How to use the Conduit Fill Calculator
- 1
Choose the conduit type (EMT, PVC Schedule 40 or Schedule 80) and the trade size you plan to use.
- 2
Add each conductor or cable with its quantity. For Cat5e, Cat6, Cat6A or coax, check the outside diameter on the spec sheet and adjust it.
- 3
Read the fill percentage and the smallest conduit that fits. Green means you're under the NEC limit for that number of conductors.
NEC conduit fill limits
NEC Chapter 9, Table 1 caps how much of a conduit's cross-section conductors can take up. The limit depends on how many conductors are in the run, because three or more wires need room to pull without damaging the insulation.
Short conduit nipples of 24 inches or less can be filled to 60%. Equipment grounding conductors count toward fill, so include them.
- 1 conductor: 53% fill
- 2 conductors: 31% fill
- 3 or more conductors: 40% fill
- Nipples 24 in or shorter: 60% fill
How conduit fill is calculated
Add up the cross-sectional area of every conductor from NEC Chapter 9 Table 5 (THHN 12 AWG is 0.0133 sq in, for example), then divide by the conduit's internal area from Table 4. A 3/4 in EMT has 0.533 sq in inside, so at 40% you have 0.213 sq in to work with: sixteen 12 AWG THHN conductors.
Different conduit types have different internal areas at the same trade size. Schedule 80 PVC has thicker walls than EMT, so the same wires can fail in 1 in Schedule 80 and pass in 1 in EMT.
Conduit fill for Cat6 and low-voltage cable
Data cable is round, so its area is π × (diameter ÷ 2)². Most designers apply the same 40% limit to cable pathways, and the TIA pathway standards land in a similar place. The catch is diameter: one maker's Cat6 is 0.22 in and another's is 0.29 in, and Cat6A is thicker again.
Always use the diameter from the spec sheet of the cable you're actually pulling. A run planned with the wrong diameter is how you end up with a conduit that can't take the last few drops.
Frequently asked questions
What is the maximum conduit fill?
40% of the conduit's internal area for three or more conductors, 31% for two and 53% for one, per NEC Chapter 9 Table 1. Nipples 24 inches or shorter can be filled to 60%.
How many 12 AWG wires fit in 1/2 inch EMT?
Nine THHN conductors. Half-inch EMT has 0.304 sq in inside; 40% of that is 0.122 sq in, and each 12 AWG THHN is 0.0133 sq in.
How many Cat6 cables fit in a 1 inch conduit?
About 7 in 1 inch EMT at 40% fill using a 0.25 in cable. Thicker Cat6 or Cat6A fits fewer, so enter your cable's real diameter.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding and bonding conductors are counted in the fill calculation.
Is this calculator code-compliant?
It uses NEC Chapter 9 Tables 1, 4 and 5 for THHN/THWN-2. Your inspector enforces the code edition your area has adopted, so confirm anything critical against it.