Conduit Fill Chart: Max THHN Conductors per EMT Size (NEC)

Pull too many wires into a conduit and you'll fight the pull, damage insulation, and cook the conductors — the NEC fill limits exist because jammed wires can't shed heat. This chart gives you the maximum number of THHN/THWN-2 conductors per EMT trade size for the most common gauges, computed from NEC Chapter 9 Tables 1, 4, and 5, plus the fill rules and a worked example you can copy for your next run.

Conduit fill is measured as the total cross-sectional area of all conductors divided by the internal area of the raceway:

Two conductors get *less* fill than three (31% vs 40%) because two round wires wedge side-by-side and jam — the geometry is worse, not better.

Maximum conductor counts per EMT trade size, computed from NEC Chapter 9 Table 4 (EMT areas) and Table 5 (THHN/THWN-2 conductor areas). Always verify against the current code tables for your exact insulation type — compact-strand, XHHW, and TFN have different diameters.

Reading the chart: find your wire gauge across the top, read down to the conduit size. A 3/4" EMT legally holds up to 16 × 12 AWG THHN conductors at 40% fill.

Conduit fill tells you what *fits*; NEC 310.15(C)(1) tells you what the wires can *carry* once they're in there. More than three current-carrying conductors in one raceway triggers ampacity adjustment:

Grounding conductors and (usually) the neutral of a balanced 120/240V circuit don't count as current-carrying. Example: nine 12 AWG THHN conductors in a 3/4" EMT fit fine (limit is 16), but their adjusted ampacity drops from the 90°C base by 70% — often pushing you to upsize wire or split the run into two conduits. This is the single most- missed step in DIY conduit work, and it fails inspections.

1. List every conductor in the run — hots, neutrals that count, and note that the equipment ground counts toward fill but not toward derating.
2. Check the count against the 40% column above (or 31%/53% for 2-wire and single-wire runs).
3. Apply the ampacity adjustment if you have 4+ current-carrying conductors, and confirm the adjusted ampacity still exceeds the breaker size.
4. Size up one trade size for long runs, more than two 90° bends (360° total max between pull points per NEC 358.26), or future expansion — your future self will thank you.

You're running one EMT from a house panel to a detached garage: a 60A feeder (3 × 6 AWG: two hots + neutral, plus a 10 AWG ground) and you want to add a 20A circuit for yard outlets (2 × 12 AWG).

• Fill check: 3 × 6 AWG + 2 × 12 AWG + 1 × 10 AWG ground. Conductor areas from NEC Table 5: 6 AWG THHN = 0.0507 in², 12 AWG = 0.0133 in², 10 AWG = 0.0211 in². Total = 3(0.0507) + 2(0.0133) + 0.0211 = 0.1998 in².
• 1" EMT at 40% = 0.346 in² → fits with room to spare. 3/4" EMT at 40% = 0.213 in² → also fits, barely (94% of the legal fill) — bad plan for a 100-foot pull with bends.
• Derating check: 5 current-carrying conductors (the feeder neutral counts here) = 80% adjustment. The 6 AWG THHN's 90°C base ampacity is 75A; 75 × 0.80 = 60A — still OK for the 60A breaker, with zero margin.
• Decision: 1" EMT. It pulls easier, keeps the derating legal, and leaves room for a future circuit. Buy an extra stick — nothing hurts like being one conduit short on a Saturday.

The fill chart above is EMT-specific because EMT is the most common residential/light-commercial raceway — but the right raceway depends on where the run lives:

• EMT ("thinwall"): indoor and covered outdoor runs, garages, basements, commercial interiors. Cheap, fast, bends with a hand bender. Not for direct burial or where subject to severe physical damage.
• PVC Schedule 40/80: direct burial and wet locations — the standard for underground feeders and solar ground runs. Note PVC Schedule 40 has a *smaller* internal area than EMT of the same trade size, so the fill counts above don't transfer — recompute from NEC Chapter 9 Table 4 (Article 352). Expansion fittings are required on long outdoor runs; PVC moves a lot with temperature.
• RMC / IMC (rigid metal): physical protection where conduit is exposed to damage — service masts, drive-under runs, industrial floors. Threads like pipe, costs like pipe.
• LFMC / LFNC (liquidtight flex): short final connections to motors, HVAC condensers, and equipment that vibrates or needs alignment slack.

