How to use NEC Table 310.17 free air ampacity vs 310.16

Free air vs raceway, in plain language

Look up Tables 310.16 and 310.17 and section 392.80 in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the tables; it does not reproduce them.

Ampacity is really a heat limit. A conductor in conduit is wrapped in air that can’t move and sits next to other warm conductors, so Table 310.16 is conservative. A single insulated conductor in free air sheds heat in every direction, so Table 310.17 gives it a much higher rating. Both tables assume a 30 °C ambient and use the same 60/75/90 °C columns.

Where contractors actually use it: cable tray. Section 392.80(A)(2) sizes single conductors 1/0 and larger in uncovered tray as a percentage of the 310.17 value — one percentage for 1/0 through 500 kcmil, a higher one for 600 kcmil and up — because conductors lying together in a tray are somewhere between free air and a conduit. If they’re laid in a single layer with a maintained space of one diameter, they can take the full free-air value; triplexed or triangular bundles use the configuration tables at 310.20. Multiconductor tray cable is a different animal and starts from 310.16.

Worked example. 500 kcmil copper at 75 °C is 380 A in conduit. As a single conductor in free air it’s 620 A. Laid in an uncovered ladder tray, 392.80(A)(2) allows 65% of the free-air value: 0.65 × 620 = 403 A — more than the conduit rating, which is why large feeders often go in tray. Then apply any ambient temperature correction and confirm the terminations and OCPD.

For tray runs, keep the grounding straight too: see the equipment grounding conductor guide (NEC 250.122). Not engineering advice — tray ampacity has several configuration-specific rules, so read 392.80 in the edition your AHJ enforces.