NEC 310.15: Ampacities for Conductors Rated 0 – 2000 Volts
What This Article Covers
NEC 310.15 contains the ampacity tables and the correction and adjustment factors that you apply to those table values. Most NEC ampacity problems are really 310.15 problems: base ampacity from the table, then derate for ambient temperature and conductor bundling.
Key Requirements
- Use Table 310.16 for raceway or cable installations
- Apply ambient temperature correction factors from Table 310.15(B)(1)
- Apply adjustment factors from Table 310.15(C)(1) when more than 3 current-carrying conductors
- Use the lowest-rated termination temperature when determining final ampacity
- Neutral counts as current-carrying when it carries the unbalanced portion of nonlinear loads
Common Field Applications
- Derating a conductor run through an attic at 50°C ambient
- Calculating allowable ampacity with 6 conductors in a single raceway
- Selecting conductors for solar PV DC feeders (high ambient + bundling)
Common Mistakes & Inspection Failures
- Forgetting that termination temperature caps the final ampacity
- Double-counting derating — apply each factor only once
- Misreading the temperature column
Related NEC Articles
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What does NEC 310.15 cover?
NEC 310.15 contains the ampacity tables and the correction and adjustment factors that you apply to those table values. Most NEC ampacity problems are really 310.15 problems: base ampacity from the table, then derate for ambient temperature and conductor bundling.
What are the key requirements of NEC 310.15?
Key requirements include: Use Table 310.16 for raceway or cable installations; Apply ambient temperature correction factors from Table 310.15(B)(1); Apply adjustment factors from Table 310.15(C)(1) when more than 3 current-carrying conductors. See the full requirements list on this page.
What are common mistakes with NEC 310.15?
Forgetting that termination temperature caps the final ampacity Double-counting derating — apply each factor only once Misreading the temperature column
Related Resources
Michael B.
IBEW Local 134 Journeyman Electrician · Licensed Electrical Contractor
Michael is a licensed electrical contractor and IBEW Local 134 journeyman with years of field experience. He built Sparky AI after ChatGPT gave him wrong NEC code information on a job — costing him $800 in callbacks. Every answer in Sparky AI is verified against the actual NEC.