Complete Guide to Parallel Conductors: NEC 310.10(H) Compliance
The Parallel Conductors Calculator determines the number of conductors per phase to meet NEC ampacity and voltage drop requirements. Supports copper and aluminum for large feeders above 1/0 AWG.
What Are Parallel Conductors?
Parallel conductors are multiple identical cables connected at both ends to share load current. Used when single conductor ampacity is insufficient (e.g., > 600A). NEC 310.10(H) mandates:
- Same length, material, size, insulation
- Same termination method
- Equal current sharing
- Minimum 1/0 AWG
Our Parallel Conductors Calculator automates sizing per NEC Table 310.16 and voltage drop formulas.
Why Use Parallel Conductors?
Benefits include:
- Higher Ampacity: 2× 500 kcmil Cu = 760A @ 75°C
- Reduced Voltage Drop: Lower resistance per phase
- Easier Pulling: Smaller cables through conduit
- Cost Savings: Al parallel vs single large Cu
Common in data centers, industrial plants, and high-rise buildings. Pair with Conduit Fill and Derating tools.
NEC 310.10(H) Requirements
Detailed rules:
| Rule | Requirement |
|---|---|
| Size | ≥ 1/0 AWG |
| Identical | Length, material, circular mils, insulation |
| Terminations | Same method, no splicing mid-run |
| Current Sharing | Balanced impedance |
Ampacity Calculation
Total ampacity = N × single conductor ampacity (Table 310.16). Example: 3× 400 kcmil Cu 75°C = 3 × 380A = 1140A.
Voltage Drop in Parallel
Effective resistance = R_single / N. VD = (2 × L × I × R) / (1000 × N) for single-phase.
Conductor Sizing Steps
- Determine Load: FLA × 1.25 (continuous)
- Select Material/Temp: Cu 75°C common
- Find Min Sets: Load / single ampacity
- Check VD: ≤ 3% feeder, 5% total
- Apply Derating: >3 current-carrying → 80%
Example: 480V 3-Phase 1200A Feeder
150 ft, Cu 75°C, 3% VD max.
- Min ampacity = 1200A
- 500 kcmil = 380A → 1200 / 380 = 3.16 → 4 sets
- 4× 500 kcmil = 1520A
- VD = 1.8% → PASS
Copper vs Aluminum
| Size | Cu 75°C | Al 75°C |
|---|---|---|
| 250 kcmil | 255A | 205A |
| 500 kcmil | 380A | 310A |
| 750 kcmil | 475A | 385A |
Al needs ~1 size up but cheaper. Use anti-oxidant joints.
Conduit and Derating
4–6 conductors → 80% ampacity. Use Conduit Fill Calculator for dimensions.
Cost Analysis
2× 500 kcmil Al often cheaper than 1× 1000 kcmil Cu. Factor pulling ease and terminations.
Installation Best Practices
- Label Phases: A-B-C per set
- Equal Length: ±5% tolerance
- Stagger Pulls: Reduce jamming
- Torque Lugs: Per manufacturer
- Thermal Imaging: Verify balance post-install
Common Mistakes
- Mixing Sizes: Violates NEC
- Ignore VD: Motor starting issues
- No Derating: Overheating
- Unequal Length: Current imbalance
- Single Conduit: For all phases → EMF heating
Advanced Considerations
- Skin Effect: >500 kcmil, use Skin Effect Calculator
- Reactance: Long runs, use Reactance Calculator
- Fault Current: Parallel increases SCC
- Tray vs Conduit: Tray allows more sets
FAQs
Can I parallel 1 AWG? No — minimum 1/0 AWG.
Neutral in parallel? Yes if required (e.g., nonlinear loads).
Ground per set? One EGC sized per 250.122.
Conclusion
Parallel conductors enable large feeder installation while meeting NEC safety standards. Our Parallel Conductors Calculator ensures compliance with ampacity, voltage drop, and derating. Optimize designs with Wire Size, Voltage Drop, and Construction tools. Build efficiently with 1000 Calculators.