Conduit Fill Calculator
Checks whether a set of conductors fits in a conduit under the fill limits of the National Electrical Code (NEC). Pick a conduit type and trade size and the page looks up its internal area. Pick THHN/THWN-2 wire of one size and a count and it looks up the conductor area. For other raceways or mixed wire, enter the areas yourself. Electricians, estimators and electrical designers use it to size conduit runs.
The fill check
Fill = Aw/Ac × 100 % Aw ≤ f·Ac
Ac is the conduit’s internal area from NEC Chapter 9 Table 4 and Aw the total area of the conductors, n × a for n wires of area a from Table 5 [1]. f is the allowed fill from Chapter 9 Table 1: 53 % for one conductor, 31 % for two and 40 % for three or more [1]. Two conductors get the lowest limit because a pair can jam across the conduit’s diameter when it is pulled through a bend. The page shows the allowed fill, your actual fill, the most conductor area the conduit may hold, and whether the run fits.
Limits and common mistakes
- Count every conductor, including the neutral and the equipment grounding conductor. Each counts toward the fill at its full area.
- The THHN list covers one wire size at a time. For mixed sizes or other insulation types, add up each conductor’s area from Table 5 and choose Custom.
- The page doesn’t apply the 60 % fill allowed for a nipple 24 in or shorter. It doesn’t derate ampacity either. More than three current-carrying conductors in one conduit need their ampacity reduced, which can force a larger wire and so a larger conduit.
- Meeting the fill limit doesn’t guarantee an easy pull. Long runs with several bends may need a size up.
Worked example
Two 30 A, 240 V water-heater circuits share one EMT run. Each circuit has two 10 AWG THHN hots and its own 10 AWG ground, for six conductors. The plan shows 1/2 in EMT. Check it.
- Choose Conduit type EMT (electrical metallic tubing) and Trade size 1/2.
- Choose Conductors THHN / THWN / THWN-2, all one size, and Conductor size 10 AWG.
- Enter Number of conductors 6.
- Press Solve. The Solution shows:
Allowed fill: 40 %
Actual fill: 41.6447 %
Max allowed conductor area: 0.1216 in²
Fits within fill limit?: No — use a larger conduit
Conduit internal area (trade size 1/2 EMT): 0.304 in²
Total conductor area (6 × 0.0211 in², 10 AWG): 0.1266 in²
Check: 6 × 0.0211 = 0.1266 in², but 40 % of 0.304 is only 0.1216 in². Choose Trade size 3/4 instead and the fill drops to 23.7523 % of 0.533 in², which fits.
