Wire ampacity chart from NEC Table 310.16

NEC Table 310.16 ampacity chart, 14 AWG to 2000 kcmil

Values in amps for insulated conductors rated up to 2000 volts, not more than three current carrying conductors in a raceway, cable or earth, at an ambient temperature of 30°C (86°F). Aluminum includes copper clad aluminum. Aluminum is not listed at 14 AWG.

From NEC Table 310.16 as published by Inch Calculator, checked row by row against Cerrowire (14 AWG to 1000 kcmil) and Portlandia Electric Supply (14 AWG to 4/0). The 14, 12 and 10 AWG values are subject to the 240.4(D) limits below. Some ampacity charts online scale aluminum as a fixed share of copper. That gives wrong numbers, so do not mix sources.

Which column you may use

The temperature column is set by the lowest rated part of the circuit, not by the wire insulation alone. NEC 110.14(C) ties the column to the terminals on the breaker and the equipment at both ends.

For circuits rated 100 amps or less, or conductors from 14 AWG through 1 AWG, use the 60°C column unless the equipment terminals are marked for 75°C. For circuits over 100 amps, or conductors larger than 1 AWG, the 75°C column usually governs. Most modern breakers and lugs are rated 75°C, so the 75°C column is the one you will use most.

The 90°C column is not a free upgrade for THHN or XHHW-2. Its value is where you start the derating math for heat and bundling, and the result must then still fit under the 75°C or 60°C value for the terminals.

Small conductor limits in NEC 240.4(D)

Whatever the table says, the overcurrent device on the smallest conductors is capped. This is why 12 AWG copper, which shows 25 A at 75°C, still goes on a 20 A breaker.

These limits apply to general branch circuits. Motor circuits under Article 430 and some other articles have their own rules.

What wire size for common amp ratings

The smallest conductor in Table 310.16 that carries each load at standard conditions, with the 240.4(D) limits applied. Computed from the table above. Above 100 A the 60°C column no longer applies by default, so only the 75°C sizes are shown. Long runs, hot locations and more than three current carrying conductors all push the size up.

Ambient temperature correction factors

Table 310.16 assumes 30°C (86°F). Hotter air means less ampacity. Multiply the table value by the factor for the column you are using. NEC Table 310.15(B)(1) lists these factors.

Raceways and cables above a sunlit rooftop also get a temperature adder under 310.15(B)(2). Within half an inch of the roof, add 33°C to the outdoor ambient.

More than three current carrying conductors

When more than three current carrying conductors share a raceway or cable for more than 24 inches, the heat adds up and each conductor must be derated with NEC Table 310.15(C)(1). Equipment grounding conductors do not count. A neutral counts when it carries unbalanced or harmonic current, such as the neutral of a single phase 3 wire circuit or of a wye circuit feeding mostly nonlinear loads.

Worked example with both corrections

Six 4 AWG THWN-2 copper conductors in one conduit, in a 40°C space, on breakers and equipment with 75°C terminals. Start from the 90°C column, 95 A. The 90°C factor for 36 to 40°C is 0.91, and six conductors take 0.80. So 95 x 0.91 x 0.80 = 69.2 A. Now check against the terminals. The 75°C value for 4 AWG copper is 85 A, which is higher, so the corrected 69.2 A governs. Each conductor may carry 69 A, not 85 A.

Aluminum vs copper equivalent sizes

Aluminum carries less current than copper of the same size, so it runs about two sizes larger. Each row gives the smallest aluminum conductor whose 75°C ampacity is at least that of the copper size, computed from the table. Aluminum terminations must be rated for it, marked AL7CU or AL9CU on lugs, and connections often need an antioxidant compound where the equipment maker calls for it.

Sizing conductors on a transformer secondary

Transformer secondaries are where the large kcmil rows matter. The full load current of a three phase transformer is kVA x 1000 divided by (1.732 x volts). A 225 kVA transformer at 480Y/277 gives 270.6 A, which 300 kcmil copper (285 A) or 500 kcmil aluminum (310 A) can carry at 75°C. The same 225 kVA unit at 208Y/120 gives 624.5 A, which needs parallel sets, for example two sets of 400 kcmil copper (2 x 335 = 670 A) or two sets of 600 kcmil aluminum (2 x 340 = 680 A).

This only shows how to read the table. The secondary overcurrent protection, the tap and feeder rules and voltage drop decide the final size. Run any rating through the kVA to amps calculator to get the current first.

Free air and medium voltage tables

Table 310.16 is for conductors in a raceway, cable or earth. Single conductors in free air get higher ampacities from Table 310.17, and other tables in Article 310 cover other installations. Conductors over 2000 volts, such as the medium voltage cable that feeds a pad mounted transformer primary, use Article 311 and its own tables, not Table 310.16.

Table numbering by NEC edition

The same table has carried different numbers. Through the 2017 NEC it was Table 310.15(B)(16). The 2020 NEC renumbered it Table 310.16, and the 2023 NEC keeps that number. The ampacity values in this chart are the ones published for the 2017 through 2023 editions. Always check the edition your jurisdiction has adopted.

What is the ampacity of 4/0 aluminum wire

4/0 aluminum is 150 A at 60°C, 180 A at 75°C and 205 A at 90°C in NEC Table 310.16. With 75°C terminals, the usual case, use 180 A.

What size wire for 100 amps

With 75°C terminals, 3 AWG copper (100 A) or 1 AWG aluminum (100 A). If the terminals are rated only 60°C, which 110.14(C) assumes for 100 A circuits unless they are marked otherwise, you need 1 AWG copper (110 A) or 1/0 aluminum (100 A). Derating or voltage drop can push either up.

What size wire for 200 amps

At 75°C, 3/0 copper (200 A) or 250 kcmil aluminum (205 A) from Table 310.16. Dwelling services and feeders can use the separate sizing in 310.12, which allows smaller conductors in that one case.

What is the ampacity of 12 AWG THHN

12 AWG copper THHN is 30 A in the 90°C column, but that number is only the starting point for derating. With 75°C terminals the limit is 25 A, and 240.4(D) caps the breaker at 20 A.

Why can I not use the 90°C column for THHN

Because the breaker and equipment terminals are almost never rated 90°C. NEC 110.14(C) limits the conductor to the rating of the lowest rated termination, so the 90°C value is used only as the base for ambient and bundling corrections.

What does 310.16 assume

Insulated conductors rated up to 2000 volts, not more than three current carrying conductors in a raceway, cable or earth, and a 30°C (86°F) ambient. Anything else needs correction or adjustment factors.

Is Table 310.16 the same as 310.15(B)(16)

Yes. It was numbered 310.15(B)(16) through the 2017 NEC and became Table 310.16 in the 2020 edition.