NEC 310: Conductors for General Wiring
What This Article Covers
NEC Article 310 covers the general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses. It contains the ampacity tables (most importantly Table 310.16) that determine what conductor size you need for a given current.
Key Requirements
- Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating
- Table 310.16 lists ampacities for not more than three current-carrying conductors in a raceway
- Ambient temperature correction factors per 310.15(B)(1)
- Adjustment factors for more than three current-carrying conductors per 310.15(C)(1)
- Termination temperature limits the usable ampacity column (75°C is most common for breakers and lugs)
Common Field Applications
- Choosing the right wire size for a 20A circuit
- Derating conductors in a hot attic or bundled conduit run
- Verifying ampacity for a feeder to a sub-panel
Common Mistakes & Inspection Failures
- Using the 90°C column when the terminations are rated 75°C
- Forgetting to apply ambient temperature correction in attics
- Counting the neutral as a current-carrying conductor when it isn't (or missing when it is)
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What does NEC 310 cover?
NEC Article 310 covers the general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses. It contains the ampacity tables (most importantly Table 310.16) that determine what conductor size you need for a given current.
What are the key requirements of NEC 310?
Key requirements include: Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating; Table 310.16 lists ampacities for not more than three current-carrying conductors in a raceway; Ambient temperature correction factors per 310.15(B)(1). See the full requirements list on this page.
What are common mistakes with NEC 310?
Using the 90°C column when the terminations are rated 75°C Forgetting to apply ambient temperature correction in attics Counting the neutral as a current-carrying conductor when it isn't (or missing when it is)
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Michael B.
IBEW Local 134 Journeyman Electrician · Licensed Electrical Contractor
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