to save money, it's the same size.'
False. Aluminum has lower conductivity and higher thermal expansion. For a 225A load at 75°C, 4/0 AWG aluminum is only rated for 180A. You must jump all the way to 300 kcmil aluminum (rated 240A at 75°C) to safely carry 225A. Furthermore, aluminum requires meticulous wire brushing and the application of an anti-oxidant compound (like Noalox) at terminations to prevent high-resistance arcing over time.
3. 'Conduit size doesn't matter as long as the wires fit.'
False. NEC Chapter 9 limits conduit fill to 40% for three or more conductors. Three 4/0 AWG THHN conductors plus a 1 AWG copper ground require roughly 1.15 square inches of cross-sectional area. A 2-inch Schedule 80 PVC conduit only allows 1.14 square inches at 40% fill. You must use 2-inch EMT (metal) or step up to 2.5-inch PVC to legally pull this feeder.
Frequently Asked Questions
What size ground wire do I need for a 225-amp feeder?
Per NEC Table 250.122, a 225A overcurrent device requires a minimum 2 AWG copper or 1/0 AWG aluminum equipment grounding conductor (EGC). If you ungrounded conductors (hot wires) to compensate for voltage drop, you must proportionally increase the size of your ground wire per NEC 250.122(B).
Can I parallel two smaller wires to get 225 amps?
NEC 310.10(H) generally prohibits paralleling conductors smaller than 1/0 AWG. You cannot parallel two 2 AWG wires to achieve 225A. You must use a single, properly sized conductor (4/0 Cu or 300 kcmil Al) unless you are using specialized engineered busways or parallel 1/0 AWG sets (which would yield 340A and is overkill/improperly protected without specific tuning).
Does the 125% continuous load rule apply here?
Yes. If your 225A load is continuous (expected to run for 3 hours or more), you must multiply 225 by 1.25, resulting in a required wire ampacity of 281.25A. In that scenario, 4/0 AWG copper (230A) is vastly undersized, and you would need to step up to 350 kcmil copper (310A at 75°C) or use parallel 1/0 AWG conductors. The sizes in this guide assume a non-continuous 225A load or a breaker sized to handle the continuous calculation natively.
