Wire Dimensions Chart — Diameter & Area by Insulation

Conduit fill is an area problem: sum the square-inch areas of every conductor in the raceway — the highlighted THHN columns are the everyday case — and compare the total against the usable fill area of your conduit on the conduit fill chart. The conduit fill calculator runs the same Table 5 data, mixed sizes and insulations included, and recommends a minimum trade size. The diameter column earns its keep on jam checks — three same-size conductors whose combined diameters sit near the conduit's inner diameter can wedge during the pull — and anywhere physical fit matters: gutters, KOs, and lug throats.

Diameters are the code's approximate values at conventional Class B stranding — a given manufacturer's THHN may run a few mils tighter or looser, so check the spec sheet when the fit is critical. Watch the insulation crossover: THHN is the slim choice through 6 AWG, but from 4 AWG up XHHW is slightly smaller, which can save a trade size on a crowded feeder. And don't borrow rows across variants — the RHW column here is the with-covering construction; the asterisked no-covering variants and all compact-stranded conductors (Table 5A) have their own, smaller numbers.