Conduit Fill Calculator

How the fill is figured

Step 1. Add up the area of every conductor, insulation included. The areas come from NEC Chapter 9, Table 5, or from the cable maker’s data sheet for a cable the table does not list.

Step 2. Find the fill limit in Table 1: 53 percent of the conduit’s internal area for one conductor, 31 percent for two, and 40 percent for three or more. A nipple 24 in. or shorter may be filled to 60 percent.

Step 3. Take the conduit’s internal area from Table 4 and use the smallest trade size where the limit is at least the total wire area.

A worked example

Ten 12 AWG THHN wires have a total area of 10 × 0.0133 = 0.133 sq in. In 1/2 in EMT the 40 percent limit is 0.122, which is too small. In 3/4 in EMT the limit is 0.213, so it fits at about 25 percent full.

A mixed pull of three 500 kcmil, one 250 kcmil and one 3 AWG THHN adds up to about 2.616 sq in. In 2-1/2 in EMT the limit is 2.343, which is too small. 3 in EMT has 3.538 and fits, at about 30 percent full.

The 0.8 rule (Table 1, Note 7)

When every conductor in the conduit is the same size, the code lets you round the maximum number of conductors up to the next whole number if the calculation ends in a decimal of 0.8 or more. Three 2 AWG THHN in 1 in EMT come to 0.347 sq in against a limit of 0.346, but the limit divided by one wire is 2.99, so three are permitted. The calculator applies this and marks the row “Note 7”. It does not apply to mixed sizes.

Where the numbers come from

Conductor areas are the approximate areas in NEC Chapter 9, Table 5 for THHN/THWN/THWN-2, XHHW/XHHW-2/XHH and THW/THW-2/TW. Conduit areas come from Table 4: each row is a nominal internal diameter, and the area at each percent is worked out from it the same way the table does. The method matches how Mike Holt Enterprises teaches it. The values are from the 2017 edition text; the same rows appear in later editions, but check the edition your project is under.

What this does not do

It does not derate ampacity for more than three current-carrying conductors, check voltage drop, size a pull box or account for bends. It does not use the bare conductor table, so add a bare ground using the cable option and its actual area. Multiconductor cable counts as one conductor with its actual area (Table 1, Note 9), so enter that area from the data sheet. Always check the result against the code edition your jurisdiction has adopted.