Introduction to 3-Phase Power
Three-phase power is the backbone of commercial and industrial electrical systems worldwide. Unlike single-phase power found in most homes, three-phase systems deliver power through three conductors carrying alternating current 120 degrees out of phase with each other. This configuration provides significant advantages including more efficient power transmission, smoother motor operation, and higher power density.
Understanding three-phase wire sizing is essential for engineers, electricians, and facility managers working with commercial buildings, manufacturing plants, data centers, and any installation requiring significant power. This guide covers the fundamentals of 3-phase systems and provides practical guidance for proper conductor sizing.
3-Phase Power Fundamentals
Voltage Relationships
In three-phase systems, two voltage values are commonly referenced: line-to-line (phase-to-phase) voltage and line-to-neutral (phase-to-neutral) voltage. The relationship between them involves the square root of 3:
V(line-to-line) = V(line-to-neutral) × √3
√3 ≈ 1.732. Example: 480V L-L system has 277V L-N voltage.
Common 3-Phase Voltages
| System Type | Line-to-Line | Line-to-Neutral | Typical Use |
|---|---|---|---|
| 208Y/120V | 208V | 120V | Small commercial, multi-family |
| 480Y/277V | 480V | 277V | Commercial, industrial |
| 240V Delta | 240V | N/A (or 120V high-leg) | Older commercial |
| 600Y/347V | 600V | 347V | Canadian industrial |
Delta vs Wye Configurations
Wye (Star) Configuration
In a wye configuration, one end of each phase winding is connected to a common neutral point. This provides access to two voltage levels and is the most common configuration for building power distribution.
- Four-wire system: Three phase conductors plus neutral
- Two voltage levels: Line-to-line and line-to-neutral
- Neutral carries unbalanced current: Size neutral for maximum unbalanced load
- Common example: 480Y/277V - 480V for motors, 277V for lighting
Delta Configuration
In a delta configuration, phase windings are connected end-to-end forming a triangle. This creates a three-wire system with only one voltage level available between phases.
- Three-wire system: No inherent neutral point
- Single voltage level: Line-to-line only
- High-leg delta: Can create 120V with center-tapped transformer
- Common use: Motor loads, older installations
High-Leg Delta Caution
3-Phase Power Calculations
Power Formulas
P = √3 × V(L-L) × I × PF
Three-phase power in watts. V(L-L) = line-to-line voltage, I = line current, PF = power factor
I = P / (√3 × V × PF)
Calculate line current from power. Essential for wire sizing.
Apparent Power (kVA)
S = √3 × V(L-L) × I / 1000
Apparent power in kVA. Used for transformer and conductor sizing.
Wire Sizing for 3-Phase Circuits
Step-by-Step Sizing Process
- Step 1: Calculate the full load current using the power formula
- Step 2: Apply NEC continuous load factor (×1.25) if applicable
- Step 3: Select wire based on ampacity from NEC Table 310.16
- Step 4: Calculate voltage drop and upsize if necessary
- Step 5: Apply derating factors for temperature and conduit fill
3-Phase Voltage Drop
VD = (√3 × K × I × D) / CM
Where K = 12.9 (copper) or 21.2 (aluminum), I = amps, D = one-way distance (ft), CM = circular mils
3-Phase Wire Size Reference
The following table shows recommended copper wire sizes for 3-phase 480V circuits at various distances (based on 3% voltage drop):
| Load (Amps) | 50 ft | 100 ft | 200 ft | 300 ft |
|---|---|---|---|---|
| 20A | 12 AWG | 12 AWG | 10 AWG | 8 AWG |
| 50A | 6 AWG | 6 AWG | 4 AWG | 2 AWG |
| 100A | 3 AWG | 1 AWG | 2/0 AWG | 3/0 AWG |
| 200A | 3/0 AWG | 250 kcmil | 350 kcmil | 500 kcmil |
| 400A | 500 kcmil | 2×3/0 AWG | 2×300 kcmil | 2×400 kcmil |
Neutral Conductor Sizing
In 4-wire wye systems, the neutral conductor carries the unbalanced current between phases. For linear loads, if phases are perfectly balanced, neutral current is zero. However, practical installations are rarely perfectly balanced.
Linear Loads
For traditional linear loads (lighting, heating, motors), the neutral can often be sized smaller than the phase conductors per NEC 220.61:
- First 200A of neutral load: 100%
- Remainder over 200A: 70%
- Minimum size: Per NEC 250.24(C) for service
Non-Linear Loads (Harmonics)
Harmonic Current Warning
Equipment Grounding Conductor
The equipment grounding conductor (EGC) is sized based on the overcurrent protection device per NEC Table 250.122:
| Overcurrent Device (Amps) | Copper EGC | Aluminum EGC |
|---|---|---|
| 60A | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
| 200A | 6 AWG | 4 AWG |
| 400A | 3 AWG | 1 AWG |
| 600A | 1 AWG | 2/0 AWG |
Motor Circuit Considerations
Three-phase motors require special consideration for wire sizing due to starting currents and continuous operation:
- Full Load Current: Use NEC Table 430.250, not nameplate
- Branch Circuit Size: 125% of motor FLA minimum
- Overload Protection: Typically 115-125% of nameplate FLA
- Short Circuit Protection: Size per NEC Table 430.52
Practical Example
Problem: Size conductors for a 75 kVA, 480V 3-phase load, 200 feet from the panelboard, with a power factor of 0.85.
- Calculate current: I = 75,000 / (√3 × 480) = 90.2A
- Continuous load: 90.2A × 1.25 = 112.8A
- Minimum for ampacity: 1 AWG copper (130A at 75°C)
- Check voltage drop at 1 AWG: Approximately 3.5% - marginal
- Upsize to 1/0 AWG: Approximately 2.8% - acceptable
- Final selection: 1/0 AWG copper, 125A breaker
Conclusion
Three-phase wire sizing requires understanding of power relationships, voltage configurations, and the special requirements of commercial and industrial loads. By following NEC requirements for ampacity and considering voltage drop for longer runs, you can design safe and efficient three-phase power distribution systems.
Use our Voltage Drop Calculator to verify your designs, and always consult with a licensed professional engineer for complex industrial installations.