Conduit Fill Calculator
Mix wire sizes and insulation types, pick the conduit, and see the fill percentage against the NEC limit — plus the smallest trade size that fits.
Conductors
Count every conductor in the raceway, including neutrals and equipment grounds (as insulated conductors).
Result
Based on NFPA 70 (NEC) 2023 tables. An estimating aid, not an engineering design: confirm sizes against the code edition your authority having jurisdiction enforces, local amendments and manufacturer data.
How conduit fill is calculated
- Add up the cross-section area of every conductor from NEC Chapter 9 Table 5 (THHN, XHHW…).
- Look up the conduit’s total internal area in Chapter 9 Table 4 for its type and trade size.
- Divide: conductor area ÷ conduit area = fill %. It must not exceed 53 % (1 wire), 31 % (2 wires) or 40 % (3+ wires).
Example: six 12 AWG THHN = 6 × 0.0133 = 0.0798 in². In 1/2" EMT (0.304 in²) that is 26.3 % — well under 40 %.
Frequently asked questions
What is the maximum conduit fill under the NEC?
NEC Chapter 9 Table 1: 53 % of the conduit’s internal area for one conductor, 31 % for two, and 40 % for three or more. The 40 % rule covers almost every branch circuit and feeder.
How many 12 AWG THHN wires fit in 3/4" EMT?
Sixteen. 3/4" EMT has 0.533 in² of internal area, so 40 % is 0.213 in²; one 12 AWG THHN is 0.0133 in², and 0.213 ÷ 0.0133 = 16.0. In 1/2" EMT the answer is nine.
Do grounds and neutrals count toward conduit fill?
Yes. Every conductor in the raceway counts toward fill, including neutrals and equipment grounding conductors. (Neutrals and grounds are treated differently for ampacity adjustment, which is a separate calculation — see the wire size calculator.)
What is the 0.8 rounding rule?
Chapter 9 Note 7: when all conductors are the same size and the calculated number of conductors comes out with a decimal of 0.8 or more, the next whole number is permitted. That is why 22 × 14 AWG THHN fit in 3/4" EMT even though 22 × 0.0097 in² is a hair over 40 %. The calculator applies it automatically.
Does conduit fill apply to short nipples?
No. A nipple of 24 inches or less between boxes or enclosures may be filled to 60 % (Chapter 9 Note 4). This calculator uses the normal limits for longer runs.
Sizing one run is easy. Counting a whole job isn't.
Aginera reads your electrical drawings and counts devices, fixtures and panels, measures conduit and feeder runs, and builds the takeoff with a sheet reference for every number.
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