The wire size for 250 amps is the minimum conductor cross-sectional area—typically 250 kcmil copper or 350 kcmil aluminum at a 75°C temperature rating—required to safely carry a 250-ampere continuous or non-continuous load without exceeding the insulation's thermal limits. When you are pulling feeders for large subpanels, commercial HVAC disconnects, or heavy workshop equipment, guessing the wire gauge isn't just a code violation; it's a fire hazard. This guide gives you the exact sizes, the math for voltage drop, and the physical realities of working with conductors this thick.

The Direct Answer: Wire Size for 250 Amps

For a standard 250A feeder protected by a 250A breaker, you must size the conductors using the 75°C column of NEC Table 310.16 (assuming your breaker and panel lugs are rated for 75°C, which almost all modern 250A+ gear is).

Copper: 250 kcmil (Rated 255A @ 75°C)
Aluminum: 350 kcmil (Rated 250A @ 75°C)
⚠️ The 60°C Trap: If you are terminating into an older panel, a fused disconnect, or a piece of equipment explicitly marked for 60°C terminations, you must use the 60°C column. This forces you to step up to 300 kcmil Copper (240A is too low, so you must jump to 350 kcmil Cu at 255A) or 400 kcmil Aluminum (265A @ 60°C). Always check the equipment lug rating before buying wire.

What Changes When You Step Up to 250A

Sizing wire for 250 amps changes the physical geometry of your entire installation. You are no longer dealing with standard NM-B Romex or easily bent THHN. Here is what shifts in a real-world installation:

  • Conduit Fill and Sizing: Three 350 kcmil THHN aluminum conductors plus a ground will not fit in 1.5-inch conduit. NEC Chapter 9 fill tables mandate a minimum 2-inch PVC or EMT conduit. If you add a neutral and pull 4 current-carrying conductors, you must derate, likely forcing you into 2.5-inch conduit.
  • Termination Lugs: Standard 200A panelboard lugs max out at 4/0 AWG. For 350 kcmil, you need panels equipped with oversized mechanical lugs or a specific 250A main breaker designed to accept up to 500 kcmil.
  • Bending Radius and Gutter Space: NEC 300.34 dictates strict bending radii for large conductors. A 350 kcmil wire requires a minimum bending radius of 5 times its diameter. Your panel gutters must be deep enough (usually 6 to 8 inches minimum) to allow the wire to sweep into the lug without kinking or stressing the termination.
  • Grounding Conductor: Per NEC Table 250.122, a 250A breaker requires a minimum 4 AWG Copper or 2 AWG Aluminum equipment grounding conductor (EGC).

Worked Numeric Example: Sizing a 250A Subpanel Feeder

Ampacity gets you the right wire for heat dissipation, but voltage drop ensures your equipment actually runs. Let's calculate a real-world scenario using the Southwire voltage drop methodology.

The Scenario: You are feeding a 250A detached workshop subpanel located 150 feet from the main service. The system is 240V single-phase. You want to use Aluminum THHN in conduit.

  1. Base Ampacity Check: 350 kcmil Aluminum is rated 250A @ 75°C. This meets the breaker size.
  2. Voltage Drop Formula: $VD = \frac{2 \times K \times I \times L}{CM}$
  3. Variables:
    • $K$ (AC resistance for Al at 75°C) ≈ 21.2
    • $I$ (Current) = 250A
    • $L$ (One-way length) = 150 ft
    • $CM$ (Circular mils for 350 kcmil) = 350,000
  4. The Math: $VD = \frac{2 \times 21.2 \times 250 \times 150}{350,000} = \frac{1,590,000}{350,000} = 4.54V$
  5. Percentage: $4.54V / 240V = 1.89\%$
Bench Note: A 1.89% drop is well under the NEC recommended 3% maximum for feeders. 350 kcmil Aluminum is the perfect pick here. However, if that workshop was 300 feet away, the drop would double to 3.78%. At that distance, you must step up to 400 kcmil Aluminum to keep the voltage drop under 3% and prevent welders and heavy motors from tripping on low-voltage brownouts.

Where You Meet This in Practice (and Common Confusions)

When discussing 250A circuits, two major points of confusion lead to failed inspections and unsafe installations.

Confusion 1: The 4/0 AWG Myth

Many DIYers and junior apprentices assume 4/0 AWG (four-aught) is the default "large service" wire. 4/0 Aluminum is rated for 180A (60°C) or 205A (75°C). It is the standard wire for a 200A residential service. It cannot safely carry 250A on a standard feeder. If you put 4/0 on a 250A breaker, the wire will melt before the breaker trips during a sustained overload.

Confusion 2: The 83% Residential Service Rule (NEC 310.12)

The NEC allows a specific exception for whole-house residential service entrance conductors. Because residential loads are highly diversified (you rarely run the oven, AC, dryer, and EV charger at 100% simultaneously), NEC 310.12 allows you to size service wires at 83% of the service rating.

For a 250A residential service, 83% of 250A is 207.5A. In this highly specific scenario, you could theoretically use a wire rated for 208A. However, this does not apply to subpanels, commercial feeders, or dedicated equipment circuits. For any standard 250A feeder, you must use the full 250A ampacity from Table 310.16.

Decision Path: Choosing Your 250A Conductor

Use this decision tree to finalize your material list before heading to the electrical supply house.

Installation Condition Required Action Final Concrete Pick
Standard 250A Feeder (Subpanel, Commercial, < 150ft) Use 75°C column ampacity. 350 kcmil Aluminum (or 250 kcmil Cu)
Long Distance Feeder (> 200 feet one-way) Step up one size to mitigate >3% voltage drop. 400 kcmil Aluminum (or 300 kcmil Cu)
Terminations rated strictly for 60°C Use 60°C column ampacity. 400 kcmil Aluminum (or 350 kcmil Cu)
Whole-House 250A Residential Service (NEC 310.12 applies) Apply 83% rule (207.5A min required). 250 kcmil Aluminum (Rated 250A @ 75°C, safely covers 207.5A)

FAQ: 250 Amp Wire Sizing Edge Cases

Can I parallel two smaller wires instead of pulling one massive 350 kcmil?

Yes, NEC 310.10(G) allows paralleling conductors for sizes 1/0 AWG and larger. To get 250A, you could run two parallel sets of 3/0 AWG Aluminum (190A each x 2 = 380A total capacity). However, paralleling requires identical lengths, identical routing, and double the lugs. Unless you already have the wire on hand or are dealing with extreme bending radius constraints in a tight retrofit, a single 350 kcmil pull is faster, cheaper, and less prone to imbalance errors.

What size breaker do I use if my calculated load is exactly 250A?

If your calculated continuous and non-continuous load totals exactly 250A, you must use a 250A breaker and wire sized for 250A. However, if the load is purely continuous (running for 3 hours or more, like a large server room HVAC), NEC 210.20 requires the breaker to be sized at 125% of the continuous load. In that case, a 250A continuous load requires a 300A breaker and wire sized for 300A (400 kcmil Aluminum).

Do I need to adjust for ambient temperature in a hot attic?

Yes. Table 310.16 assumes an ambient temperature of 30°C (86°F). If you are routing your 250A feeder through an attic that reaches 120°F (49°C) in the summer, you must apply a derating factor of 0.82 for THHN/THWN-2. A 350 kcmil Aluminum wire derated by 0.82 drops to 205A, which is insufficient. You would need to step up to 500 kcmil Aluminum or reroute the conduit through a conditioned space.