Conduit Size Chart & Calculator
Estimate conduit size for common wire counts and conduit types. Use it as a planning reference and verify final selections with current code tables.
Conduit Sizing Calculator
Conduit Sizing for Electrical Installations
Real talk: conduit sizing matters because oversized gets expensive fast, and undersized creates hard pulls and higher fill percentages. Common planning guidance starts around the 40% fill limit for three or more conductors. Three #12 THHN in 1/2-inch EMT is a common example where space gets tight fast.
Too many wires crammed in conduit? They get hot from friction, insulation degrades, and you're looking at a fire waiting to happen. Undersized conduit also makes pulling wires a nightmare. The 40% rule exists for safety and practicality.
Conduit Types: Pick the Right One
- EMT: Lightweight, affordable, standard for most indoor residential. Not rated for burial. Fast to install.
- PVC Schedule 40: Non-metallic, won't rust, good for wet areas and underground where code allows. Cheaper than metal.
- PVC Schedule 80: Thicker walls, tougher. Required for underground or high-traffic areas. Smaller inner diameter than Sch 40.
- IMC: Better corrosion resistance than EMT. Hybrid choice when you need toughness without rigid's cost.
- Rigid Metal: Built like a tank. Outdoor, hazardous locations, industrial. Expensive and labor-intensive to install.
The 40% Fill Rule (It's Not Negotiable)
NEC 300.17: wires must not exceed 40% of conduit area. Why? Wire pulls get impossible above 40%. Heat builds up. The extra space allows future additions. Special cases: two conductors get 50%, one gets 53%, but anything over three wires stays at 40%.
Conduit Sizes and Interior Dimensions
| Trade Size | EMT ID (in) | PVC 40 ID (in) | PVC 80 ID (in) | 40% Fill Area (sq in) |
|---|---|---|---|---|
| 1/2" | 0.622 | 0.622 | 0.546 | 0.122 |
| 3/4" | 0.824 | 0.824 | 0.722 | 0.213 |
| 1" | 1.049 | 1.049 | 0.920 | 0.346 |
| 1 1/4" | 1.380 | 1.380 | 1.210 | 0.598 |
| 1 1/2" | 1.610 | 1.610 | 1.426 | 0.814 |
| 2" | 2.067 | 2.067 | 1.835 | 1.342 |
| 2 1/2" | 2.731 | 2.731 | 2.469 | 2.343 |
| 3" | 3.356 | 3.356 | 3.068 | 3.540 |
| 4" | 4.166 | 4.166 | 3.834 | 5.452 |
Installing Conduit the Right Way
Add up your wire cross-sectional areas. Multiply conduit's internal area by 0.40. Your total wires must fit. Use lubricant when pulling anything over a couple wires. Secure every 3 feet and within 12 inches of boxes. That's the minimum.
