NEC Annex B provides application information for ampacity calculations. This annex is designed to assist with the practical application of the ampacity tables found in Chapter 9 and provides additional guidelines and considerations for calculating conductor ampacity in various installation conditions. Here is an overview of the key points covered in Annex B:
Purpose and Scope
Annex B supplements the ampacity tables by offering additional information on how to apply the values in these tables under different conditions. This includes adjustments for ambient temperature, the number of conductors, and specific installation environments.
Ambient Temperature Correction Factors
Annex B provides detailed correction factors for adjusting conductor ampacity based on ambient temperature. These factors ensure that the conductor’s capacity is not exceeded due to high ambient temperatures which can affect the conductor’s ability to dissipate heat.
Adjustment Factors for More Than Three Current-Carrying Conductors
This section details the necessary ampacity adjustments when more than three current-carrying conductors are installed together in a raceway or cable. It includes tables that specify the percentage of the base ampacity to use based on the number of conductors.
Conditions of Use
Annex B explains how to apply the ampacity tables to different installation conditions such as:
- Direct Burial in Earth: Provides specific guidelines and correction factors for conductors buried directly in the ground.
- Conduits Exposed to Sunlight on Rooftops: Includes considerations for the additional heating effects of sunlight on rooftop conduits, requiring further ampacity adjustments.
- Cables in Free Air: Outlines how to calculate ampacity for cables installed in free air, considering factors like wind speed and air temperature.
Sample Calculations
To aid understanding, Annex B includes sample calculations demonstrating how to apply correction and adjustment factors to determine the appropriate conductor size for various scenarios. These examples illustrate the step-by-step process of adjusting ampacity based on specific installation conditions.
Tables and Figures
The annex contains numerous tables and figures that provide visual aids and reference points for making accurate ampacity calculations. These tables include:
- Ambient temperature correction factors.
- Adjustment factors for more than three conductors.
- Ampacity tables for different installation conditions.
For more detailed information and specific application guidelines, consulting the NEC Handbook and additional resources provided by the NFPA is highly recommended. These resources offer comprehensive insights and practical examples to ensure safe and effective electrical installations.