Conduit Fill + Sizing Quick Reference
Why this matters
Conduit fill violations are common failed inspections. NEC has specific maximum-fill percentages (53% one wire, 31% two wires, 40% three or more) that techs forget in the field. Knowing the basic conduit + wire size matching from memory saves the trip back to the truck. This is the field card.
NEC fill limits
Per NEC Chapter 9 Table 1:
- 1 conductor: max 53% fill
- 2 conductors: max 31% fill
- 3+ conductors: max 40% fill
For most jobs (3+ wires in conduit), the 40% limit applies.
Common conduit types
EMT (Electrical Metallic Tubing): thin-wall steel, most common interior. IMC (Intermediate Metal Conduit): thicker than EMT, outdoor + commercial. RMC (Rigid Metal Conduit): heaviest steel, severe-duty. PVC Schedule 40/80: outdoor + underground, non-metallic. LFMC: liquid-tight flex. FMC: flex ("Greenfield"). ENT: flexible plastic interior. Internal diameter varies - fill calcs use conduit-specific table value.
Maximum wires per conduit (40% fill, EMT)
For THHN/THWN wire (typical):
| Conduit size | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG |
|---|---|---|---|---|---|---|---|
| 1/2" EMT | 12 | 9 | 5 | 3 | 1 | 1 | 1 |
| 3/4" EMT | 22 | 16 | 10 | 6 | 4 | 2 | 1 |
| 1" EMT | 35 | 26 | 16 | 9 | 7 | 4 | 3 |
| 1-1/4" EMT | 61 | 45 | 28 | 16 | 12 | 7 | 5 |
| 1-1/2" EMT | 84 | 61 | 38 | 22 | 16 | 10 | 7 |
| 2" EMT | 138 | 101 | 63 | 36 | 26 | 16 | 11 |
| 2-1/2" EMT | 241 | 176 | 111 | 64 | 46 | 28 | 20 |
| 3" EMT | 374 | 273 | 172 | 99 | 71 | 44 | 31 |
For PVC (Schedule 40), slightly higher fills per same conduit size due to thinner wall.
Maximum wires per conduit (40% fill, PVC Schedule 40)
| Conduit size | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|
| 1/2" | 11 | 8 | 5 | 2 | 1 |
| 3/4" | 21 | 15 | 9 | 5 | 4 |
| 1" | 34 | 25 | 15 | 8 | 6 |
| 1-1/4" | 60 | 44 | 27 | 15 | 11 |
| 1-1/2" | 82 | 60 | 37 | 21 | 15 |
| 2" | 135 | 99 | 62 | 35 | 25 |
XHHW-2 + most other wire types: very similar to THHN; verify in NEC Table.
Quick rules of thumb
For typical residential branch circuits:
- 15A circuit: 2 #14 conductors + ground = fits 1/2" EMT easily
- 20A circuit: 2 #12 conductors + ground = fits 1/2" EMT easily
- 30A circuit: 2 #10 conductors + ground = needs 3/4" EMT typically
- 50A circuit: 2 #6 conductors + ground = needs 1" EMT
- 100A subfeed: 3 conductors (2 hots + neutral) + ground = needs 1-1/4" EMT typical
- 200A subfeed: 3 conductors (1/0 + 2/0) + ground = needs 2" EMT
Conduit bend radius
Minimum bend radius (per NEC + manufacturer):
- 1/2" EMT: 4" radius minimum
- 3/4" EMT: 4.5" radius
- 1" EMT: 5.75" radius
- 1-1/4" EMT: 7.25" radius
- 1-1/2" EMT: 8.25" radius
- 2" EMT: 9.5" radius
No more than 360° total bends between pull points (NEC 358.26).
Use proper bender for size. 90° bend takes specific deduct (deduct from end of bend to mark).
Standard bend types + pull boxes
90° (right angle): most common. Offset: two short bends to move off centerline. Saddle: three bends to go over obstacle. Kick: less than 90° slight redirection. Backbend: returns to original direction. 4-bend rule: no more than 360° between pull points or 4 quarter-bends. Long runs: pull box every 100 - 200 ft.
Common conduit mistakes + supports
Mistakes: over-fill (NEC violation, derating required), wrong bend radius (kinked conduit, wire damage), skipping deburring (sharp edge cuts insulation), more than 360° between pulls, wrong conduit for environment, no support per code, cross-threading over-tightening (damaged threads, no ground continuity).
Support requirements: EMT (NEC 358.30) within 3 ft of each box + every 10 ft along run. IMC + RMC (NEC 344.30): same. PVC (NEC 352.30): within 3 ft of each box, every 3 ft for 1/2" - 1", every 5 ft for 1-1/4" - 1-1/2", etc. per Table 352.30.
Outdoor + wet location + pulling
EMT outdoors: allowed but must use fittings rated for location. PVC outdoors: standard, UV-resistant. LFMC: liquid-tight outdoor + wet. Underground: PVC Schedule 40/80 most common, RMC for some.
Pulling lubricant required for runs > 50 ft (Polywater J, IDEAL Yellow 77 - reduce friction 50 - 80%). Apply to wire + into conduit at start. NEVER use motor oil/grease (chemical incompatibility). Single-wire pull: drop or push through. Multi-wire: fish tape OR previously-installed pull rope. Long run: power puller + lubricant. Bundled pulls: tape wires at lead end + lubricate generously.
Calculation + truck stock + pricing
When not in table: total wire cross-section / conduit cross-section ≤ 0.40 (40% fill). Wire areas in NEC Ch 9 Table 5; conduit areas in NEC Table 4. Example: 5 × #12 THHN (0.0181 sq in each) = 0.0905 sq in. 1/2" EMT internal 0.304 × 0.40 = 0.1216 sq in available. 0.0905 < 0.1216 → OK.
Truck stock: 1/2" + 3/4" EMT (8 - 10 ft sticks), couplings + connectors, 90° elbows, LB conduit bodies, larger sizes 1-1/2" + 2", PVC 90° sweep for underground, strut + clamps. Specialty: long pull rope, fish tape 50 - 100 ft, lubricant, bender + level.
Pricing per linear ft installed: 1/2" EMT 3/4" 1" 2" . PVC similar; RMC 30 - 50% more. Includes wire + materials + labor.
The single highest-value conduit habit is the FUTURE-CAPACITY PLAN. Customer needs 1 circuit today + might add 2 more in 2 years (smart home, EV charger, addition). Run 3/4" or 1" instead of 1/2" - 5% more cost upfront, eliminates pulling new conduit later. Customer pays a little more on initial install + saves thousands on second pull. Pros think ahead; amateurs install minimum + create future problems.
References
- NEC Chapter 9 Tables 1, 4, 5 (conduit + wire dimensions + fill)
- NEC 358 (EMT) + 352 (PVC) + 344 (RMC) + 342 (IMC) + 350 (FMC) + 356 (LFMC) + 362 (ENT)
- Manufacturer technical data
- Manuall internal: Conductor Ampacity Reference, Service Entrance + Meter Base