Conduit fill limits exist so wire can be pulled without stripping its insulation and so it can shed heat. NEC Chapter 9 Table 1 allows 53 % of the inside area for one conductor, 31 % for two and 40 % for three or more; Table 4 gives the inside area of every raceway and Table 5 the area of every insulated wire.
How the count is worked out
Add the Table 5 area of every conductor, including the insulated ground, and compare it with the allowed share of the Table 4 area. ½" EMT has 0.304 in² inside, so 40 % is 0.122 in²: 9 × 12 AWG THHN (0.0133 in² each) fit, 5 × 10 AWG.
Note 7: when every conductor is the same size and the count works out to a fraction of .8 or more, round up. That is why two 6 AWG THHN fit in ½" EMT: 31 % of 0.304 in² ÷ 0.0507 in² = 1.85, which rounds to 2.
fill % = Σ (Table 5 area × count) ÷ Table 4 total area limit: 1 wire 53 % · 2 wires 31 % · 3+ 40 % · nipple ≤ 24" 60 %
Jamming
Three conductors of one size can wedge in a bend when the conduit’s inside diameter is 2.8 to 3.2 times the wire’s outside diameter (Chapter 9 Table 1, Informational Note 2). The calculator flags it; a size up, or a fourth conductor, avoids it.
The drawing
The cross-section packs the conductors bottom-up, the way a pull settles in a horizontal run. It is drawn to scale, not 1:1; the fullscreen mat in Mode 01 draws ½" to 2" EMT and PVC outside diameters at true size.
Straps, supports and bends
Every raceway has a securing rule and a support spacing. EMT is secured within 3 ft of each box and supported at least every 10 ft (358.30); PVC within 3 ft and then every 3 ft for ½" to 1" or 5 ft for 1¼" to 2" (Table 352.30); flexible metal conduit within 1 ft and every 4.5 ft (348.30). A straight 50 ft run of EMT therefore takes 6 straps, of 1" PVC 16.
Between pull points (boxes and conduit bodies) the bends may total 360°, the same as four quarter bends, in every raceway type (358.26, 352.26, 348.26, 344.26). Offsets and kicks count too: a 30° offset is 60° of bend. PVC also needs expansion fittings where the temperature swing would move a long straight run (352.44).