Conduit Fill: How Many Wires Fit
You can't just stuff conduit until it's full. The NEC caps how much of the pipe's cross-section your wires may occupy — and the number is smaller than most people guess.
The 40% rule, in one paragraph
NEC Chapter 9, Table 1 sets the limits: one conductor may fill 53% of the raceway's cross-section, two conductors 31%, and three or more conductors 40%. (Short nipples 24 inches or less get 60%.) The odd-looking 31% for two wires accounts for how two round conductors jam against each other and the pipe wall — they're harder to pull than three.
Why does the rule exist? Two reasons: heat (conductors packed tight can't shed it) and the pull (overfilled conduit shreds insulation on the way through). The percentages aren't suggestions — they're what the inspector measures.
The two tables you actually need
Conduit fill math uses two NEC Chapter 9 tables: Table 4 gives the cross-sectional area of each conduit type and size, and Table 5 gives the cross-sectional area of each conductor type and size (insulation included — that's what actually takes up space). Divide the usable area by the conductor area, round down.
For the most common job — copper THHN in EMT — the pre-computed answers look like this:
| EMT size | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|
| 1/2″ | 12 | 9 | 5 | 3 | 2 |
| 3/4″ | 21 | 16 | 10 | 5 | 4 |
| 1″ | 35 | 26 | 16 | 9 | 6 |
| 1-1/4″ | 61 | 44 | 28 | 16 | 11 |
| 1-1/2″ | 84 | 61 | 38 | 22 | 15 |
| 2″ | 138 | 101 | 63 | 36 | 26 |
Maximum THHN/THWN-2 copper conductors at 40% fill, computed from NEC Chapter 9 Tables 4 and 5. Different insulation types (XHHW, compact strand) have different diameters — always verify against the tables for your exact wire.
Count everything in the raceway: phase conductors, neutrals, equipment grounding conductors, control wires — all of it counts toward fill. A ground wire takes up the same space as a hot wire even though it carries no normal current.
Fits doesn't mean works — the derating trap
Here's the part that bites contractors: conduit fill tells you what fits, not what the wires can carry. Once you have more than three current-carrying conductors in a raceway, NEC 310.15(C)(1) makes you derate the ampacity:
- 4–6 current-carrying conductors → 80% of ampacity
- 7–9 → 70%
- 10–20 → 50%
So those 16 × 12 AWG THHN in 3/4″ EMT are legal on fill — but at 50% of the 90°C ampacity (30A), each conductor is good for 15 amps. The small-conductor rule (240.4(D)) caps 12 AWG at 20 amps anyway, and 15 is less than 20, so those circuits land on 15-amp breakers. Sixteen conductors, sixteen 15-amp circuits — a very different job than sixteen 20-amp circuits.
(Equipment grounding conductors count for fill but generally don't count as current-carrying for derating. Neutrals sometimes do — in a 3-phase wye with nonlinear loads, for example. When the count is close, do the full calculation.)
The Chicago note: Chicago's electrical code requires conduit for most wiring — Romex is essentially out. So Chicago electricians live in Chapter 9 every single day. If you're pulling EMT in the Chicago area, these tables are your daily bread. Want the ampacity side of the story? Here's the wire gauge chart — what size wire for every amperage.
Clearing out a shop — reels of THHN, partial spools, project leftovers that never got pulled? We buy contractor wire surplus — anywhere in the country. Fast, free quote.
Conduit fill questions, answered.
How many 12 AWG THHN wires fit in 3/4-inch EMT?
Sixteen. A 3/4″ EMT has 0.533 in² of cross-section (NEC Chapter 9, Table 4); at 40% fill that's 0.213 in² usable. One 12 AWG THHN conductor occupies 0.0133 in² (Table 5). 0.213 ÷ 0.0133 = 16.015 — and you always round down, so 16.
Why is the fill limit 40% for three or more wires?
Two reasons: heat dissipation and pulling. Conductors packed tighter can't shed heat, and overfilled conduit shreds insulation during the pull. The percentage limits keep both problems away.
Do ground wires count toward conduit fill?
Yes. Every conductor in the raceway counts toward fill — phase conductors, neutrals, equipment grounding conductors, and control wires. A ground takes up the same physical space as a hot wire.
What's the difference between conduit fill and derating?
Fill (Chapter 9) limits how many conductors physically fit. Derating (310.15(C)(1)) reduces how much current each conductor may carry once more than three current-carrying conductors share a raceway. Both apply at the same time — passing one doesn't excuse the other.
Can you mix wire sizes in the same conduit?
Yes — add up the cross-sectional areas of all the conductors (each from Table 5) and compare the total against the raceway's allowed fill area (Table 4 × the Table 1 percentage). Different insulation types have different areas, so use the table for each exact wire.
Does the 40% rule apply to flexible conduit and PVC too?
The fill percentages in Table 1 apply to all raceways; what changes is the raceway area, which comes from the Table 4 section for that specific conduit type (EMT, rigid, PVC, flexible metal, and so on each have their own table).
What happens if conduit is overfilled?
Best case: the inspector fails it and you repull. Worst case: damaged insulation, overheated conductors, and a pull that jams halfway through a run with no way to finish except starting over. Conduit is cheap — re-pulls are not.
I know wire. That's how I know what it's worth.
Reading a jacket is the first thing we do when a roll comes in — the letters tell us the type, the gauge, the rating, and whether it's resale inventory or scrap. That's also how we know when usable wire is worth more than the yard pays for it.