B ampacity

Keywords: b ampacity
Description: Learn the principles of ampacity, temperature ratings and derating factors.

Ampacity is the maximum current (measured in amperes or more simply, amps) an insulated conductor can safely carry without exceeding its insulation and jacket temperature limitations. As the amount of current passing through a conductor is increased, the amount of heat produced in the conductor increases. The heat created in the conductor must be dissipated to the environment. If the heat cannot escape, the temperature of the conductor would continue to increase until the cable exceeds its temperature rating, and the cable would deteriorates.

To prevent premature cable failure, the NEC® (National Electrical Code) 1 as well as other industry groups such as IEEE (Institute of Electrical and Electronics Engineers) and ICEA (Insulated Cable Engineers Association) have published tables of ampacities that cover many of the installation conditions frequently encountered in real life. For example, IEEE has published a book called IEEE Std 835-1994 Standard Power Ampacity Tables that contains thousands of ampacity tables. General ampacity ratings for conductors can be found in the NEC 2014 Article 310 Conductors for General Wiring.

* Reprinted with permission from NFPA 70®-2014, National Electrical Code®, Copyright © 2013, National Fire Protection Association, Quincy, MA. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

[1] NFPA 70®, National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Quincy, MA.

The ambient temperature is vital when determining ampacity ratings because at higher temperatures heat transfers out of the cable at a slower rate. If the ambient temperature differs from the ampacity tables’ stated ambient temperatures (typically 30°C or 40°C) in NEC 2014 Article 310, a correction factor may need to be applied. NEC 2014 Table 310.15(B)(2)(A) Ambient Temperature Correction Factors Based on 30°C (86°F) is one of the tables where adjustment factors can be found. For example, a 10 AWG 3C 600 V cable is installed in an ambient temperature of 50°C. According to NEC 2014 310.15(B)(2)(A), the cable’s ampacity would need to be multiplied by a correction factor of 0.75, assuming the 75°C column was used in the ampacity table. The adjusted ampacity would be 26 amps.

1. Why do flexible cords or portable power cables have higher ampacity ratings compared to NEC 2014 Section 310 Conductors for General Wiring?

Flexible cords and portable power cables are not permitted by NEC 2014 Section 400.8 to be installed in raceways, concealed by walls or floors, or installed in dropped ceilings. These cable types are assumed to be installed in applications where there is adequate space and free air. Because of these anticipated installation conditions, overheating from being enclosed in a small space is not a large concern. Therefore, these cables are permitted to carry more current per gauge size compared to cable and wire types mentioned in NEC 2014 Section 310 Conductors for General Wiring, which typically are installed in applications with limited air space or air movement. Ampacity tables for flexible cords and cables can be found in NEC 2014 Section 400.5 Ampacities for Flexible Cords and Cables.

2. What are the ampacity requirements for conductors used in continuous duty applications for single motors?

According to NEC 2014 Section 430.22 Single Motor, conductors shall not be rated less than 125 percent of the motor full-load current. For example, if the FLC of a motor is 120 amps, the ampacity rating would be 150 amps (120*1.25) at the minimum.

3. I need higher ampacity and want to save money and weight by using a welding cable because they have higher ampacity ratings. Which welding cable do I select?

Welding cable ampacity ratings depend on many factors including the length of the welding circuit, rated output of the welding power source, and the duty cycle of the welding source. Unless the variables are known and can be accounted for, it is not recommended to use welding cables in nonwelding applications.

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