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The gauge of the conductors being pulled through the conduit.
This calculator assumes standard THHN/THWN insulation, the most common type for conduit runs.
The trade size of the EMT conduit.
This uses standard EMT (thin-wall) conduit internal area figures.
Maximum conductors
16 conductors (maximum)
This uses standard EMT and THHN area tables — different conduit types (PVC, rigid) or conductor insulation types have different area values. Always verify against the current NEC Chapter 9 tables for your specific materials.
- Available fill area (40% of conduit)
- 0.21 in²
They open the calculator with your figures already in it
Conduit Fill Calculator: 16 conductors (maximum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- NEC Chapter 9, Table 1: conduit fill is limited to 40% of internal area for 3 or more conductors
- Approximate THHN conductor areas and EMT conduit internal areas, per NEC Chapter 9 tables
- NEC Chapter 9, Notes to Tables, Note 7: where all conductors are the same size, a result ending in a decimal of 0.8 or larger is taken to the next whole number
Inputs used
- Conductor Size (THHN)
- 12 AWG
- Conduit (EMT) Size
- 3/4 in
Intermediate steps
- Available fill area (40% of conduit)
- 0.21 in²
Confidence note: This uses standard EMT and THHN area tables — different conduit types (PVC, rigid) or conductor insulation types have different area values. Always verify against the current NEC Chapter 9 tables for your specific materials.
What this calculation does not cover
- Fill is not ampacity. This says how many conductors physically fit under the 40% rule; it does not derate them, and the NEC requires conductor ampacity to be adjusted downward once more than three current-carrying conductors share a raceway. A bundle that passes fill can still need a larger conductor or a smaller breaker — this is not a load calculation and it does not size the circuit.
- EMT and THHN/THWN only. PVC, IMC, rigid and flexible raceways have different internal areas at the same trade size, and other insulations — XHHW, TW, RHW, anything with a thicker jacket — have different conductor areas. Change either and the count changes.
- Every conductor in the raceway counts toward the 40%, including neutrals and the equipment grounding conductor, and the count assumes they are all the same size. A bundle mixing gauges has to be totalled area by area against 40% of the conduit's internal area rather than read off a single-gauge count.
- Fitting is not pulling. Nothing here accounts for run length, the number and radius of bends, pulling tension, lubricant or conductor jamming — a conduit loaded to the code maximum across a long run with several bends can be impractical to pull.
- These are US NEC figures for a run of three or more conductors. A single-conductor run is permitted 53% fill and a short nipple between enclosures 60%, but a two-conductor run is held to 31% — less than the 40% applied here, not more. Outside North America conduit is sized by different methods entirely, so check the code edition and any local amendments your jurisdiction has adopted.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations3
- NEC Chapter 9, Table 1: conduit fill is limited to 40% of internal area for 3 or more conductors
- Approximate THHN conductor areas and EMT conduit internal areas, per NEC Chapter 9 tables
- NEC Chapter 9, Notes to Tables, Note 7: where all conductors are the same size, a result ending in a decimal of 0.8 or larger is taken to the next whole number
Which documents these citations point at
- National Electrical Code (NFPA 70) — Chapter 9 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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Protection this work requires
- Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.
Tools and safety for this job
To first-fix electrical. Generic types, no brands, no prices.
- Assume every cable is live until proved dead at the point you will touch, with a two-pole tester you have just proved on a known source.
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