Electrician Prep

NEC Ampacity Chart

NEC 310.16

Interactive NEC Table 310.16 ampacity chart. Filter by material, insulation type, and temperature rating.

About this calculator

NEC Table 310.16 lists the ampacity of insulated conductors based on insulation temperature rating, conductor material (copper or aluminum), and AWG or kcmil size. It is the table electricians reach for most. This interactive chart lets you filter by material and temperature column instead of paging through the printed table or memorizing the columns.

Formula

Table 310.16 ampacity values are based on no more than three current-carrying conductors in a raceway, an ambient temperature of 30°C (86°F), and a 30% load diversity assumption built into the underlying calculations. Real installations require correction (310.15(B)) and adjustment (310.15(C)) factors when conditions differ.

Adjusted ampacity = base 310.16 ampacity × ambient temp correction factor × conductor count adjustment factor

Reference: NEC Table 310.16, 310.15(B), 310.15(C)

How to use

  1. Pick a conductor material - copper or aluminum.
  2. Pick a temperature rating column - 60°C, 75°C, or 90°C.
  3. Optionally filter by insulation type (THHN, XHHW, etc.).
  4. Read the base ampacity for each AWG or kcmil size.
  5. Apply correction and adjustment factors separately if your installation conditions differ from the table assumptions.

Worked example

Setup

A #6 AWG copper THHN conductor in a raceway. Need to know its base ampacity at the 75°C column.

Calculation

Look up #6 copper, 75°C column in Table 310.16: 65 A. THHN is rated 90°C dry, but the 75°C column is what you use because most terminations are limited to 75°C per NEC 110.14(C).

Answer

Base ampacity is 65 A at 75°C. With four current-carrying conductors in the same raceway, apply the 0.80 adjustment factor: 65 × 0.80 = 52 A.

Frequently asked questions

Why are there three temperature columns?
Insulation temperature ratings determine how hot the conductor can run before damaging the insulation. 60°C insulation (TW) is older and less common; 75°C (THW, THWN) and 90°C (THHN, THWN-2, XHHW-2) are the modern standards. Higher ratings allow more current for the same wire size, but terminations on most equipment are still limited to 75°C.
When does the 60°C column apply?
NEC 110.14(C)(1)(a) says circuits rated 100 A or less, or marked for #1 AWG and smaller conductors, must use the 60°C column unless the equipment terminations are specifically rated for 75°C or higher. Most modern equipment is rated 75°C, so this restriction mostly affects older panels and small residential equipment.
Does this chart include all conductor sizes?
Table 310.16 covers wire sizes from #14 through 2,000 kcmil. Smaller conductors (#16, #18) are governed by NEC 402 and 725 for fixture wires and Class 2/3 circuits. Larger sizes are uncommon in normal commercial and industrial work.