For a standard 100A 3-phase circuit in the US, you need 3 AWG copper wire (assuming 75°C rated terminals). The required insulation colors depend entirely on your system voltage: a 208Y/120V system uses Black, Red, and Blue; a 480Y/277V system uses Brown, Orange, and Yellow; and a 240V High-Leg Delta uses Black, Orange (high leg), and Blue. Sizing and coloring 3-phase feeders requires cross-referencing ampacity tables with voltage-specific color codes to pass inspection and prevent catastrophic cross-voltage wiring errors.
The 3-Phase Wire Color and Ampacity Chart (Up to 100A)
The table below merges copper wire ampacity limits with mandatory phase conductor colors. How to read this table: Locate your target amperage in the 60°C or 75°C column (see the next section to determine which applies). Read left to find the minimum AWG size, then read right to select the correct THHN/THWN-2 insulation colors based on your facility's nominal voltage.
Data sourced from NFPA NEC Table 310.16 (Ampacities of Insulated Conductors) and NEC Articles 215.12, 230.56, and 210.4 (Color Codes and High-Leg Identification).
| AWG Size (Cu) | 60°C Ampacity | 75°C Ampacity | 208Y/120V Colors | 480Y/277V Colors | 240V High-Leg Colors |
|---|---|---|---|---|---|
| 14 | 15A | 20A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 12 | 20A | 25A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 10 | 30A | 35A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 8 | 40A | 50A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 6 | 55A | 65A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 4 | 70A | 85A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 3 | 85A | 100A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 2 | 95A | 115A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
| 1 | 110A | 130A | Blk / Red / Blu | Brn / Org / Yel | Blk / Org / Blu |
Decoding the Columns: Temperature Ratings and Derating
Which column applies to your installation? Under NEC 110.14(C)(1)(a), if your circuit is rated 100A or less, you must default to the 60°C column unless the breaker and equipment terminals are explicitly marked for 75°C. Most modern 100A molded-case breakers (like Square D PowerPact or Eaton FD series) are rated 75°C, allowing you to use the 75°C column (3 AWG for 100A). If you are connecting to older equipment, unmarked lugs, or NM-B cable, you are legally bound to the 60°C column, requiring 1 AWG for a 100A load.
How derating rows modify the base value: The ampacities above assume no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). If you pull two 3-phase circuits in a single conduit (6 current-carrying conductors), OSHA Standard 1910.304 and NEC 310.15(C)(1) mandate an 80% derating factor. A 3 AWG wire rated at 100A drops to 80A (100 x 0.80). To maintain a legal 100A capacity under these conditions, you must upsize to 1 AWG copper (130A x 0.80 = 104A).
Decision Tree: Picking Your Exact Wire and Color
Use this decision path to terminate your planning phase with a concrete materials list. Trace your system voltage down to the final pick.
| System Voltage | Configuration | Phase Colors Required | Neutral & Ground | Concrete 100A Pick (75°C) |
|---|---|---|---|---|
| 208Y/120V | Standard Commercial Wye | Black, Red, Blue | White (Neu), Green (Gnd) | 3 AWG THHN (5 wires total) |
| 480Y/277V | Industrial / Lighting Wye | Brown, Orange, Yellow | Gray (Neu), Green (Gnd) | 3 AWG THHN (5 wires total) |
| 240/120V | High-Leg Delta | Black, Orange*, Blue | White (Neu), Green (Gnd) | 3 AWG THHN (5 wires total) |
*Note: In a High-Leg Delta, the Orange wire MUST be the high leg (208V to neutral) and must be tagged or durably marked at every termination point per NEC 230.56.
What This Chart Cannot Tell You (Edge Cases & Code Limits)
While the chart above satisfies NEC ampacity and identification rules, it omits three critical engineering variables that dictate real-world performance:
- Voltage Drop Over Distance: NEC 310.15(B) provides an informational note recommending a maximum 3% voltage drop for feeders. At a full 100A load on a 208V 3-phase circuit, 3 AWG copper will hit a 3% drop at approximately 115 feet. If your conduit run is 150 feet, 3 AWG is legally allowable for ampacity but practically insufficient; you must upsize to 1/0 AWG to prevent motor overheating and dimming lights.
- Aluminum vs. Copper: This chart strictly applies to copper. If you are using aluminum conductors (like XHHW-2 AL) to save money on long feeder runs, the 75°C ampacity for 100A requires a minimum of 1/0 AWG Aluminum, and your color coding rules remain identical.
- Continuous Loads: If your 3-phase load is continuous (running for 3 hours or more, such as HVAC compressors or industrial lighting), NEC 210.20 requires the branch circuit rating to be 125% of the continuous load. A 100A continuous load requires a 125A breaker and wire sized for 125A (1 AWG Copper at 75°C), not 3 AWG.
Quick-Jump Reference: Most Queried 3-Phase Configurations
Bookmark this section for rapid jobsite lookups. These assume copper wire, 75°C terminals, no derating, and runs under 100 feet.
- 100A 208V 3-Phase: 3 AWG Cu | Black, Red, Blue, White, Green
- 100A 480V 3-Phase: 3 AWG Cu | Brown, Orange, Yellow, Gray, Green
- 60A 208V 3-Phase: 6 AWG Cu | Black, Red, Blue, White, Green
- 60A 480V 3-Phase: 6 AWG Cu | Brown, Orange, Yellow, Gray, Green
- 30A 208V 3-Phase: 10 AWG Cu | Black, Red, Blue, White, Green
- 30A 480V 3-Phase: 10 AWG Cu | Brown, Orange, Yellow, Gray, Green
- 200A 480V 3-Phase (Bonus): 2/0 AWG Cu | Brown, Orange, Yellow, Gray, Green
Always verify your local Authority Having Jurisdiction (AHJ) requirements, as some municipalities enforce stricter voltage drop limits or mandate specific phase-color taping practices at terminations regardless of the insulation jacket color.
