Temperature Limitation of Conductors Based on Philippine Electrical Code

The temperature limitations of conductors are critical in ensuring electrical safety and preventing insulation degradation or fire hazards. In the Philippine Electrical Code (PEC), specific guidelines govern the use of conductors based on their temperature ratings.

General Limitation:

• According to the PEC, no conductor should be used in a manner that allows its operating temperature to exceed the designated temperature for its type of insulation​. This restriction ensures that the insulation will not deteriorate due to excessive heat.

Temperature Ratings:

• Conductors are categorized based on their maximum allowable temperature under both normal and abnormal operating conditions. Common temperature ratings include:

• 60°C (140°F)
• 75°C (167°F)
• 90°C (194°F)
• Special applications may go up to 150°C (302°F)

Consideration of Termination and Equipment Ratings:

• Conductors with higher temperature ratings may be used, but the system design must ensure that the termination points, devices, and equipment connected to the conductor are rated for the same or higher temperatures. For instance, conductors connected to equipment with a 75°C rating should not exceed that temperature, even if the conductor insulation allows for a higher rating.

Correction Factors:

• When ambient temperatures exceed the standard 30°C (86°F), the conductor ampacity (the maximum amount of electrical current a conductor can carry) must be adjusted using correction factors to avoid overheating. These correction factors are detailed in tables and vary based on the conductor insulation’s maximum operating temperature​.

Correction Factors for Ambient Temperature 30°C (Based on Philippine Electrical Code)

The Correction Factors Table adjusts the ampacity of conductors based on the ambient temperature, which ensures that the conductor operates safely without exceeding its insulation temperature rating.

How to Use the Table:

1. Identify the conductor’s insulation rating (60°C, 75°C, or 90°C).
2. Determine the ambient temperature where the conductor is installed.
3. Find the corresponding correction factor.
4. Multiply the conductor’s ampacity (from the ampacity tables) by the correction factor to get the adjusted ampacity for the given temperature.

Example:

• For a 75°C-rated conductor at an ambient temperature of 45°C, the correction factor is 0.82.
• If the conductor’s ampacity at 75°C is 100A, the adjusted ampacity would be:
  100A × 0.82 = 82A

This adjusted ampacity ensures the conductor operates within safe temperature limits.

Allowable Ampacities of Insulated Conductors (COPPER) Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F) Conductor Types TW, THHW, THWN and THHN.

Usage:

This table should be referenced when selecting conductors based on expected current loads, ensuring that the conductor chosen can handle the current without overheating. Always apply appropriate correction factors if the ambient temperature differs from the standard 30°C (86°F).

How to use the Table of Allowable Ampacities of Insulated Conductors.

Using the Table of Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, based on ambient temperature of 30°C (86°F), and covering types TW, THHW, THWN, and THHN, involves following specific steps to ensure that the conductors chosen meet both the current and temperature requirements for your installation. Here’s how to effectively use the table:

Steps to Use the Table:

• Identify the Conductor Type:
  • Choose the conductor type based on the location and environment of the installation. The common types are:
    • TW (60°C): For dry or damp locations with lower heat tolerance.
      • THHW (75°C wet and dry): For wet and dry locations requiring heat and moisture resistance.
    • THWN (75°C wet, 90°C dry): Suitable for wet locations like underground installations.
    • THHN (90°C dry): For high-heat applications in dry locations.

• Determine the Conductor Size:
  • The table provides ampacities for different conductor sizes (in mm²). Choose the conductor size based on your load requirements, ensuring the conductor’s ampacity matches the expected current load of the installation.

• Check the Ampacity for the Conductor Type and Size:
  • Look up the corresponding ampacity for the conductor size and type under the appropriate temperature column (60°C, 75°C, or 90°C). Ensure that the ampacity matches the current demand of your circuit.
• Example: If you have a THHN conductor and you need to carry 30A, a 3.5 mm² (12 AWG) conductor can handle 30A at 90°C.