Two rules that apply everywhere: underground runs need burial depth per NEC Table 300.5 (18" for PVC under residential driveways, 6" for rigid in many cases), and every raceway that enters a building from underground needs sealing against moisture and gas migration. Budget a pull string and a few extra couplings — conduit math on paper always meets physics in the trench.

PVC Schedule 40 has a smaller internal area than EMT at the same trade size, so it holds fewer conductors — recompute, don't assume. Maximum counts below are computed the same way as the EMT chart: NEC Chapter 9 Table 4 (PVC Schedule 40 raceway areas) divided by Table 5 (THHN/THWN-2 conductor areas) at 40% fill.

Worked check: 1" PVC Schedule 40 at 40% fill gives you 0.333 in² of usable area; nine 8 AWG THHN conductors take 9 × 0.0366 = 0.329 in² — legal, and tighter than you'd want on a long buried pull. Schedule 80 has thicker walls and a smaller bore than Schedule 40, so its counts drop again — pull those from NEC Chapter 9 Table 4 directly. The NEC's own Annex C tables can differ from computed values by a conductor here and there because they use measured dimensions; when the inspector and your math disagree, show the math.

Fill is only half of an underground run — cover depth is the other half, and it fails more inspections than fill does. Minimum cover (grade to top of raceway or cable) for the common residential and light-commercial cases:

Two things this table doesn't say but the field will teach you: cover is measured to the top of the conduit, so a deep trench with large-bore PVC eats into your depth fast, and "18 inches" means 18 inches below final grade — if the landscaper re-grades later, your legal cover can disappear. We've dug up plenty of "18-inch" runs that were 14 inches by the time the sod went down. Size the wire before you dig with our wire size calculator, and keep the ampacity chart open for the derating check.

What percentage can you fill a conduit?

53% for one conductor, 31% for two, 40% for three or more (NEC Chapter 9, Table 1). Short nipples 24 inches or less may go to 60%.

How many 12 AWG wires fit in 1/2 inch EMT?

Nine THHN/THWN-2 conductors at 40% fill. But nine current-carrying conductors would derate to 50% ampacity — in practice, keep 1/2" EMT to one or two circuits.

Does the ground wire count in conduit fill?

Yes — every conductor in the raceway, including equipment grounding conductors, counts toward the cross-sectional fill. Grounds do *not* count as current-carrying conductors for derating.

Is EMT, PVC, or rigid conduit different for fill?

Yes — each raceway type and size has its own internal area in NEC Chapter 9 Table 4. PVC Schedule 40 has a slightly smaller internal area than EMT of the same trade size, so it holds fewer conductors. This chart is EMT-specific.

How many bends can I have in one conduit run?

A maximum of 360° of bends total (for example, four 90s) between pull points. More than that and you need a junction box or conduit body.

Can I mix different wire sizes in the same conduit?

Yes — mixed fills are normal. Add up the cross-sectional area of every conductor from NEC Chapter 9 Table 5 and keep the total under 40% of the raceway's internal area from Table 4. The worked example above does exactly that for a 6 AWG feeder plus a 12 AWG branch circuit.

Does THHN count the same as THWN-2 for conduit fill?

Yes. THHN and THWN-2 share the same approximate dimensions in NEC Chapter 9 Table 5 (modern wire is dual-rated THHN/THWN-2 anyway), so one fill chart covers both. XHHW-2 and RHH run thicker — recompute from Table 5 before you reuse these counts.

Portlandia Electric Supply stocks raceway, fittings, and the THHN/THWN-2 to fill them — like Cerro 8 AWG THHN/THWN-2 stranded copper 500' spools and larger feeders like Cerro 1/0 AWG THHN/THWN-2. Browse the Electrical Components collection and Allied Tube & Conduit, and check our wire ampacity chart to confirm your derated conductors still carry the load. Questions on a tricky fill? Call the counter.

Working from a full conduit & wire schedule? Send us your BOM — we'll cross every raceway, fitting, and conductor against NEC fill and derating rules and quote it same day. Get a wholesale quote or browse the contractor portal for account pricing.