NEC ReferenceNEC 2023 · 2026 · Updated Apr 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 major section with practical interpretation for electricians.

Table 310.16 Explained

Table 310.16 is the most-used table in the NEC. It gives allowable ampacities for insulated conductors in raceway, cable, or earth at 30°C ambient. Three temperature columns: 60°C (TW, UF), 75°C (THW, THWN, XHHW), and 90°C (THHN, XHHW-2). The 90°C column gives the highest ampacity but can only be used when both terminations are rated for 90°C.

110.14(C) Termination Temperature Limits

This is the rule most people miss. For circuits 100A and under, conductor ampacity must be based on 60°C column values unless all terminations are listed for 75°C (most modern equipment is). For circuits over 100A, 75°C column applies. You can still use 90°C wire - you just cannot use the 90°C ampacity for sizing.

Temperature Correction (310.15(B)(1))

Table 310.16 assumes 30°C (86°F) ambient temperature. If ambient is higher (like in an attic or hot climate), multiply the ampacity by correction factors from Table 310.15(B)(1). At 40°C: multiply by 0.91 for 75°C wire. At 50°C: multiply by 0.82. This is why attic wiring often needs to be upsized.

Bundling Adjustment (310.15(C)(1))

When more than 3 current-carrying conductors are in a raceway or cable, ampacity is reduced. 4-6 conductors: 80%. 7-9 conductors: 70%. 10-20 conductors: 50%. Grounds and neutrals (that only carry unbalanced loads) do not count. This is why conduit fill matters for ampacity, not just physical space.

How to Size a Conductor (Step by Step)

Step 1: Determine circuit amperage (include 125% for continuous loads). Step 2: Start with Table 310.16 at the appropriate temperature column. Step 3: Apply ambient temperature correction. Step 4: Apply bundling adjustment. Step 5: Verify the final ampacity meets or exceeds the circuit requirement.

Step 6: Check voltage drop for long runs. Our ampacity pages do all of this automatically.

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 2023 is the current edition. Adoption varies by state. Check your state adoption status on our state code pages.

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 2023 references verified April 2026