Electrical
Calculate NEC conduit fill percentage, allowable wire count, and raceway capacity for EMT and Schedule 40 PVC with THHN copper conductors.
Select your conduit type, trade size, and conductor gauge, then enter wire count. The calculator applies NEC Chapter 9 fill limits to check raceway capacity.
Calculation and worked example reviewed 2026-11-12. See our calculation methodology.
Total conductor area 0.0532 sq in of 0.533 sq in raceway.
NEC Chapter 9 limit: 40% max fill for 3+ wires.
Formula and example
This estimator converts your entered jobsite inputs (conduit type & trade size, thhn copper wire gauge, number of conductors) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.
where:
Conductor area = wire count × wire area (NEC Ch 9 Table 5); allowed fill area = conduit area (Table 4) × allowed % (1 wire: 53%, 2 wires: 31%, 3+ wires: 40%); fill % = (conductor area ÷ conduit area) × 100; max wires = floor(allowed fill area ÷ wire area).
Four 12 AWG THHN conductors in 3/4 in EMT raceway occupy 4 × 0.0133 = 0.0532 sq in. In a 0.533 sq in conduit, fill is 9.98%—well within the 40% NEC maximum allowable limit of 16 conductors.
Maximum number of conductors permitted in common raceways under NEC Chapter 9 Table 1 (40% limit for 3 or more wires).
| Conduit Trade Size | 14 AWG THHN | 12 AWG THHN | 10 AWG THHN |
|---|---|---|---|
| 1/2" EMT | 12 wires | 9 wires | 5 wires |
| 3/4" EMT | 22 wires | 16 wires | 10 wires |
| 1" EMT | 35 wires | 26 wires | 16 wires |
| 1-1/4" EMT | 61 wires | 45 wires | 28 wires |
Assumes identical THHN/THWN-2 copper conductors without outer jackets. Multiconductor cables, mixed conductor sizes, grounding wires, and compact aluminum stranding require individual cross-sectional summation under NEC Chapter 9 Note 2.
Under NEC Chapter 9 Table 1, maximum allowable fill is 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors to prevent heat buildup and insulation damage during pulling.
Under NEC Chapter 9, Table 1, raceways with three or more conductors must not exceed 40% total cross-sectional area fill. This ensures sufficient heat dissipation and prevents mechanical damage to cable insulation while pulling.
Yes. All conductors routed through the raceway—including equipment grounding conductors and neutral wires—must be included in the total cross-sectional area calculation.
Up to 16 THHN 12 AWG copper conductors can fit in a 3/4" EMT conduit under the standard 40% fill limit (0.213 sq in allowable area divided by 0.0133 sq in per wire).
Method references: NFPA 70: National Electrical Code Chapter 9 Tables