310.15(C) Ampacity Adjustment Factors

Summary

This video explains the process of adjusting ampacities for current-carrying conductors, particularly when dealing with multiple wires in a raceway or cable. It covers key regulations from sections 310.15c and 110.14c, highlighting how to safely de-rate the ampacities based on the number of conductors, the size of the conductors, and the potential need for temperature corrections. The video also discusses factors such as conductor types, short raceways, and special cases like signal conductors and neutral wires. Practical scenarios, including branch circuits and wireway design, are explored to ensure safe, efficient electrical system planning.

Takeaways

• 😀 Conductors in raceways with more than three current-carrying wires require ampacity adjustments due to heat buildup.
• 😀 The heat generated by conductors in a raceway is managed by reducing their ampacity based on the number of conductors involved.
• 😀 The NEC 310.15C adjustment factor decreases the allowable ampacity as the number of current-carrying conductors increases, ranging from 80% for 4-6 conductors to 50% for 10-20 conductors.
• 😀 Conductors rated for 90°C insulation (e.g., THHN) are commonly used, but their ampacity can be adjusted based on the number of conductors in the raceway.
• 😀 Neutral conductors in multi-phase systems must be included in ampacity calculations as they are also current-carrying conductors.
• 😀 Ground conductors do not carry current under normal conditions, so they are not counted in ampacity adjustment calculations.
• 😀 Spare wires not yet in use must be considered in ampacity calculations because they will add heat when energized in the future.
• 😀 Short raceways (less than 24 inches) do not require ampacity adjustments, as the heat dissipation is sufficient over such short distances.
• 😀 Signal or control wires do not contribute to the heat load in a raceway, so they are excluded from ampacity adjustment calculations.
• 😀 For systems with gutters or wire ways, de-rating is required if more than three current-carrying conductors are present in non-metallic wire ways, but in metallic wire ways, de-rating is needed only when there are over 30 current-carrying conductors.

Q & A

What is the primary purpose of de-rating ampacity in a raceway with more than three current-carrying conductors?

The primary purpose of de-rating ampacity is to prevent overheating of the conductors. When multiple current-carrying conductors are placed together in a raceway, they generate heat, which can cause damage if not managed properly. De-rating reduces the ampacity to ensure safe operation and heat dissipation.

Which sections of the NEC are crucial for understanding ampacity adjustments in a raceway with multiple conductors?

The key sections are 310.15(C), which addresses the adjustment factors for ampacity based on the number of current-carrying conductors, and 110.14(C), which requires matching the conductor's ampacity to the lowest temperature rating of any connected equipment.

How do you calculate the ampacity of conductors when there are more than three current-carrying conductors in a raceway?

To calculate the ampacity, you begin with the conductor's original ampacity based on its insulation rating. Then, multiply this by a derating factor from Table 310.15(C) corresponding to the number of conductors. For example, with nine conductors, the factor would be 0.7 (70%), reducing the conductor's ampacity.

What is the derating factor applied when there are 9 current-carrying conductors in a raceway?

The derating factor for 9 current-carrying conductors is 0.7, meaning that the conductor's original ampacity is reduced to 70% of its rated value.

How does the conductor size affect ampacity adjustments in a raceway with multiple conductors?

The conductor size determines its original ampacity. Larger conductors have higher ampacities, and they will be derated based on the number of current-carrying conductors. If there are different conductor sizes (e.g., #4 and #12), each size is derated separately according to the same factor, and the new ampacities are calculated accordingly.

What is the role of the ground wire in ampacity adjustment calculations?

Ground wires do not carry current under normal conditions and do not contribute to heat generation. Therefore, ground wires are not counted when performing ampacity adjustment calculations for current-carrying conductors.

How does the presence of spare conductors affect the ampacity adjustment calculation?

Spare conductors need to be counted in the ampacity adjustment calculation because, while they may not currently carry current, they will contribute to heat once energized. This ensures future heat load is considered in the design.

What is the significance of Section 310.15(C)(1) regarding short raceways?

Section 310.15(C)(1) specifies that for raceways shorter than 24 inches, ampacity adjustment is not required because the heat dissipation is assumed to be sufficient in such a short distance, preventing significant temperature rise.

What happens if conductors in a raceway have varying ampacity along their length?

If a conductor’s ampacity changes along its length (for example, due to a junction box or splice), the adjustment factors need to account for the varying ampacity. Section 310.15(C)(2) provides guidelines for handling this scenario.

What are the ampacity adjustment rules for metallic versus non-metallic gutters and wireways?

For non-metallic gutters and wireways, de-rating is required when there are more than three current-carrying conductors. In contrast, metallic wireways do not require de-rating until there are more than 30 current-carrying conductors. This difference is due to the better heat dissipation properties of metal.