Conduit Fill Calculator

Raceway area. The inside area is π/4 × inside diameter². Inside diameters come from manufacturer data: Wheatland Tube’s published figures for EMT, IMC and rigid metal conduit, and the average inside diameter Spears publishes for Schedule 40 and Schedule 80 PVC, which has the same outside diameter and minimum wall that CANTEX and Prime Conduit publish for their conduit. ¾ in EMT, for example, is 0.824 in inside, or 0.533 sq in.

Wire area. Each wire is π/4 × outside diameter². THHN/THWN-2 diameters are from Cerrowire’s spec sheet, using stranded wire for 14 to 10 AWG. Solid wire in those sizes is a little smaller, so the result errs on the safe side. Bare wire is concentric Class B stranding (ASTM B8): 7 strands for 14 to 2 AWG, 19 for 1 to 4/0 AWG and 37 for 250 to 500 kcmil, which puts 3, 5 or 7 strands across, each strand √(circular mils ÷ strands). A 12 AWG THHN is 0.130 in, or 0.0133 sq in.

Fill limit. NEC Chapter 9, Table 1 allows 53% of the raceway area for one conductor, 31% for two and 40% for more than two. Note 4 allows 60% in a nipple 24 in or shorter, and Note 3 counts grounding and bonding conductors. When every wire is the same type and size, the calculator finds the most that fit and applies Note 7: a count ending in a decimal of 0.8 or more rounds up. Mixed sizes are checked by total area against the limit. The smallest size that passes is the first trade size, ½ to 4 in, that clears the same check.

Jam ratio. With exactly three wires of one size, the calculator divides the raceway inside diameter by the wire’s outside diameter and flags a ratio from 2.8 to 3.2, per Chapter 9, Table 1, Informational Note No. 2.

Check close calls in the code book. Inspectors work from NEC Chapter 9, Tables 4 and 5, and Table 8 for bare wire. The published dimensions used here match those tables closely but not to the last digit, so a result within about 1% of the limit gets a note to check it against the code book and the edition your jurisdiction enforces. Not covered: flexible metal conduit, XHHW and other insulations, aluminum wire, multiconductor cable, ampacity and derating.

Price. Wire to buy for each size is run length × number of wires × (1 + makeup), rounded up to the foot. The 10% makeup default is a starting allowance for loops and terminations, not a code number; set your own. The raceway is the run length × your price per foot for that type and size, and each wire size is its footage × your price per foot. Nothing is priced with numbers you did not enter.

Pricing a whole job? The electrical estimate template has a device takeoff that turns circuits into wire footage by cable type, and a customer-facing estimate grouped by phase.