How to Calculate Conduit Fill for Mixed Wire Sizes
When a raceway carries more than one conductor size, the National Electrical Code (NEC) Annex C tables do not apply, because every Annex C column assumes a single conductor size. You do the calculation by hand: look up each conductor's area in NEC 2020 Chapter 9 Table 5, multiply by quantity, sum the areas, and compare the total against the raceway's allowable area, which is the Table 4 internal area times the Table 1 fill percentage. Two worked feeder examples follow.
The Conduit Fill Calculator takes mixed bundles directly: add one row per conductor size and insulation, and it returns the total area, fill percentage, and minimum trade size. Use it to check the hand math below.
The four steps
- Look up each conductor's area in NEC Chapter 9 Table 5 by insulation type and size.
- Multiply each area by its quantity and add the results. That sum is the total conductor area.
- Count every conductor in the raceway, grounds included, and pick the Table 1 fill limit: 53% for one, 31% for two, 40% for three or more.
- For each candidate raceway, multiply its Table 4 total area by that percentage. If the allowable area is at or above your total conductor area, the conduit passes.
Two details trip people up. The equipment grounding conductor (EGC) counts, both toward the total area and toward the conductor count that selects the 31% or 40% bracket. And the drop to 31% at exactly two conductors is real; three conductors and up all use 40%.
Example 1: a 60 A subpanel feeder
Two #6 THHN ungrounded conductors, one #6 THHN neutral, and one #10 THHN equipment grounding conductor. Three conductors at one size plus a smaller ground, so Annex C has no single column for it.
Table 5: #6 THHN = 0.0507 in², #10 THHN = 0.0211 in²
Conductors = 3 × 0.0507 + 1 × 0.0211
= 0.1521 + 0.0211 = 0.1732 in²
Count = 4 conductors → 40% fill
1/2 in EMT (0.304): 0.304 × 0.40 = 0.1216 in² → fails
3/4 in EMT (0.533): 0.533 × 0.40 = 0.2132 in² → passes
The bundle needs 3/4 inch EMT. Actual fill is 0.1732 / 0.533 = 32.5%, comfortably inside the 40% ceiling, while 1/2 inch EMT falls short at an allowable 0.1216 in². Note that the count of four includes the ground; drop the EGC and you would still be at three conductors and still in the 40% bracket, but with a real EGC in the pipe the fill number itself changes.
The same conductors change size in Schedule 80 PVC
The conductor total is a property of the wire, not the raceway, so it stays 0.1732 in². What changes is the Table 4 internal area, which differs by raceway type for the same trade size. Schedule 80 PVC has a thicker wall and less room than EMT.
3/4 in PVC Sch 80 (0.409): 0.409 × 0.40 = 0.1636 in² → fails
1 in PVC Sch 80 (0.688): 0.688 × 0.40 = 0.2752 in² → passes
The identical four conductors that fit 3/4 inch EMT need 1 inch Schedule 80 PVC, because 3/4 inch Sch 80 allows only 0.1636 in² and the bundle is 0.1732 in². In 1 inch Sch 80 the fill is 0.1732 / 0.688 = 25.2%. This is why the raceway type has to be chosen before the fill is final: Schedule 40 PVC, Schedule 80 PVC, EMT, IMC, and RMC each publish their own Table 4 areas, and a Schedule 80 wall can cost a trade size against EMT on the same pull.
Example 2: a three-phase feeder with a smaller ground
Three #2 THHN phase conductors, one #2 THHN neutral, and one #8 THHN equipment grounding conductor in EMT. Five conductors, two different sizes.
Table 5: #2 THHN = 0.1158 in², #8 THHN = 0.0366 in²
Conductors = 4 × 0.1158 + 1 × 0.0366
= 0.4632 + 0.0366 = 0.4998 in²
Count = 5 conductors → 40% fill
Minimum area needed = 0.4998 / 0.40 = 1.2495 in²
1 in EMT (0.864): 0.864 × 0.40 = 0.3456 in² → fails
1-1/4 in EMT (1.496): 1.496 × 0.40 = 0.5984 in² → passes
The minimum is 1-1/4 inch EMT, filled to 0.4998 / 1.496 = 33.4%. The shortcut of dividing the total area by the fill fraction, 0.4998 / 0.40 = 1.2495 in², gives the minimum internal area directly, so you can scan Table 4 for the first trade size that clears it rather than testing sizes one at a time.
Two shortcuts that go wrong
Because Annex C is so convenient, it gets misapplied to mixed pulls in two ways. Reading the count for the largest conductor size and ignoring the smaller ones understates the area only when the small conductors are numerous, and it is not what the tables say to do. Treating every conductor as the largest size is conservative and safe but routinely oversizes the raceway, wasting material on a long run. Neither is the code method. Summing the actual Table 5 areas is exact, and for a single mixed pull it takes about a minute. For same-size pulls, the shortcut is legitimate and the counts are already tabulated; see how many #12 THHN fit in 3/4 EMT for the single-size method and a full EMT chart.
NEC reference
NEC 2020. Fill percentages from Chapter 9 Table 1 (53% for one conductor, 31% for two, 40% for three or more). Conduit internal areas from Chapter 9 Table 4, 100% total-area column, read for the specific raceway type. Conductor areas from Chapter 9 Table 5, THHN/THWN/THWN-2 column. Annex C tables apply only to raceways of a single conductor size; mixed sizes use the Table 1, Table 4, and Table 5 method shown here.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Get the full guide
NEC Code Quickstart
This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.
Related
- Conduit Fill Calculator: enter a mixed bundle and read the fill percentage and minimum trade size.
- How Many #12 THHN Fit in 3/4 EMT: the single-size method, worked, with a full EMT count chart.
- Wire Size Calculator: size the conductors for ampacity and voltage drop before sizing the raceway.