NEC Cable Sizing
The NEC’s primary ampacity table looks intimidating at first glance. Here is how to actually read it, column by column.
NEC Table 310.15(B)(16) is the workhorse ampacity table for most US-code conductor sizing — and one of the most frequently misread tables in the entire code, mostly because its column structure isn't self-explanatory at first glance.
| Column | What it represents |
|---|---|
| Conductor size (AWG / kcmil) | Standard American Wire Gauge sizes, transitioning to thousand circular mils (kcmil) for larger conductors |
| 60°C, 75°C, 90°C temperature-rating columns | Each conductor size has three different ampacity values depending on the insulation's temperature rating — not the ambient temperature |
| Copper vs. Aluminum/Copper-clad Aluminum sections | The table is split entirely by conductor material — copper ampacity and aluminum ampacity for the same AWG size are meaningfully different, not a simple ratio |
The 60/75/90°C columns refer to the insulation's temperature rating (THHN is 90°C, THWN is 75°C, for example) — not the installation's ambient temperature. Ambient temperature correction is a completely separate multiplier applied afterward, from a different table entirely. Conflating the two is one of the most common NEC sizing mistakes.
It's tempting to always read from the 90°C column since it gives the highest ampacity figure for a given size. But the terminal — the breaker or device the conductor connects to — is usually only rated for 60°C or 75°C conductors. NEC 110.14(C) requires using the ampacity from the column matching the lowest-rated termination in the circuit, even if the conductor's insulation itself is rated higher. Using the 90°C column's higher number when your terminations are only 75°C-rated is a genuine, common compliance error.
Once you've pulled the base ampacity from the correct temperature column, the same two corrections from every other code still apply: an ambient temperature correction factor and a conductor-count/grouping adjustment factor — both from their own separate NEC tables, applied as multipliers against the base figure.
Work through your own conductor size, insulation rating, and termination rating in the NEC Cable Sizing calculator.