NEC ReferenceNEC 2023 · 2026 · Updated May 2026

NEC Article 310: Conductor Sizing & Ampacity

NEC Article 310: Conductors covers 310.16 Explained through to Size a Conductor (Step by Step). This guide explains each section with practical interpretation, common mistakes, and how these rules apply to real projects.

Table 310.16 Explained

NEC Table 310.16 is the most referenced table in the entire code. It provides allowable ampacities for insulated conductors rated 2000V or less, installed in raceway, cable, or earth, at an ambient temperature of 30C (86F). The table has three temperature columns: 60C (types TW, UF), 75C (types THW, THWN, XHHW), and 90C (types THHN, THWN-2, XHHW-2).

Common values every electrician should know: 14 AWG copper at 60C = 15A, 12 AWG at 60C = 20A, 10 AWG at 75C = 35A, 8 AWG at 75C = 50A, 6 AWG at 75C = 65A, 4/0 AWG at 75C = 230A. The 90C column provides the highest ampacity values but can only be used when both ends of the conductor terminate on equipment rated for 90C, which is uncommon.

See our complete ampacity tables for every wire size with pre-computed derating.

110.14(C) Termination Temperature Limits

NEC 110.14(C) controls which temperature column you can use from Table 310.16, and it is the rule most electricians and engineers miss. For circuits rated 100A or less (or conductors 1 AWG and smaller), the ampacity must be based on the 60C column unless all terminations (breakers, receptacles, switches) are listed and marked for 75C.

Most modern equipment is rated 75C, so the 75C column is typically usable for circuits 100A and under in new installations. For circuits over 100A, the 75C column applies. You can still install 90C-rated wire (THHN) on any circuit. The advantage is not higher ampacity but rather more room for derating: you start with the 90C value, apply temperature and bundling corrections, then check that the result still meets the 75C column limit.

Temperature Correction (310.15(B)(1))

Table 310.16 ampacities assume a 30C (86F) ambient temperature. When conductors are in hotter environments, the ampacity must be reduced using correction factors from Table 310.15(B)(1). At 40C ambient: the correction factor is 0.88 for 75C-rated wire and 0.91 for 90C-rated wire.

At 50C: factors drop to 0.75 for 75C wire and 0.82 for 90C wire. This matters most in attics (which routinely exceed 50C in summer), outdoor conduit exposed to direct sun, and industrial environments. For example, 12 AWG THHN in a 50C attic: start at 30A (90C column), multiply by 0.82 = 24.6A, which still exceeds the 20A 60C column limit, so the wire remains adequate.

Use our ampacity calculator to apply temperature correction automatically.

Bundling Adjustment (310.15(C)(1))

When more than 3 current-carrying conductors share a raceway or cable, heat dissipation is reduced and ampacity must be adjusted. NEC Table 310.15(C)(1) specifies: 4-6 conductors = 80% of table ampacity, 7-9 conductors = 70%, 10-20 conductors = 50%, 21-30 conductors = 45%, 31-40 conductors = 40%.

Equipment grounding conductors do not count. Neutral conductors carrying only unbalanced current from other conductors in the same raceway do not count per 310.15(E)(1). This derating is cumulative with temperature correction: if you have 6 conductors in a hot attic, apply both factors.

The 90C conductor rating provides headroom for these reductions, which is the real advantage of using THHN instead of TW wire.

How to Size a Conductor (Step by Step)

Proper conductor sizing follows a specific sequence. Step 1: Determine the circuit load amperage, applying 125% for continuous loads per 210.19(A)(1). Step 2: Look up the conductor ampacity from Table 310.16 at the appropriate temperature column for your wire type.

Step 3: Apply the ambient temperature correction factor from Table 310.15(B)(1) if the environment exceeds 30C. Step 4: Apply the bundling adjustment factor from Table 310.15(C)(1) if more than 3 current-carrying conductors share a raceway. Step 5: Verify the final derated ampacity meets or exceeds the circuit load from Step 1 and does not fall below the termination temperature column limit per 110.14(C).

Step 6: Check voltage drop for long runs using our voltage drop calculator. All of our ampacity pages perform these calculations automatically for every common scenario.

Key NEC Tables Referenced

Frequently Asked Questions

What does NEC Article 310 cover?

Article 310 covers conductors requirements. Key sections include Table 310.16 Explained and 110.14(C) Termination Temperature Limits. It applies to all electrical installations in the United States.

Where can I find the full text of Article 310?

The full NEC is published by NFPA and available at nfpa.org. Many libraries provide free access. Your local building department also has copies available for reference during permit applications.

Does Article 310 apply to existing wiring?

The NEC applies to new installations, modifications, and additions. Existing wiring that was code-compliant when installed is generally grandfathered unless a renovation triggers an upgrade requirement. Check with your local AHJ (authority having jurisdiction).

When did the current Article 310 requirements take effect?

NEC 2026 is the latest published edition (effective September 2025). NEC 2023 remains the most widely adopted edition by states. Adoption varies - check your state adoption status on our state code pages.

What is the ampacity of 6 AWG copper?

6 AWG copper THHN has a base ampacity of 75A at 90C per Table 310.16. After the 110.14(C) termination limit, usable ampacity is 65A at 75C. This wire is commonly used for 60A circuits (EV chargers, hot tubs, subpanels).

Can I use the 90C column for wire sizing?

Only if both ends of the conductor terminate on equipment rated and marked for 90C. Most residential breakers and devices are rated 75C, so the 75C column controls the final ampacity per 110.14(C). However, you can start with the 90C value when calculating derating for temperature or bundling.

What is NEC Table 310.16?

Table 310.16 is the primary wire ampacity chart in the NEC. It lists allowable ampacities for insulated conductors rated up to 2000V in raceway, cable, or earth at 30C ambient. Three columns (60C, 75C, 90C) cover different insulation types.

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
This guide provides practical interpretation of NEC requirements. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction. WireRef is a reference tool and does not replace licensed professional judgment.

NEC references verified May 2026