A 350 amp wire size refers to the specific conductor cross-sectional area—measured in thousands of circular mils (kcmil)—required to safely carry 350 amperes of electrical current without exceeding the thermal limits of its insulation or terminations. In a real installation, selecting this gauge dictates your physical conduit diameter, the bending radius required at junction boxes, and the specific lug torque values at your panelboard. What people most commonly confuse is the wire’s 90°C insulation rating with the 75°C termination limit of the breaker or panel, a mistake that leads to dangerous undersizing and failed inspections.

The Baseline Numbers: Sizing for 350 Amps

Standard AWG wire sizing effectively stops at 4/0 AWG, which maxes out at 230 amps for copper at 75°C. To handle 350 amps, you must move into kcmil (formerly MCM) territory. According to NEC Table 310.16, the minimum wire sizes required to achieve an ampacity of at least 350A in the 75°C column are:

Copper (Cu): 500 kcmil (Rated 380A at 75°C)
Aluminum (Al): 700 kcmil (Rated 375A at 75°C)

Note that 350 kcmil wire is not rated for 350 amps. A 350 kcmil copper conductor is only rated for 310 amps at 75°C. The naming convention (kcmil) refers strictly to the physical cross-sectional area of the metal, not its current-carrying capacity.

The 75°C Termination Rule vs. 90°C Derating

The most frequent error on high-amperage jobsites is sizing wire based on the 90°C column of the ampacity table. While you can buy 90°C wire (like THHN or XHHW-2), NEC 110.14(C) mandates that the ampacity of the circuit is limited by the lowest temperature rating of any connected termination, conductor, or device.

Almost all breakers, panelboard lugs, and disconnect switches rated 100A or higher are tested and rated for 75°C terminations. Therefore, you must use the 75°C column to select your base wire size.

The Highway Analogy: Think of a 90°C THHN wire as a multi-lane highway capable of moving massive traffic (current). However, the breaker lug is a toll booth rated only for 75°C. No matter how wide the highway is leading up to it, the toll booth dictates the maximum safe flow. You can only use the 90°C column to apply derating factors (like ambient temperature or conduit fill adjustments) before hitting the toll booth, but your final adjusted ampacity cannot exceed the 75°C limit of the lug.

Worked Numeric Example: A 400A Panel Feeder

Let’s size a feeder for a detached workshop subpanel. The panel is rated 400A. The calculated load is 280A continuous and 70A non-continuous, totaling 350A. The run is 150 feet through underground PVC conduit at 240V.

Step 1: Base Ampacity Sizing
We need a wire rated for at least 350A in the 75°C column. We select 500 kcmil Copper THHN (rated 380A) or 700 kcmil Aluminum XHHW-2 (rated 375A).

Step 2: Voltage Drop Calculation
NEC 310.15(B) recommends keeping voltage drop under 3% for feeders. Using the formula VD = (2 × K × I × L) / CM:

  • Copper (K=12.9): (2 × 12.9 × 350 × 150) / 500,000 = 2.7V drop (1.1% on 240V). Passes easily.
  • Aluminum (K=21.2): (2 × 21.2 × 350 × 150) / 700,000 = 3.18V drop (1.3% on 240V). Passes easily.

Step 3: Conduit Sizing and Bending Radius
A single 500 kcmil THHN copper wire has a cross-sectional area of roughly 0.707 square inches. Three current-carrying conductors plus a ground will require over 2.2 square inches of fill space. According to NEC Chapter 9, Table 4, you need a minimum of 2.5-inch Schedule 80 PVC to stay under the 40% fill limit. However, due to the extreme stiffness of 500 kcmil cable, most electricians will upsize to 3-inch PVC and use sweeping 36-inch radius bends to avoid damaging the insulation during the pull.

Where You Meet 350A in Practice

You won't find 350A circuits in standard residential bedroom wiring. This amperage is reserved for heavy-duty infrastructure:

  • Luxury Residential Subpanels: Large estates with dual HVAC systems, heated driveways, pool equipment, and multiple Level 2 EV chargers often require 350A to 400A feeders from the utility transformer to the main house panel.
  • Commercial DC Fast Chargers: Level 3 EV charging stations (like Tesla Superchargers or Electrify America cabinets) frequently pull 300A to 400A per pedestal, requiring heavy kcmil feeders from the site switchgear.
  • Industrial Welding and Motors: Large fabrication shops running multi-operator arc welding bays or 200+ HP 3-phase motors rely on 350A+ circuits to handle inrush currents and continuous duty cycles.

Decision Path: Choosing Your Conductor

When specifying materials for a 350A feeder, use this decision tree to select the right conductor and installation method.

Condition / Constraint If True... Select This Wire
Budget is the primary constraint Aluminum is roughly 40-50% cheaper than copper by weight and volume. 700 kcmil Aluminum XHHW-2
Termination space is extremely tight Aluminum requires larger lugs and more physical clearance inside the panel. 500 kcmil Copper THHN
Pulling by hand over long distances with bends Single 500/700 kcmil cables are too stiff to bend without a mechanical winch. Two parallel sets of 3/0 AWG Copper THHN (NEC 310.10(H) allows paralleling 1/0 and larger)
High ambient temperature (e.g., rooftop conduit) Must apply 90°C derating factors before hitting the 75°C lug limit. Upsize to 600 kcmil Cu or 750 kcmil Al to absorb the derating penalty.
Pro-Tip on Paralleling: Pulling a single 500 kcmil copper cable through conduit is a back-breaking job that often requires a capstan winch and specialized cable lubricant. For runs over 100 feet with multiple bends, electricians frequently parallel two sets of 3/0 AWG copper (rated 200A each x 2 = 400A total). This yields a mathematically equivalent ampacity while allowing you to bend the wires by hand.

Frequently Asked Questions

Is 350A a standard breaker size?

Yes, per NEC 240.6(A), 350 amps is a standard fixed-trip breaker and fuse size. However, in the commercial panelboard market, manufacturers typically skip from 300A directly to 400A mains. If you are feeding a panel, you will likely buy a 400A panelboard and protect the 350A calculated load with a 350A or 400A molded case breaker, depending on the exact continuous load calculations.

What torque spec is required for 500 kcmil lugs?

NEC 110.14(D) mandates that connections must be torqued to the manufacturer's specifications. For 500 kcmil copper in a standard mechanical lug, torque values typically range from 375 in-lbs to 450 in-lbs (approx. 31 to 37 ft-lbs). You cannot guess this with a standard wrench; you must use a calibrated torque wrench or a hydraulic torque tool to ensure the lug does not overheat under a 350A load.

Do I need a neutral wire for a 350A feeder?

It depends on the load. If the subpanel serves 120V/240V single-phase loads (like lighting and standard receptacles), you must pull a neutral. Per NEC 220.61, the neutral can sometimes be sized smaller than the phase conductors based on the maximum unbalanced load, but it can never be smaller than the required equipment grounding conductor. If the panel exclusively serves 240V or 480V 3-phase equipment (like large motors or DC fast chargers), a neutral is not required.

The Final Verdict: Your Default Pick

While engineering constraints always dictate the final design, you need a reliable starting point for estimating and material ordering. For 90% of 350A feeder installations, your default pick should be 700 kcmil Aluminum XHHW-2 for cost-efficiency and weight savings. If you are navigating tight conduit bends, long pulling distances, or space-constrained termination lugs where aluminum won't fit, switch to two parallel sets of 3/0 AWG Copper THHN. This parallel copper approach gives you the necessary 400A ampacity headroom while remaining manageable for a two-person crew to pull by hand.