Before pulling wire, check that the conductors will fit in the conduit.
For a conduit-fill calculation, you need the conduit type and size, plus the wire size, insulation type, and number of conductors.
Quick Conduit Fill Example
Example: three #10 THHN conductors in 1/2-inch EMT.
Get the allowable fill percentage from NEC Chapter 9, Table 1. For more than two conductors, the maximum fill is 40%.
Get the inside area of 1/2-inch EMT from NEC Chapter 9, Table 4.
1/2-inch EMT inside area = 0.304 sq. in.
Get the area of one #10 THHN conductor from NEC Chapter 9, Table 5.
One #10 THHN conductor = 0.0211 sq. in.
0.0211 × 3 conductors = 0.0633 sq. in. total wire area
Now calculate the actual conduit-fill percentage.
0.0633 ÷ 0.304 = 0.2082
Actual conduit fill = 20.8%
Because the actual conduit fill is 20.8%, and the maximum allowed fill is 40%, the three conductors fit within the limit for this example.
How Sparky Toolbox Helps
Instead of looking up conductor area, conduit area, and fill percentage by hand, enter the conduit and wire details into Sparky Toolbox.
The Conduit Fill Calculator shows:
- Total wire area
- Conduit area
- Fill percentage
- Whether the planned fill is within the limit
That gives you a quick answer before ordering wire or starting the pull.
Important Reminder
Calculator results are planning tools. Always confirm conductor details, conduit type, project requirements, approved plans, and local inspection requirements before installation.