NEC conduit fill calculator (Chapter 9 Tables 1, 4, 5)

Enter the size and number of THHN/THWN conductors and we show whether they fit in an EMT or PVC Schedule 40 conduit according to the NEC: the fill ratio, the allowed limit (53, 31 or 40 %) and the smallest suitable trade size. An informative calculation; NEC mode is in beta.

Draw your plan Already have an account? Sign in

What does the conduit fill calculator compute?

It shows whether THHN/THWN conductors fit in an EMT or rigid PVC (Schedule 40) conduit according to the NEC. It divides the total conductor area (Chapter 9 Table 5) by the full internal area of the conduit (Table 4) and compares the result with the allowed fill (Table 1):

• 1 conductor: at most 53 %.
• 2 conductors: at most 31 %.
• 3 or more conductors: at most 40 %.

If you do not choose a conduit size, the calculator suggests the smallest trade size (from ½ in to 2 in) in which the conductors stay within the limit.

Worked example: ten 12 AWG THHN conductors in EMT

One 12 AWG THHN conductor is 0.0133 in², so ten conductors are 0.133 in² together. With ten conductors the allowed fill is 40 %.

The smallest suggested conduit is therefore ¾ in EMT. The calculator does the same with the conductors you enter.

What it does not compute

The calculator only knows THHN/THWN conductors from 14 to 2 AWG and EMT and PVC Schedule 40 conduits from ½ in to 2 in trade size. It does not check ampacity derating for many conductors, the number of bends or other conduit types. It is an informative calculation, not a permit document: compliance is decided by the authority having jurisdiction (AHJ) and the NEC edition in force, and the values must be checked against the official text. The program's NEC mode is in beta.

Automatically in the drawing

In an NEC project the drawing editor uses the same tables to check conduit fill and to suggest the conduit size. See the US electrical plan page, and the box fill calculator for sizing boxes.

Draw your plan