Conduit Fill Calculator
Pick a conduit type, a THHN conductor size and how many wires you are pulling — the tool returns the minimum EMT, RMC or PVC Schedule 40 trade size per NEC Chapter 9 fill rules. No sign-up, instant results.
Conduit fill chart — NEC Chapter 9 Table 4 internal areas (in²)
| Trade size | EMT total (in²) | EMT @ 40% | RMC total (in²) | RMC @ 40% | PVC Sch. 40 total (in²) | PVC Sch. 40 @ 40% |
|---|---|---|---|---|---|---|
| 1/2" | 0.304 | 0.122 | 0.314 | 0.125 | 0.304 | 0.122 |
| 3/4" | 0.533 | 0.213 | 0.549 | 0.220 | 0.533 | 0.213 |
| 1" | 0.864 | 0.346 | 0.887 | 0.355 | 0.864 | 0.346 |
| 1-1/4" | 1.496 | 0.598 | 1.526 | 0.610 | 1.496 | 0.598 |
| 1-1/2" | 2.036 | 0.814 | 2.071 | 0.829 | 2.036 | 0.814 |
| 2" | 3.356 | 1.342 | 3.401 | 1.360 | 3.356 | 1.342 |
| 2-1/2" | 5.858 | 2.343 | 5.935 | 2.374 | 4.788 | 1.915 |
| 3" | 8.846 | 3.538 | 8.920 | 3.568 | 7.393 | 2.957 |
| 3-1/2" | 11.545 | 4.618 | 11.642 | 4.657 | 9.887 | 3.955 |
| 4" | 14.753 | 5.901 | 14.862 | 5.945 | 12.730 | 5.092 |
Fill percentages per NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for three or more. Areas above are computed from the Table 4 internal diameters (EMT / RMC / PVC Schedule 40) and rounded to three decimals, matching the published table values. Conductor cross-sections use NEC Chapter 9 Table 5 (THHN/THWN, approximate areas in in²).
Verify against the NEC edition adopted in your jurisdiction — this tool reflects the general (NEC 2023) values for planning, not a code inspection. For uniform conductor groups the Annex C count tables may differ by one conductor from an area calculation due to rounding of published areas.
How the Calculator Works
Conduit sizing in the NEC is an area comparison, done in three steps:
1. Conductor area. Each insulated conductor has a cross-sectional area from NEC Chapter 9 Table 5 — for example 12 AWG THHN is 0.0133 in², 10 AWG is 0.0211 in². Multiply by the conductor count.
2. Fill percentage. Chapter 9 Table 1 sets the maximum occupation of the conduit cross-section: 53 % for one conductor, 31 % for two, and 40 % for three or more. (Conduit nipples up to 24 in. may be filled to 60 %.)
3. Conduit area. NEC Chapter 9 Table 4 lists each trade size’s total internal area and the 53 / 31 / 40 % allowances; the chart below reproduces those values for EMT, RMC and PVC Schedule 40. The answer is the smallest trade size whose applicable allowance is at least the total conductor area.
Example: 6 × 10 AWG THHN = 6 × 0.0211 = 0.127 in². The 40 % column of 1/2" EMT is 0.122 in² — just short — so 3/4" EMT (0.213 in²) is the minimum size.
When Engineers Use This Calculator
Branch-circuit homeruns. Three #12 THHN (phase, neutral, ground) in EMT is the classic residential and light-commercial case — one glance at the 40 % column settles whether 1/2" is enough or 3/4" is warranted.
Panel and feeder schedules. When a submittal lists mixed conductor sizes in one raceway, area-based calculation is the only method the NEC accepts for mixed groups (Annex C lookup tables only cover uniform groups).
Control wiring in cabinets. Bundles of #14 and #16 control wires between terminals and cable glands add up fast; checking fill before fabrication prevents rework on the panel build.
Upgrades and retrosfits. Adding a circuit to an existing conduit? Re-run the fill with the new total count — the old installation may already be near the 40 % limit.
Frequently Asked Questions
What is the conduit fill rule in the NEC?
NEC Chapter 9 Table 1: one conductor may fill up to 53 % of the conduit’s internal cross-section, two conductors up to 31 %, and three or more up to 40 . Conductor areas come from Table 5, conduit areas from Table 4, and the comparison of the two is the fill calculation this page performs.
How many 12 AWG THHN wires fit in 1/2" EMT?
Nine. 9 × 0.0133 in² = 0.120 in², just inside the 0.122 in² that 40 % of a 1/2" EMT allows; ten wires would exceed it. The wire fill chart above shows every trade size the same way.
Do I count the equipment grounding conductor?
Yes — every conductor in the raceway counts toward fill, including grounds and neutrals, each at its own Table 5 area. An oversized ground specified for fault performance can push the total over the limit on an otherwise comfortable run.
What if my calculation disagrees by one wire with a fill chart?
Published areas are rounded to three or four decimals, so a hand calculation sitting exactly on the boundary can differ by one conductor from the Annex C tables (which are computed from unrounded values). When it matters, size up — larger conduit also reduces pulling tension and leaves room for future circuits.
Does conduit fill affect ampacity?
Indirectly. Fill itself is a physical limit, but more than three current-carrying conductors in one raceway trigger the NEC 310.15(C)(1) adjustment factors that derate ampacity — typically the binding constraint before fill is. Pair this calculator with the wire size calculator to check both.
Related Guides
- Wire Size Calculator — NEC 310.16 ampacity table with voltage-drop check.
- Voltage Drop Calculator — size the conductor electrically before the raceway physically.
- Electrical Control Panel Design Specification — wiring, raceway and layout requirements for panel builds.
- Membrane Cable Glands for Enclosures — where the conduit meets the cabinet.
- Transformer kVA Calculator — full-load current at the source end of the feeder.
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