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):

For PVC (Schedule 40), slightly LOWER fills per same conduit size: the wall is thicker than EMT, so the internal area is smaller (Chapter 9 Table 4, 1/2 in.: EMT 0.304 sq in vs PVC 40 0.285 sq in).

Maximum wires per conduit (40% fill, PVC Schedule 40)

XHHW-2 + most other wire types: very similar to THHN; verify in NEC Table.

Quick rules of thumb

For typical residential branch circuits:

Grounds sized per NEC 250.122. Sizes below are the smallest EMT the 40% fill math allows; going one size up is a fair call for pull ease and future capacity, but do not tell an inspector the smaller one fails.

• 15A circuit: 2 #14 + #14 ground = 0.029 sq in. 1/2" EMT (0.122 available), lots of room
• 20A circuit: 2 #12 + #12 ground = 0.040 sq in. 1/2" EMT, lots of room
• 30A circuit: 2 #10 + #10 ground = 0.063 sq in. 1/2" EMT
• 50A circuit: 2 #6 + #10 ground = 0.123 sq in. Just past 1/2", so 3/4" EMT
• 100A subfeed: 3 #3 copper + #8 ground = 0.329 sq in. 1" EMT (0.346 available)
• 200A subfeed: 3 #2/0 copper + #6 ground = 0.718 sq in. 1-1/2" EMT. In 4/0 aluminum the same feeder is 1.02 sq in and needs 2"

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 (0.0133 sq in each) = 0.0665 sq in. 1/2" EMT internal 0.304 × 0.40 = 0.1216 sq in available. 0.0665 < 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 scales roughly with trade size: 3/4" runs about 1.3x the 1/2" rate, 1" about 1.8x, 2" about 3x. PVC lands close to EMT; RMC runs 30 - 50% over EMT. Rates include wire, materials, and labor.

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