Home energy · electrical reference
Conduit fill, smallest size that fits.
Enter how many THHN or THWN-2 conductors of each size run in one conduit. Get the smallest trade size that stays inside the NEC fill limit.
Short answer
Add up the cross-section area of every conductor and keep it at or under 40% of the conduit's internal area when there are three or more wires (53% for one, 31% for two). Five 6 AWG THHN wires total 0.2535 sq in, which needs 1 in EMT (0.346 sq in at 40%); 3/4 in EMT (0.213) is too small.
Inputs
Result
Worked example
Three 12 AWG THHN conductors total 3 x 0.0133 = 0.0399 sq in, well under the 0.122 sq in allowed in 1/2 in EMT. Nine fit (0.1197 sq in); the tenth pushes the total to 0.133 sq in, so the run needs 3/4 in EMT (0.213 sq in at 40%).
Formula, assumptions and sources
Fill = total conductor area / conduit internal area. NEC Chapter 9 Table 1 sets the limit at 53% of the internal area for one conductor, 31% for two and 40% for more than two. Table 4 gives each raceway's internal area and Table 5 gives approximate conductor areas; the table here uses the THHN/THWN row. Mixed sizes are simply added. The tool does not model conduit nipples (60%), conductors of other insulation types, compact-stranded wire, bends, derating for more than three current-carrying conductors, or the equipment grounding conductor you may need to add to the count.
Table values here follow the NEC as reproduced in the sources below (editions 2008 to 2020 era); fill percentages, tables and allowances can change between editions, so confirm against the edition your jurisdiction has adopted.
Code and safety notes
This tool follows the U.S. National Electrical Code (NFPA 70). The Canadian Electrical Code (CSA C22.1) and local amendments differ in several tables and rules, so Canadian installations must be checked against the CEC and the authority having jurisdiction.
Planning aid only, not electrical advice. Electrical work can cause fire and shock; the adopted code edition, the equipment listing and a licensed electrician or inspector decide what is allowed.
FAQ
What is the 40% conduit fill rule?
NEC Chapter 9 Table 1 limits the total conductor area to 40% of the conduit's internal area when more than two conductors share it. One conductor may fill 53% and two may fill 31%.
How many 12 AWG wires fit in 1/2 inch EMT?
Using THHN areas, nine 12 AWG conductors (0.1197 sq in) fit under the 0.122 sq in limit for 1/2 in EMT. Check the other limits too, such as the 20 A circuit and derating rules.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding conductors occupy space in the raceway and are included in the fill calculation, so add them to the count.
Is this the Canadian Electrical Code?
No. This tool uses NEC tables. The CEC has its own conductor fill tables and rules, so Canadian jobs need the CEC and the local authority.