TradeToolsElectrical · in the TradeTools app
Conduit Fill Calculator
How much of a raceway the conductors may take up, and the smallest trade size that fits. Mixed conductor sizes and types, nipples at 60 %, and the room left for more of the last size you added.
NEC Ch. 9 Tables 1, 4 and 5
App Store in review
Worked example
Seven 12 AWG THHN in 1/2" EMT — will they fit?
| Conduit | EMT |
|---|---|
| Trade size | 1/2" |
| Nipple (24 in or shorter) | off |
| Conductors | 7 × 12 AWG THHN/THWN-2 |
| Fill | 30.6 % |
|---|---|
| Allowed | 40 % (more than two conductors) |
| Conductor area | 0.0931 in² |
| Conduit area | 0.304 in² |
| Room left for more of the last size | 2 |
| Smallest conduit that fits | 1/2" |
7 × 0.0133 in² = 0.0931 in² of conductor against 0.304 in² of EMT is 30.6 %, under the 40 % Table 1 allows for more than two conductors. Two more 12 AWG THHN would still fit — nine in total, which is the count Annex C Table C.1 lists for the same case.
How it's calculated
The table behind the number
Three lookups and one division. Table 5 gives the area of each insulated conductor by insulation type; the app adds them up. Table 4 gives the internal area of the conduit for its type and trade size. Table 1 sets the limit: 53 % for one conductor, 31 % for two, 40 % for more than two, and 60 % for a nipple of 24 inches or less (Note 4). The app divides conductor area by conduit area, compares it with the limit, reports how many more of the last conductor would fit, and names the smallest trade size of the same conduit type that passes.
- NEC 2023 Chapter 9, Table 1Maximum fill: 53 % one conductor, 31 % two, 40 % over two; Note 4 nipples ≤ 24 in at 60 %
- NEC 2023 Chapter 9, Table 4Internal area of conduit and tubing by type and trade size (EMT 1/2" 0.304 in², 3/4" 0.533 in², 1" 0.864 in²)
- NEC 2023 Chapter 9, Table 5Area of insulated conductors by type and size (THHN/THWN-2: 12 AWG 0.0133 in², 10 AWG 0.0211 in², 6 AWG 0.0507 in²)
- NEC 2023 Annex CConductor counts for same-size, same-type conductors only
Questions
Before you trust the number
Can I use Annex C instead?
Only when every conductor is the same size and insulation type. Mix sizes — a feeder with three 6 AWG and a 10 AWG ground — and Annex C has no row for you; the app does the Table 4 and Table 5 arithmetic directly. One nuance: the app counts whole conductors under the limit and does not apply Note 7 rounding, so for same-size runs Annex C can list one more than the app's "room left".
Is conduit fill the same as derating?
No. Fill is about pulling conductors in without damage. Ampacity adjustment for more than three current-carrying conductors is 310.15(C)(1), a separate rule and a separate calculator (Wire Size).
What about a nipple?
A conduit nipple of 24 inches or less between enclosures may be filled to 60 % (Chapter 9, Table 1, Note 4). Flip the Nipple switch and the limit changes.