A conduit size chart lists the outside diameter (OD), inside diameter (ID), and fill capacity for electrical conduit types: EMT, IMC, rigid metal (GRC), and PVC. Understanding conduit dimensions is essential for selecting the right trade size and calculating how many wires will fit per NEC fill rules.
EMT Conduit Dimensions & Fill Data
| Trade Size | OD (in) | ID (in) | OD (mm) | Fill Area 40% (in²) | Max #12 THHN wires |
|---|
NEC Conduit Fill Rules (Art. 358 / 344)
How to Use the Conduit Size Chart
Selecting the correct conduit size involves two main steps: choosing the conduit type for your application and calculating the required interior fill area for the number and size of wires you need to run.
Step 1: Choose the Conduit Type
EMT is the most common in residential and commercial applications for dry locations. PVC Schedule 40 or 80 is used outdoors, underground, and in wet/corrosive locations. IMC and rigid (GRC) are used in industrial and harsh environments, or where mechanical protection is required.
Step 2: Calculate Wire Fill
Sum the cross-sectional areas of all conductors (from NEC Chapter 9 Table 5). Apply the NEC fill limit: 40% of the conduit's interior area for 3+ conductors. Find the conduit trade size whose 40% fill area equals or exceeds your total wire area. When in doubt, go up one trade size for easier pulling.
Common Wire Sizes (THHN Cross-Section)
#14 AWG = 0.0097 in² | #12 AWG = 0.0133 in² | #10 AWG = 0.0211 in² | #8 AWG = 0.0366 in² | #6 AWG = 0.0507 in² | #4 AWG = 0.0824 in² | #2 AWG = 0.1158 in² | #1/0 AWG = 0.1855 in²
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What is the difference between EMT, IMC, and rigid conduit?
EMT (Electrical Metallic Tubing) is thin-wall conduit used in exposed or concealed installations in dry locations. IMC (Intermediate Metal Conduit) has thicker walls than EMT and can be used outdoors. Rigid Metal Conduit (RMC/GRC) has the thickest walls and is used in harsh environments. PVC is non-metallic and used in underground, wet, and corrosive locations.
What is the NEC conduit fill rule?
The National Electrical Code (NEC) limits conduit fill to 53% of the interior area for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors. These limits prevent overheating and ensure wires can be pulled through without damage. Always follow local codes, which may be more restrictive.
What size conduit do I need for 4 x 12 AWG wires?
Four 12 AWG THHN conductors in 1/2" EMT is within the NEC 40% fill limit for 3+ conductors. Each 12 AWG THHN wire has a cross-sectional area of 0.0133 in². Four wires = 0.0532 in², which fits within the 40% fill (0.122 in²) of 1/2" EMT. When in doubt, go up one trade size for easier wire pulling.
Can I use PVC conduit underground?
Yes, Schedule 40 and Schedule 80 PVC conduit are approved for direct burial and underground applications. Schedule 80 has a thicker wall and is used where mechanical protection is needed. Always check NEC burial depth requirements, which vary by circuit voltage and whether the conduit is under a road.