Conduit Fill Calculator

Calculate conduit fill percentage and check NEC Chapter 9 compliance. Supports EMT conduit sizes and THHN wire gauges.

Fill Percentage
9.98%
Max Allowed Fill
40%
(3+ wires)
Max Wires Allowed
16
12 AWG THHN in 3/4" EMT
Total Wire Area
0.0532 in²
PASS — NEC Chapter 9 Compliant

Fill of 9.98% is within the 40% maximum for 3+ conductors per NEC Table 1.

Per NEC Chapter 9, Table 1 — fill limits: 53% (1 wire), 31% (2 wires), 40% (3+ wires). Wire areas per Table 5 (THHN). Conduit areas per Table 4 (EMT).
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How It Works

This calculator determines the fill percentage of a conduit based on the number and size of wires installed, then checks compliance with NEC Chapter 9, Table 1.

Formula: Fill % = (Number of Wires × Wire Area) / Conduit Area × 100. The maximum allowed fill depends on the number of conductors: 53% for one wire, 31% for two wires, and 40% for three or more wires.

Wire areas are based on THHN insulation per NEC Chapter 9, Table 5. Conduit areas are for EMT (Electrical Metallic Tubing) per NEC Chapter 9, Table 4.

The calculator also shows the maximum number of wires of the selected gauge that can fit in the chosen conduit while staying within the NEC fill limit.

Frequently Asked Questions

What is conduit fill and why does it matter?

Conduit fill is the percentage of a conduit's internal area occupied by wires. Overfilling conduit makes pulling wire difficult, can damage insulation, and causes overheating due to reduced air circulation. The NEC sets strict limits on how full a conduit can be.

What are the NEC conduit fill limits?

Per NEC Chapter 9, Table 1: one wire can fill up to 53% of the conduit area, two wires can fill up to 31%, and three or more wires can fill up to 40%. These limits apply to conduits, raceways, and nipples over 24 inches long.

What is NEC Chapter 9 Table 1?

NEC Chapter 9, Table 1 defines the maximum percentage of conduit cross-sectional area that can be filled with conductors. Tables 4 and 5 in Chapter 9 provide the actual cross-sectional areas for various conduit types and wire sizes, which are used to calculate fill percentage.

Does wire insulation type affect conduit fill?

Yes. Different insulation types (THHN, THWN, XHHW, etc.) have different outer diameters. THHN/THWN-2 is the most common for building wire and has a relatively compact insulation. This calculator uses THHN wire areas from NEC Chapter 9, Table 5.

Can I mix wire sizes in the same conduit?

Yes, you can mix wire sizes in the same conduit. When mixing sizes, add up the total cross-sectional area of all wires and compare it to the conduit area using the 40% fill limit (since you will have 3+ conductors). For mixed sizes, calculate each wire's area individually.