The article explores the relationship between conductor insulation and ampacity, focusing on heat generation, temperature ratings, and NEC standards for determining safe operating limits. It also highlights different insulation types, correction factors for ampacity, and the uses and restrictions of flexible cords and cables.
Heat is generated whenever current flows through a conductor. The amount of heat depends upon the value of current and resistance of the conductor. As the heat is generated, the operating temperature of the conductor increases. This rise in temperature continues until the rate at which heat leaves the conductor equals the rate at which heat is produced. When the two are equal, the temperature remains constant.
What is Conductor Ampacity?
“The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.” is the definition of ampacity as defined by the NEC. National Electrical Code and NEC are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02269. The ampacity of a conductor is limited by the temperature that its insulation can tolerate without deteriorating or losing its insulating quality.
When considering the ampacity of a conductor, it is also necessary to consider its operating temperature. Table 310.17 of the NEC lists the Ampacities of Single-Insulated Conductors Installed in Free Air. It is based on an ambient temperature of 30C°. The maximum allowable operating temperature is listed at the top of each column. This value is determined by the type of insulation used. The operating temperatures range from 60C° to 90C°.
Similar to NEC Table 310.17 is Table 310.16. Table 310.16 lists the Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried). Due to the heat accumulation from two or three current-carrying conductors, as well as the conductors being enclosed in raceway, cable, or earth, the ampacities will be lower than those for the same size conductor in Table 310.17.
Correction Factors
The operating temperature of a conductor depends upon the ambient temperature and the amount of current flowing in the conductor. For example, NEC Table 310.16 and Table 310.17 indicate that insulation types of TW and UF have a maximum operating temperature of 60C°(140F°). Insulations that are subjected to temperatures higher than their maximum operating temperatures will deteriorate.
The values indicated in NEC Table 310.16 and Table 310.17 are for use where the ambient temperature does not exceed 30C°(86F°). For conductors installed in areas where the ambient temperature is above 30C°(86F°), the ampacity is determined by applying the correction factor indicated in Table 310.15(B)(1) Ambient Temperature Correction Factors Based on 30C°(86F°).
Conductor Ampacity Calculation Example 1
A No. 12 AWG copper conductor is covered with RHW insulation. If this conductor is installed in an area where the ambient temperature is 122F°(50C°), what is the ampacity of the conductor?
NEC Table 310.16 indicates that the load current rating of No. 12 RHW copper wire is 25 A [however, the *footnote at the bottom of the table refers the reader to see 240.4(D)]. Here the reader will discover that under 240.4(D)(5), the overcurrent protection for 12 AWG copper is 20 A. This limits the load current rating of the conductor to 20 A, provided that not more than three conductors are installed in the same raceway and the maximum ambient temperature does not exceed 86F°(30C°).
NEC Table 310.16 also indicates that type RHW insulation is rated at 75C°. Looking at NEC Table 310.15(B)(1), finding the intersection of the 75C° column with the 122F° row (on the right side of the table), we find the temperature correction factor to be 0.75. Therefore, 20A × 0.75 = 15A. The maximum safe ampacity for No. 12 RHW copper wire is 15 A. A correction factor must also be applied when there are more than three conductors in the raceway.
Conductor Ampacity Calculation Example 2
If the raceway in the preceding example contains six conductors, what is the ampacity of the conductors?
The Adjustment Factors for More Than Three Current-Carrying Conductors are listed in NEC Table 310.15(C)(1). For four to six conductors, the correction factor is 80. This is the Percent of Values in Table 310.16 through 310.19 as Adjusted for Ambient Temperature if Necessary.
Therefore, take the ampacity of the conductor and apply the correction factors for temperature and number of conductors:
Corrected Ampacity = Initial Ampacity × Temperature Correction Factor × Four to Six Conductors Correction Factor
Corrected Ampacity = 20A × 0.75 × 0.8 = 12A
The safe ampacity for this conductor is 12 A.
Types of Insulations
Plastic is the most widely used insulating material for electrical conductors due to its affordability, durability, and excellent insulating properties. For general-purpose wiring in residential and commercial buildings, the most commonly employed insulation types are TW, THW, and THWN. These insulation types are versatile, offering reliable protection for standard electrical installations under typical environmental conditions.
In addition to these general-purpose insulations, specialized options are available for specific applications. Type TFN insulation, for example, is primarily designed for wiring in lighting fixtures and similar applications where space constraints and lightweight wiring are critical considerations. It offers flexibility and is tailored for low-voltage, high-temperature environments often encountered in lighting systems.
Similarly, Type MTW insulation is specifically engineered for wiring in machine tools and other industrial applications exposed to harsh conditions, such as exposure to oil, water, or other corrosive liquids. MTW insulation provides enhanced resistance to moisture and chemicals, ensuring durability and safety in environments where standard insulation types might deteriorate.
NEC Table 400.4 (Flexible Cords and Flexible Cables) and Table 402.3 (Fixture Wires) list the various insulations that meet the NEC standards. These tables provide information as to the trade name, type letter, voltage, AWG or kcmil, number of conductors, insulation, nominal insulation thickness, braid on each conductor, outer covering, use, and maximum operating temperature. For installing electrical conductors, it is necessary to consider all factors that may affect the size and material of the conductors, and the type of insulation used.
Flexible Cords and Cables
In the electrical industry, several types of cords and cables are commonly used, each designed to meet specific needs and applications. Among these, nonmetallic sheathed cables, often referred to as NM cables, are widely used in residential and light commercial wiring. They are favored for their ease of installation and cost-effectiveness, particularly in situations where the wiring is protected by walls or other structural elements.
Hard service cords are durable and robust, making them suitable for industrial and heavy-duty applications. They are typically used to power equipment and tools that require high resilience to wear and environmental factors.
Portable power cables are designed for temporary installations or equipment that needs to be moved frequently. These cables are flexible, rugged, and capable of withstanding rough handling, making them ideal for use in construction sites, entertainment venues, and portable power equipment.
Data processing cables, on the other hand, are specialized for transmitting signals and data in computing and telecommunications systems. They are constructed to minimize interference and maintain high signal integrity, which is critical in modern digital and communication networks.
Lastly, elevator cables are specifically engineered for use in vertical motion equipment such as elevators and hoists. They are designed to endure continuous flexing and bending while maintaining electrical connectivity and safety.
The conductors in all these cords and cables are composed of fine strands of copper or other conductive materials. The use of fine strands increases their flexibility, allowing these cables to bend, twist, and move easily without breaking.
Flexible cords and cables are permitted for use as:
- pendants;
- wiring of luminaires;
- connections of portable luminaires, portable and mobile signs, or appliances;
- elevator cables;
- wiring of cranes and hoists;
- and more as listed in NEC Article 400 (specifically Article 400.10).
They may not be used:
- as a substitute for the fixed wiring of a structure;
- where run through holes in walls, structural ceilings, suspended ceilings, dropped ceilings, or floors;
- and more as listed in NEC Article 400 (specifically Article 400.12).
Various types of cords and cables are shown in Figure 1 and Figure 2.
Figure 1. Various wires and cable assemblies.
Figure 2. Cables in Different Sizes