Conduit Fill Calculator (NEC 40% Rule)

How to do it

1. 1
   Step 1: Open Conduit Fill Calculator (NEC 40% Rule) and add your input

   Type your numbers or text, add your file, or pick the options you need. Nothing is uploaded: everything stays in your browser.

2. 2
   Step 2: What Conduit Fill Calculator (NEC 40% Rule) does

   Check conduit fill for THHN wires and find the smallest EMT size that meets the NEC limit.

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   Step 3: Read the result and compare

   The result appears at once. Change one value to see how the answer moves, then keep the figure that fits your case.

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   Step 4: Copy or save it

   Copy the result or download it. For an important decision, confirm it with the official source or an adviser.

About this tool

Enter how many THHN conductors of each size you will pull. The calculator adds their cross-sectional area, applies the NEC fill limit and finds the smallest EMT conduit that stays within it.

The rule

NEC Chapter 9 Table 1 limits conduit fill to 53% of the area for one conductor, 31% for two and 40% for three or more. Fill is the total area of the conductors divided by the internal area of the conduit.

Areas used

THHN conductor areas in square inches: 14 AWG 0.0097, 12 AWG 0.0133, 10 AWG 0.0211, 8 AWG 0.0366, 6 AWG 0.0507, 4 AWG 0.0824, 2 AWG 0.1158. EMT internal areas: ½ inch 0.304, ¾ inch 0.533, 1 inch 0.864, 1¼ inch 1.496, 1½ inch 2.036, 2 inch 3.356 square inches.

Limits

This covers THHN/THWN in EMT. Other conduit types and wire insulation have different areas. Conductor derating and bend limits are not checked.

Worked example

Six 12 AWG and three 10 AWG THHN conductors have a total area of 0.1431 in². With nine conductors the limit is 40%, and ½ inch EMT allows only 0.1216 in², so ¾ inch EMT is needed at 26.8% fill.

Tips for best results

Check every box before you rely on a result, because one wrong digit changes the whole answer. Use the exact figures from your payslip, bill, certificate or contract, and keep a note of the date you calculated. Rates, thresholds and rules change from time to time, so for tax, fees and legal matters treat the output as an estimate and confirm with the official source named on this page. You can run as many scenarios as you like; nothing is saved or sent, and refreshing the page clears the boxes.