Conduit fill calculator
Add the conductors you plan to pull, including the equipment ground. The calculator totals their area and checks it against the 53 / 31 / 40% limits in Chapter 9 Table 1.
Within Chapter 9 Table 1. Near the limit, pulling is hard on long runs with bends; many electricians go up a trade size.
Up to 7 conductors here. If more than three are current-carrying, their ampacity must be reduced per 310.15(C)(1) (80% for 4–6, 70% for 7–9). Grounding conductors don't count toward that.
How conduit fill is calculated
- Look up each conductor's area in Chapter 9, Table 5 (for THHN/THWN-2: 14 AWG 0.0097 in², 12 AWG 0.0133 in², 10 AWG 0.0211 in², 8 AWG 0.0366 in²).
- Add them up, including equipment grounding conductors (Note 3).
- Divide by the raceway's internal area from Table 4 and compare with Table 1: 53% for one conductor, 31% for two, 40% for three or more.
Example: 16 × 12 AWG THHN = 0.2128 in². 3/4 in. EMT has 0.533 in², so 0.2128 ÷ 0.533 = 39.9%, just inside 40%. A 17th conductor pushes it to 42.4%.
What this calculator covers
THHN/THWN-2 conductors 14 AWG through 4/0 in EMT and rigid PVC Schedule 40, the most common combination in commercial and residential work. Other insulation types (THW, XHHW, RHH) are larger and need their own Table 5 values; compact conductors use Table 5A. A nipple 24 in. or shorter may be filled to 60% (Note 4).
Questions people ask
What is the maximum conduit fill?
NEC Chapter 9 Table 1: 53% for one conductor, 31% for two, 40% for three or more. A nipple 24 in. or shorter may be filled to 60% (Chapter 9, Note 4).
How many 12 AWG THHN fit in 3/4 EMT?
16. 3/4 in. EMT has 0.533 in² of internal area (Table 4); 40% is 0.213 in²; one 12 AWG THHN is 0.0133 in² (Table 5).
Does the ground wire count?
Yes. Equipment grounding and bonding conductors count toward fill (Chapter 9, Note 3). They do not count as current-carrying for ampacity adjustment.
Does conduit fill affect ampacity?
Separately, yes. More than three current-carrying conductors in one raceway triggers the adjustment factors of NEC 310.15(C)(1), whatever the fill percentage.