Checks whether a set of conductors or cables fits within a conduit, tubing, or trunking raceway without exceeding the maximum allowable fill percentage \u2014 the rule that keeps conductors coolable and pullable without insulation damage.
Standards followed
NEC Chapter 9 Table 1 (fill percentages)NEC Chapter 9 Table 4 (conduit internal area)NEC Chapter 9 Table 5 (conductor area)IEC / BS 7671 (IET On-Site Guide) 40%/45% space-factor practice
What it can do
- NEC path: select conduit type & trade size, add any number of conductor rows (insulation + AWG size + quantity), and get the fill percentage against the correct 1/2/3+ conductor fill tier.
- Suggests the minimum standard trade size that keeps the fill percentage compliant.
- IEC/BS 7671 path: circular conduit (about 40%) or rectangular trunking (45%) space-factor check from actual cable outer diameters.
Example problem
Six 12 AWG THHN conductors are pulled into a 3/4" EMT conduit. Check the fill percentage.
- Look up each conductor's individual area from NEC Chapter 9 Table 5 (12 AWG THHN) and sum: total conductor area = 0.0798 in².
- With 3+ conductors, the applicable fill tier is 40% of the conduit's internal area.
- 3/4" EMT internal area (Table 4) = 0.533 in² → 40% allowable = 0.2132 in².
- Fill % = 0.0798 / 0.533 = 14.97%.
14.97% fill, well within the 40% limit — pass, with 1/2" EMT suggested as the minimum size that would still work.
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