• Verify the Ambient Temperature:
  • The table assumes an ambient temperature of 30°C (86°F). If the installation environment has a higher ambient temperature, you will need to apply a correction factor to the ampacity (found in the Correction Factors Table).
• For example, if the ambient temperature is 45°C, a THWN conductor rated at 75°C will have a correction factor of 0.82. Multiply the table ampacity by the correction factor to get the adjusted ampacity.
• Adjusted Ampacity = Table Ampacity × Correction Factor
• Example: For a THWN conductor rated at 75°C with a table ampacity of 115A at 45°C: Adjusted Ampacity = 115A×0.82 = 94.3A
• So, the conductor can safely carry 94.3A under these conditions.

• Adjust for More than Three Current-Carrying Conductors:
  • If more than three current-carrying conductors are installed in a raceway, cable, or buried in the earth, you must apply an additional derating factor. This ensures the conductor won’t overheat due to the increased heat produced by multiple conductors.
• Refer to the latest PEC for additional derating factors based on the number of conductors.

• Cross-Check Equipment and Termination Ratings:
  • Ensure that the termination points (such as circuit breakers, panels, etc.) are rated for the temperature at which the conductors are operating. If the termination point is rated for 75°C, even if you use a 90°C conductor, the conductor ampacity should be based on the 75°C rating.

Temperature of Conductors Under Normal Operating Conditions

Under normal operating conditions, the normal temperature of conductors depends on the type of insulation and the environmental factors. According to the Philippine Electrical Code (PEC), typical temperature ratings for common conductor insulation types include:

• 60°C (140°F)
• 75°C (167°F)
• 90°C (194°F)

The temperature rating is crucial because it determines how much current a conductor can safely carry without damaging the insulation. Conductors must be selected and installed based on their temperature limits to avoid overheating and potential system failures.

For instance, if the ambient temperature exceeds 30°C (86°F), a correction factor is applied to the conductor’s ampacity. This ensures that the operating temperature does not exceed the insulation’s rating under elevated conditions

Maximum Operating Temperature Rating of Conductor Type TW, THHW, THWN, and THHN including their corresponding applications, and insulation types:

TW, THHW, THWN, and THHN are commonly used insulated conductor types

1. TW (Thermoplastic, Moisture-Resistant)

• Max Operating Temperature: 60°C (140°F) in dry locations.
• Application: TW conductors are typically used for wiring in dry and damp locations. They are moisture-resistant but have a lower temperature rating compared to other conductors.
• Insulation: Moisture-resistant thermoplastic.
• Common Use: Residential and commercial building wiring in low-temperature environments, especially in dry indoor applications.

2. THHW (Thermoplastic, Heat, and Moisture Resistant)

• Max Operating Temperature: 75°C (167°F) and 90°C (194°F) in both wet and dry locations.
• Application: THHW conductors are suitable for wet and dry locations and can handle higher temperatures. They offer both heat and moisture resistance.
• Insulation: Heat and moisture-resistant thermoplastic.
• Common Use: General-purpose wiring in residential, commercial, and industrial applications where resistance to moisture and heat is required, including conduits and other enclosed systems.

3. THWN (Thermoplastic, Heat and Water-Resistant, Nylon-coated)

• Max Operating Temperature: 75°C (167°F) in wet and dry locations.
• Application: THWN conductors are designed for wet locations and underground conduit installations. They are water and heat-resistant, ideal for outdoor use or wet environments.
• Insulation: Heat and water-resistant thermoplastic with a nylon coating.
• Common Use: Outdoor wiring, underground wiring in conduit, and industrial applications that are subject to moisture or exposure to water.

4. THHN (Thermoplastic, High Heat Resistant, Nylon-coated)

• Max Operating Temperature: 90°C (194°F) in dry locations.
• Application: THHN is one of the most common types of wiring used in buildings today, and it is known for its high heat resistance. It is generally used in dry locations where high-temperature ratings are required.
• Insulation: Heat-resistant thermoplastic with a nylon coating.
• Common Use: Residential and commercial building wiring, typically in dry indoor environments such as conduit wiring, lighting, and appliances.

Source: Philippine Electrical Code