3 Phase Wire Size Chart: NEC Ampacity & Derating Reference

When sizing conductors for a 3-phase feeder or branch circuit, the direct answer depends on your continuous load, termination temperature ratings, and the baseline ampacities established in the National Electrical Code (NEC). For a standard 100A 3-phase feeder with 75°C terminations, you need 3 AWG copper or 1 AWG aluminum. However, pulling a single number from a chart without accounting for installation conditions is a fast track to overheated lugs and failed inspections.

This reference guide provides a data-dense lookup table based on NEC Table 310.16, explains exactly which temperature column applies to your specific hardware, and breaks down the derating math required when you pull multiple conductors through a single raceway.

How to Read the NEC 3-Phase Ampacity Table

Before looking at the numbers, you must understand the temperature columns. Wire insulation (like THHN or XHHW-2) is often rated for 90°C, but your circuit breaker lugs and panel busbars are typically rated for only 75°C (or 60°C on older or smaller residential-style equipment).

The Golden Rule of Termination: NEC 110.14(C) dictates that the final ampacity of your wire cannot exceed the temperature rating of the lowest-rated component in the circuit. If your breaker lugs are rated for 75°C, you must use the 75°C column to select your base wire size, even if the wire itself has 90°C insulation.

Bookmark Quick-Jumps for Common 3-Phase Loads

These are the most frequently queried 3-phase breaker sizes and their corresponding minimum copper wire sizes (assuming standard 75°C commercial terminations and no derating):

• 60A 3-Phase: 6 AWG Copper (65A in the 75°C column).
• 100A 3-Phase: 3 AWG Copper (100A exact match in the 75°C column).
• 200A 3-Phase: 3/0 AWG Copper (200A exact match in the 75°C column).
• 400A 3-Phase: 600 kcmil Copper (requires looking beyond this chart; 400A often requires parallel runs of 3/0 or 250 kcmil depending on the exact lug ratings and derating).

Applying Derating Factors to Your 3-Phase Feeder

The ampacities in the table above assume you have no more than three current-carrying conductors (CCCs) in a raceway. In a 3-phase system, the number of CCCs depends on your system topology and load type.

On a 3-phase, 3-wire Delta system, you have exactly 3 CCCs. No derating is required. However, on a 3-phase, 4-wire Wye system supplying a mix of single-phase and 3-phase loads, the neutral carries unbalanced current. Under NEC 310.15(C)(1), that neutral counts as a 4th CCC. Four CCCs in a single conduit triggers an 80% derating factor.

Worked Numeric Example: 125A Load on a 4-Wire Wye System

1. Identify the Load: 125A continuous/non-continuous mixed load on a 208Y/120V system.
2. Count CCCs: 3 phases + 1 neutral = 4 CCCs.
3. Find the Derating Factor: NEC Table 310.15(C)(1) dictates 80% for 4-6 conductors.
4. Select Base Wire using the 90°C Column: We need a wire whose 90°C ampacity, when multiplied by 0.80, equals at least 125A. (125 / 0.80 = 156.25A required at 90°C).
5. Check the Chart: 1 AWG Copper has a 90°C rating of 145A (145 x 0.80 = 116A — Too small). 1/0 AWG Copper has a 90°C rating of 170A (170 x 0.80 = 136A — Passes the derating check).
6. Verify Termination Rating (The 75°C Check): Does 1/0 AWG Copper meet the 125A load in the 75°C column? Yes, the 75°C column lists 1/0 AWG at 150A, which is greater than our 125A load.
7. Final Verdict: You must pull 1/0 AWG Copper (or 2/0 AWG Aluminum) for this run.

For a deeper dive into how harmonic loads (like LED drivers and VFDs) can cause the neutral to count as a CCC even on balanced systems, refer to this technical breakdown of NEC ampacity tables by EC&M.

What This 3 Phase Wire Size Chart Cannot Tell You

While NEC Table 310.16 is the undisputed starting point for wire sizing, treating it as the final word will lead to operational failures in specific scenarios. Here is what the chart leaves out, and how to address it.

1. Voltage Drop Over Long Runs

Ampacity tables only tell you the size of wire required to prevent the insulation from melting and the breaker lugs from overheating. They do not account for voltage drop. If you are running a 100A 3-phase feeder to a detached workshop 300 feet away, 3 AWG copper is legally compliant for ampacity, but the voltage drop will exceed the NEC's recommended 3% limit for feeders. For long runs, you must calculate voltage drop using Chapter 9, Table 8 (DC resistance) or Table 9 (AC impedance) and typically upsize the wire by one or two AWG sizes to maintain equipment efficiency and prevent motor burnout.

2. Conduit Fill and Physical Jamming

Three phases, a neutral, and a ground mean five physical wires in the pipe. Chapter 9, Table 1 limits conduit fill to 40% for three or more conductors. Furthermore, if you are pulling large 3-phase conductors through multiple bends, you must calculate the jam ratio. If the conduit inner diameter is less than 2.5 times the diameter of a single conductor, the wires will physically jam at the elbows, regardless of what the ampacity chart says.

3. Short-Circuit Withstand Ratings

In high-fault-current industrial environments (e.g., 65kA or 100kA available fault current at the service entrance), smaller conductors can vaporize before the upstream breaker clears the fault. While the breaker's AIC (Ampere Interrupting Capacity) rating handles the device itself, the wire must have adequate thermal mass to withstand the let-through energy. This requires checking the specific short-circuit withstand curves provided by the wire manufacturer, not just the NEC ampacity chart.

Code Caveat: The values and rules presented here reflect standard NEC-style guidance (NFPA 70). Your local Authority Having Jurisdiction (AHJ) or municipal inspector always has final authority and may enforce local amendments, specific utility requirements, or stricter voltage drop mandates that supersede baseline national tables.