Beginner
Conduit Fill Formula
NEC conduit fill calculation — maximum conductor area as percentage of conduit area for 1, 2, or 3+ conductors.
Formulas
Fill% = (Σ A_conductor / A_conduit) × 100
Conduit fill percentage
1 conductor = 53% max fill
Single conductor fill limit
2 conductors = 31% max fill
Two conductor fill limit
3+ conductors = 40% max fill
Three or more conductor fill limit
Variable Definitions
A_conductor=Cross-sectional area of each conductor (with insulation)(square inches (in²))
A_conduit=Internal area of conduit(square inches (in²))
Fill%=Percentage of conduit area used(%)
Worked Example
Can you fit 4 × 10 AWG THHN (area = 0.0211 in² each) in ½" EMT (area = 0.304 in²)?
- Total conductor area = 4 × 0.0211 = 0.0844 in²
- Fill = 0.0844 / 0.304 × 100 = 27.8%
- Limit for 3+ conductors = 40%
- 27.8% < 40% — fits ✓
Answer: Yes, 27.8% fill (limit 40%)
When to Use
- •Selecting conduit size for a given number of conductors
- •Verifying existing conduit has room for additional wires
- •Designing conduit runs for new installations
- •Calculating nipple fills (≤24" conduits get 60% fill)
Standards & References
- NEC Chapter 9, Table 1 — Percent Fill
- NEC Chapter 9, Table 4 — Conduit Dimensions
- NEC Chapter 9, Table 5 — Conductor Areas
- NEC 344/348/350/352/358 — Various conduit types