Conduit Fill Calculator
Free conduit fill calculator for EMT, PVC, IMC, and RMC. NEC Chapter 9 Table 1 limits, with a worked example. No signup.
About this calculator
Why fill limits exist
Cramming too many conductors into a raceway damages insulation during the pull, traps heat so conductors can't cool, and makes future pulls nearly impossible. NEC Chapter 9, Table 1 caps fill by conductor count: one conductor 53%, two conductors 31%, three or more 40% of the raceway's interior cross-section.
How the math works
Fill is the total conductor cross-sectional area (NEC Chapter 9, Tables 5 and 5A for insulated conductors, Table 8 for bare) divided by the raceway's interior area (Table 4), compared against the Table 1 percentage for your conductor count. This calculator handles single-size and mixed-size conductor combinations across EMT, PVC, IMC, and RMC.
Worked example
Three 4 AWG THHN conductors plus one 8 AWG bare equipment ground in 1-inch EMT:
- 4 AWG THHN ≈ 0.0824 in² each → 3 × 0.0824 = 0.2472 in²
- 8 AWG bare ≈ 0.0130 in²
- Total = 0.2602 in² vs. the 1" EMT 40% allowance of 0.346 in² → passes at about 75% of allowed fill.
Field notes
- Fill is not derating: even when fill passes, more than three current-carrying conductors triggers ampacity adjustment per 310.15(C)(1).
- Short nipples 24 inches or less get a 60% allowance (Chapter 9, Note 4) — derating still applies.
- When pulling three large conductors, check the jam ratio: conduit ID ÷ conductor OD between 2.8 and 3.2 risks jamming, so size up the raceway if you land in that window.
Common questions
What does a conduit fill calculator check?
It adds the cross-sectional area of the insulated conductors and compares that total with the raceway area allowed by NEC Chapter 9, Table 1: 53% for one conductor, 31% for two, and 40% for three or more.
Does passing conduit fill mean the wires are big enough?
No. Fill is a space check. Ampacity still comes from NEC Table 310.16, and more than three current-carrying conductors need the adjustment in 310.15(C)(1).
For design, installation, or safety decisions, verify assumptions against the applicable code edition, equipment documentation, and site requirements.
Open the interactive tool