Conduit Sizing Guide
NEC Chapter 9 conduit fill tables and calculations for proper electrical raceway sizing
NEC Chapter 9 Requirements
Conduit fill is limited by NEC Chapter 9 to prevent overheating and ensure proper wire installation. These limits apply to all raceway types including EMT, rigid, and flexible conduit.
Conduit Fill Percentages
NEC Chapter 9, Table 1 specifies maximum fill percentages based on the number of conductors:
| Number of Conductors | Maximum Fill Percentage | Typical Applications |
|---|---|---|
| 1 | 53% | Service entrance, large feeders |
| 2 | 31% | 120V/240V circuits |
| 3 or more | 40% | Multi-wire branch circuits |
Common Conduit Types and Dimensions
EMT (Electrical Metallic Tubing)
Most common for commercial and residential installations. Lightweight and cost-effective.
| Trade Size | Internal Diameter | Area (sq in) | 40% Fill | 31% Fill | 53% Fill |
|---|---|---|---|---|---|
| 1/2" | 0.622" | 0.304 | 0.122 | 0.094 | 0.161 |
| 3/4" | 0.824" | 0.533 | 0.213 | 0.165 | 0.283 |
| 1" | 1.049" | 0.864 | 0.346 | 0.268 | 0.458 |
| 1 1/4" | 1.380" | 1.496 | 0.598 | 0.464 | 0.793 |
| 1 1/2" | 1.610" | 2.036 | 0.814 | 0.631 | 1.079 |
| 2" | 2.067" | 3.356 | 1.342 | 1.040 | 1.779 |
Rigid Metal Conduit (RMC)
Heavy-duty applications, outdoor installations, and areas requiring maximum protection.
| Trade Size | Internal Diameter | Area (sq in) | 40% Fill | 31% Fill | 53% Fill |
|---|---|---|---|---|---|
| 1/2" | 0.632" | 0.314 | 0.125 | 0.097 | 0.166 |
| 3/4" | 0.836" | 0.549 | 0.220 | 0.170 | 0.291 |
| 1" | 1.063" | 0.887 | 0.355 | 0.275 | 0.470 |
| 1 1/4" | 1.394" | 1.526 | 0.610 | 0.473 | 0.809 |
| 2" | 2.083" | 3.408 | 1.363 | 1.057 | 1.806 |
Wire Areas for Common Sizes
NEC Chapter 9, Table 5 provides conductor dimensions including insulation for conduit fill calculations:
THWN-2 Copper Conductor Areas
| AWG | Area (sq in) |
|---|---|
| 14 | 0.0097 |
| 12 | 0.0133 |
| 10 | 0.0211 |
| 8 | 0.0366 |
| 6 | 0.0507 |
| 4 | 0.0824 |
| 2 | 0.1158 |
| 1 | 0.1562 |
Common Installation Examples
Wire area: 3 × 0.0133 = 0.040 sq in
Minimum conduit: 1/2" EMT (0.122 sq in @ 40%)
Wire area: 2 × 0.0824 = 0.165 sq in
Minimum conduit: 3/4" EMT (0.165 sq in @ 31%)
Wire area: 0.2223 sq in
Minimum conduit: 1" EMT (0.458 sq in @ 53%)
Derating for Conduit Fill
When more than three current-carrying conductors are in the same conduit, ampacity must be derated per NEC 310.15(B)(3)(a):
Ampacity Adjustment Factors
Special Considerations
Equipment Grounding Conductors
- • Do not count toward fill calculations
- • May be bare or insulated
- • Still take up physical space
- • Consider in practical installations
Neutral Conductors
- • Count if carrying unbalanced current
- • Don't count in balanced 3-wire circuits
- • Always count in multiwire branch circuits
- • Consider harmonic currents
Installation Best Practices
Tips for Successful Conduit Installation
Before Installation:
- • Calculate fill percentages accurately
- • Consider future circuit additions
- • Plan pulling routes and angles
- • Select appropriate lubricants
During Installation:
- • Use proper pulling techniques
- • Avoid exceeding pull tension limits
- • Install pull boxes as needed
- • Maintain minimum bend radii
Need Help with Wire and Conduit Sizing?
Use our wire size calculator to determine conductor requirements, then apply conduit fill tables for proper raceway sizing.
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