Conduit Fill Calculator & Chart | NEC Compliant
NEC Chapter 9, Tables 1, 4 & 5 • THHN / THWN-2 insulation
Conductors
THHN / THWN-2Results
PASSTotal Conductors
3
Wire Area Used
0.0399 in²
Remaining Space
0.1733 in²
What is Conduit Fill?
Conduit fill refers to the percentage of a conduit's internal cross-sectional area occupied by electrical conductors. The National Electrical Code (NEC) regulates conduit fill to prevent damage to wire insulation during installation and to ensure adequate heat dissipation during operation.
NEC Chapter 9, Table 1 establishes three fill limits based on the number of conductors: 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors. These percentages apply to all conduit types including EMT, PVC, RMC, FMC, and LFMC.
Exceeding these limits makes it difficult to pull conductors through the raceway without damaging insulation and increases the risk of overheating under load. Proper conduit fill calculation is essential for every electrical installation — from residential branch circuits to commercial feeders.
How to Calculate Conduit Fill
Select your conduit type and trade size
Identify the conduit type (EMT, PVC Schedule 40/80, RMC, FMC, or LFMC) and trade size. Look up the total internal cross-sectional area from NEC Chapter 9, Table 4.
Determine wire cross-sectional areas
For each conductor, find the cross-sectional area from NEC Chapter 9, Table 5 based on wire gauge and insulation type. THHN/THWN-2 is the most common type used in conduit.
Calculate total wire area
Multiply each wire's cross-sectional area by its quantity, then sum all values:
Total Area = Σ (wire area × quantity)
Compare against NEC fill limits
Divide total wire area by conduit area and multiply by 100. Compare the result against NEC limits: 53% (1 wire), 31% (2 wires), or 40% (3+ wires). If you exceed the limit, upsize the conduit.
NEC Conduit Fill Chart
Maximum number of THHN/THWN-2 conductors in EMT conduit at 40% fill (3+ conductors). Reference: NEC Chapter 9, Tables 4 & 5.
| Trade Size | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|
| 1/2" | 12 | 9 | 5 | 3 | 2 |
| 3/4" | 22 | 16 | 10 | 5 | 4 |
| 1" | 35 | 26 | 16 | 9 | 6 |
| 1-1/4" | 61 | 44 | 28 | 16 | 11 |
| 1-1/2" | 83 | 61 | 38 | 22 | 16 |
| 2" | 138 | 100 | 63 | 36 | 26 |
Values are approximate and based on THHN/THWN-2 wire areas. Always verify with the specific NEC edition adopted in your jurisdiction.
NEC Conduit Fill Limits (Table 1)
| Number of Conductors | Maximum Fill Percentage |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
Common Conduit Sizing Questions
What size conduit for 6/3 wire?
For a standard 6/3 NM-B (Romex) cable, you need a minimum of 1" PVC or EMT conduit if you are running the entire sheath through it. If you strip the outer jacket and pull individual 6 AWG THHN conductors, you can use 3/4" conduit for up to 4 wires.
What size conduit for 12/2 wire?
Running a 12/2 NM-B flat cable through conduit for physical protection requires a minimum 1/2" EMT (or PVC) conduit for a short sleeve. If it's a complete conduit run, a 3/4" conduit is recommended to make pulling easier.
What size conduit for 10/2 wire?
For a standard 10/2 NM-B Romex cable, you should use at least a 3/4" conduit. Standard 10/2 flat cable has a major diameter that exceeds the allowable 40% fill limit for 1/2" trade size conduits.
What size conduit for 8/3 wire?
An 8/3 NM-B cable requires a minimum of 1" conduit. Sizing up to 1" is necessary because the cable outer diameter is roughly 0.64 inches, which exceeds the max physical opening and fill limits of a 3/4" conduit.
Frequently Asked Questions
What is the maximum conduit fill percentage allowed?
Under NEC Chapter 9, Table 1, the maximum conduit fill is 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. The 40% limit is the most common guideline for standard branch and feeder runs.
Does ground wire count in conduit fill calculations?
Yes, ground wires (both insulated and bare conductors) always count toward the conduit fill limit under the National Electrical Code. You must include the cross-sectional area of all grounding wires when calculating fill.
How do you calculate conduit fill?
To calculate conduit fill, find the total cross-sectional area of all your conductors using NEC Table 5 (or Table 5A for compact conductors). Then, divide that total area by the total internal cross-sectional area of the conduit (from NEC Table 4) to find the fill percentage.
Where are conduit fill rules found in the NEC?
Conduit fill regulations are located in NEC Chapter 9, Table 1, and the tables in Chapter 9, Annex C. Specific wire sizing dimensions are in Tables 4 and 5.
How many 12 AWG wires fit in 3/4 EMT?
A 3/4" EMT conduit has an internal area of 0.533 square inches. Under the 40% fill limit, the allowable area is 0.213 square inches. Since a 12 AWG THHN conductor is 0.0133 square inches, you can safely pull up to 16 conductors in 3/4" EMT. Use our voltage drop calculator and breaker size calculator to verify your overall circuit design.