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NEC Article 310 Wire Sizing Guide

Complete reference guide to National Electrical Code ampacity tables and conductor sizing requirements

Understanding NEC Article 310

Article 310 of the National Electrical Code covers conductors for general wiring and is fundamental to proper wire sizing. This article provides the ampacity tables that form the basis of safe electrical installations.

Key Sections of Article 310

310.12 - Insulation

Specifies insulation types and temperature ratings. Common ratings include:

  • 60°C: TW, UF
  • 75°C: THW, THWN, USE
  • 90°C: THHN, XHHW, USE-2

310.15 - Ampacity

Provides methods for determining conductor ampacity including:

  • • Temperature correction factors
  • • Adjustment factors for bundling
  • • Installation method considerations
  • • Ambient temperature effects

Table 310.16 - Allowable Ampacities

The most commonly referenced table for conductor ampacities. This table applies to not more than three current-carrying conductors in raceway, cable, or earth.

AWG/kcmilCopper 60°CCopper 75°CCopper 90°CAluminum 60°CAluminum 75°CAluminum 90°C
14152025---
12202530152025
10303540253035
8405055304045
6556575405055
4708595556575
2951151307590100
111013015085100115
1/0125150170100120135
2/0145175195115135150
3/0165200225130155175
4/0195230260150180205

Temperature Correction Factors

When ambient temperature differs from 30°C (86°F), ampacities must be corrected using Table 310.15(B)(1):

For 75°C Rated Conductors

21-25°C (70-77°F):1.05
26-30°C (78-86°F):1.00
31-35°C (87-95°F):0.94
36-40°C (96-104°F):0.88
41-45°C (105-113°F):0.82

Practical Application

If you have a 20A circuit with 12 AWG copper wire (25A at 75°C) in a 40°C attic:

Corrected Ampacity = 25A × 0.88 = 22A

The corrected ampacity (22A) still covers the 20A circuit — and 12 AWG copper is capped at 20A of overcurrent protection by NEC 240.4(D) regardless.

Adjustment Factors for Multiple Conductors

When more than three current-carrying conductors are installed in the same raceway, cable, or trench, ampacity must be reduced per NEC 310.15(C)(1) (Table 310.15(B)(3)(a) in the 2017 and earlier codes):

4-6 conductors: 80%
7-9 conductors: 70%
10-20 conductors: 50%
21-30 conductors: 45%

Important Notes

  • • Neutral conductors carrying only unbalanced current are not counted
  • • Equipment grounding conductors are not counted
  • • Conductors that are not simultaneously energized are not counted
  • • Both temperature correction and adjustment factors may apply to the same installation

Special Applications

Motor Circuits (Article 430)

  • • Use 125% of motor full-load current for conductor sizing
  • • Branch circuit conductors must have ampacity ≥ 125% of FLA
  • • Feeder conductors use 125% of largest motor plus sum of others

Continuous Loads (Article 210)

  • • Loads expected to operate for 3+ hours continuously
  • • Conductor ampacity must be ≥ 125% of continuous load
  • • Plus 100% of non-continuous loads

NEC Article 310 FAQ

What is NEC Article 310?

NEC Article 310 governs conductors for general wiring — their insulation types, ampacity ratings and how those ratings are adjusted for temperature and grouping. Table 310.16 is the most-used ampacity table for up to three current-carrying conductors in a raceway or cable.

When do I use the 60, 75 or 90°C column of Table 310.16?

Use the column that matches the lowest-rated termination in the circuit, per NEC 110.14(C). Most breakers and lugs are listed for 75°C, so the 75°C column is the usual answer even when 90°C-rated insulation (like THHN) is installed. The 90°C column is generally only used as a starting point for derating.

What is the 240.4(D) small-conductor rule?

NEC 240.4(D) caps overcurrent protection for small conductors regardless of the Table 310.16 value: 14 AWG copper at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A. This is why a 20-amp circuit uses 12 AWG copper even though 14 AWG shows 20 A in the 75°C column.

Do temperature and conduit fill both reduce ampacity?

Yes. Ambient temperature correction (Table 310.15(B)(1)) and the conductor-count adjustment (310.15(C)(1)) are applied together when both conditions exist. Start from the 90°C ampacity, multiply by both factors, then confirm the result does not exceed the termination-temperature column.

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