How the conduit fill calculator works
Add conductor rows, counting every conductor that occupies the raceway. Sum insulated areas and select the allowed-area column based on conductor count or qualified nipple status. For identical conductors, use the supported count-rounding rule; mixed conductors use strict area comparison.
Occupied area = Σ(quantity × insulated conductor area). Normal allowance: 53% for one conductor, 31% for two, 40% for more than two. Qualified nipples ≤24 in: 60%. Identical conductors: floor(allowed / single area), adding one only when the fraction is at least 0.8.
Worked example
Four #12 THHN/THWN-2 copper conductors in 1/2 in EMT occupy 4 × 0.0133 = 0.0532 in². The published 40% allowed area is 0.122 in²; actual fill is 100 × 0.0532 / 0.304 = 17.50%. Identical-conductor ratio is 0.122 / 0.0133 = 9.17, giving nine allowed conductors by the count rule. Four pass; the smallest listed EMT trade size is 1/2 in.
NEC references
NEC 2023: Chapter 9 Table 1; Chapter 9 Table 4; Chapter 9 Table 5; Chapter 9 Notes 3, 4, 6, 7; 310.15(C)(1). These are reference identifiers, not reproduced code text. Read our methodology and verification limits.
Limitations
Four raceway types only: EMT, PVC Schedule 40, PVC Schedule 80 and RMC. No compact aluminum, bare grounds, multiconductor cable, isolated protection sleeves or special manufacturer dimensions. A passing fill check does not establish pulling feasibility, bends, grounding or ampacity. Nipple qualification requires both enclosure connections and length excluding fittings.
For estimation only. Results compare loads under the selected NEC method; they do not guarantee permit approval or code compliance. Verify with your local adopted code and a licensed electrician.
FAQ
Do grounds and neutral conductors count?
Yes, their insulated physical areas and counts enter this fill calculation. Ampacity current-carrying count is a different question.
Can I mix THHN and XHHW-2?
Yes, for the supported round copper sizes. Mixed insulation uses area comparison without identical-conductor rounding.
Can I choose 60% for a short conduit?
Only for a qualifying nipple no longer than 24 inches connecting enclosures. Confirm the installation and enter actual length.
Why can identical conductors pass slightly above the area value?
The supported count rule can round up when its fractional count reaches 0.8. The result identifies when that rule is used.
Is PVC 40 interchangeable with PVC 80?
No. Their available interior areas differ, so choose the actual raceway type.
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