What is conduit fill percentage?

Conduit fill percentage is the ratio of total wire cross-sectional area to the conduit's internal area. NEC limits fill to 53% for 1 wire, 31% for 2 wires, and 40% for 3 or more wires to allow heat dissipation and easy pulling.

What are the NEC conduit fill limits?

NEC Chapter 9 Table 1 specifies: 53% fill for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors. These limits apply to all conduit types including EMT, IMC, and PVC.

What conduit size do I need?

Calculate total wire area, then select conduit where fill doesn't exceed 40% for 3+ wires. Example: Three #12 THHN wires (0.0133 sq in each = 0.0399 total) need 3/4" EMT (0.213 sq in x 0.40 = 0.085 allowed).

What's the difference between EMT, IMC, and PVC?

EMT: thin-wall metal for dry indoor use. IMC: intermediate metal for wet/corrosive areas. PVC: plastic for underground/corrosive environments. Each has different internal areas affecting fill calculations.

Do ground wires count toward conduit fill?

Yes, equipment grounding conductors count toward the fill calculation. However, the NEC allows using a reduced fill limit for grounding wires in certain applications.

NEC Chapter 9 · Electrical Code

Conduit Fill Calculator

Verify wire fill percentage for NEC and utility code compliance — prevent overheating and failed inspections.

100% FreeNEC CompliantCode VerificationSafety First

Select conduit size, wire gauge, and number of conductors

Electrical Standard

NEC 2020/2023 Chapter 9 Table 1 limits apply

Conduit Trade Size

Internal area per NEC Chapter 9 Table 4

Wire Gauge (AWG — THHN/THWN)

Cross-sectional area includes insulation (NEC Annex C)

Number of Conductors *

Include all current-carrying conductors AND grounding wires

NEC Chapter 9 Table 1 — Fill Limits at a Glance

Maximum conductor fill percentages for all conduit types

1 Conductor53% max

53%

47% empty

Asymmetric wire can shift; larger margin prevents insulation damage

2 Conductors31% max

31%

69% empty

Two wires wedge together — pulling tension spikes without clearance

3+ Conductors40% max

40%

60% empty

Standard multi-wire installation; extra space for heat dissipation

⚡ Note: The NEC counts 4+ conductors at the same 40% limit as 3 — the jump down to 31% only applies when you have exactly 2 wires. Always count ground wires toward fill.

Conduit Types — EMT vs. IMC vs. PVC

Different conduit types, same fill rules — but different internal areas for the same trade size

EMT

Electrical Metallic Tubing

Uses: Dry indoor, commercial & residential

✓ Lightest metal option, doubles as ground path

✗ Not suitable for direct burial or wet locations

½" internal area: 0.211 in²

¾" internal area: 0.533 in²

IMC

Intermediate Metal Conduit

Uses: Wet, outdoor, corrosive areas

✓ Stronger than EMT, weather/corrosion resistant

✗ Heavier and more expensive than EMT

½" internal area: 0.342 in²

¾" internal area: 0.586 in²

PVC

Polyvinyl Chloride

Uses: Underground, concrete encasement

✓ Corrosion-proof, lowest cost material

✗ Temperature-sensitive, no grounding path

½" internal area: 0.217 in²

¾" internal area: 0.508 in²

1The Fill Percentage Formula

\text{Fill \%} = \frac{\sum A_{\text{wires}}}{A_{\text{conduit}}} \times 100

Where $\sum A_{\text{wires}}$ is the total cross-sectional area of all conductors (including their insulation), and $A_{\text{conduit}}$ is the conduit's internal area from NEC Chapter 9 Table 4. For example, three #12 THHN wires ( $3 \times 0.0181 = 0.0543\text{ in}^2$ ) in ¾" EMT (0.349 in²) gives $(0.0543 / 0.349) \times 100 = 15.6\%$ — well within the 40% limit.

2Why These Limits Exist

The NEC limits aren't arbitrary — they come from physics. Conductors carrying current generate heat, and that heat needs somewhere to go. Pack a conduit too full and you create an insulated bundle that can't shed heat fast enough, degrading insulation and potentially starting fires. The 40% rule for 3+ wires also leaves room to pull wires without excessive tension that could nick insulation against the conduit's inner wall.

3Derating — The Hidden Companion Rule

Fill percentage is only half the story. When you run more than 3 current-carrying conductors in a single conduit, NEC 310.15(C)(1) requires you to derate the ampacity of each conductor. At 4–6 conductors you multiply ampacity by 0.80; at 7–9 conductors, by 0.70. This means a 12 AWG wire rated 20 A might only be allowed to carry 16 A in a shared conduit. Always run both calculations.

4Practical Sizing Tips

Electricians typically target 25–35% fill — not the full 40% — to allow pulling lubricant to work effectively and to make future wire additions easier. If you're right at the 40% limit, step up one conduit size. The material cost difference between ¾" and 1" conduit is small; a failed inspection or a heat-damaged wire job is not.

Quick Reference — Wire Areas & Conduit Capacity

THHN/THWN wire areas (NEC Annex C) — how many wires fit at 40% fill

AWG	Area (in²)	½" (0.211)	¾" (0.349)	1" (0.581)	1¼" (0.922)
14 AWG	0.0139	6	10	16	26
12 AWG	0.0181	4	7	12	20
10 AWG	0.0243	3	5	9	15
8 AWG	0.0366	2	3	6	10
6 AWG	0.0507	1	2	4	7
4 AWG	0.0824	1	1	2	4

Values assume 40% fill limit (3+ conductors). For 1 conductor use 53%; for 2 conductors use 31%.

Wire Pulling Best Practices

Stay at 25–35% fill to make pulling easier
Use UL-listed wire pulling lubricant
Pull all conductors simultaneously — never one at a time
Keep bends to ≤360° total between pull points
Install conduit before pulling wires whenever possible
Use pulling eyes, not knots, on large conductors

Common Code Violations

Exceeding 40% fill to save on conduit cost
Forgetting to count ground wires in the fill total
Using wire areas for the wrong insulation type
Mixing wire types without recalculating fill
Skipping ampacity derating for 4+ conductors
Using EMT in wet/outdoor locations without approval

Authoritative Electrical Code Resources

NFPA 70 (NEC)

Official National Electrical Code

Southwire Calculator

Professional conduit fill tables

EC&M Magazine

Conduit fill guide for electricians

Frequently Asked Questions

Conduit fill percentage is the ratio of total wire cross-sectional area (including insulation) to the conduit's internal area, expressed as a percent. NEC Chapter 9 Table 1 limits it to 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors — to allow heat dissipation and safe wire pulling.

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