Understanding the Rules for Bundling Multiconductor Cable Assemblies: Applying NEC 310.15(C)(1)(d)

When bundling multiconductor cable assemblies, the National Electrical Code (NEC) provides specific guidance to ensure safety and proper ampacity calculations. One key provision is 310.15(C)(1), which addresses adjustment factors for ampacity when multiple current-carrying conductors are installed together. This article focuses on interpreting the rules for bundling multiconductor cable assemblies of Type AC (Armored Cable) and Type MC (Metal-Clad Cable), understanding when adjustment factors apply, and how to use permissive provisions effectively, particularly under 310.15(C)(1)(d).

Key Rules for Bundling Conductors

When multiple current-carrying conductors are bundled together, they generate heat, which can reduce their ampacity. Table 310.15(C)(1) specifies adjustment factors based on the total number of current-carrying conductors:

• 1-3 conductors: No adjustment factor.
• 4-6 conductors: Adjustment factor of 80%.
• 7-9 conductors: Adjustment factor of 70%.
• 10-20 conductors: Adjustment factor of 50%.
• 21 or more conductors: Adjustment factor of 45%.

However, NEC 310.15(C)(1)(d) provides an important provision specifically for Type AC and Type MC cables that meet certain conditions.

The Permissive Provision Under 310.15(C)(1)(d)

Adjustment factors come into play when multiconductor cables, such as MC cables, are installed without maintaining spacing continuously for over 24 inches. "Not maintaining spacing" refers to situations where cables are bundled, stacked, or otherwise installed in close proximity without adequate separation to allow for proper heat dissipation. This increases the potential for heat buildup, reducing the ampacity of the conductors.

If cables are tied together, stacked, or routed along a surface in contact for more than 24 inches without adequate spacing, adjustment factors from Table 310.15(C)(1) must be applied unless specific provisions, such as those under 310.15(C)(1)(d), are met.

The provision allows certain bundled multiconductor cable assemblies of Type AC and MC cables to avoid adjustment factors entirely, provided they meet these conditions:

1. The cables do not have an overall outer jacket.
2. Each cable contains no more than three current-carrying conductors.
3. The conductors are 12 AWG copper.
4. No more than 20 current-carrying conductors are installed without maintaining spacing, are stacked, or are supported on bridle rings.

This provision applies exclusively to Type AC and MC cables. For other types of cables, the general rules under 310.15(C)(1) and its associated adjustment factors must be followed.

Understanding 310.15(C)(1)(d)(4): The 20-Conductor Rule

The specific requirement under 310.15(C)(1)(d)(4) states that the total number of current-carrying conductors in the bundle must not exceed 20 unless they are all 12 AWG conductors. If the number exceeds 20 but the conductors meet all other conditions, the adjustment factor applied is 60% rather than invalidating the provision entirely.

Example:

Bundle Composition:

• 12 MC cables with 12 AWG copper conductors (2 current-carrying conductors per cable).
• Total current-carrying conductors: 12 cables × 2 conductors = 24 conductors.

Analysis:

Since the total number of current-carrying conductors exceeds 20, but all conductors are 12 AWG and meet all other conditions, the provision can still be applied with a 60% adjustment factor. This adjustment ensures safe operation while accounting for the increased heat generated by the additional conductors.

Practical Application of the Rule

Scenario 1: Bundling with 20 or Fewer Conductors

Bundle Composition:

• 10 MC cables with 12 AWG copper conductors (meeting all provision criteria).

Total Current-Carrying Conductors:

• From 12 AWG cables: 10 cables × 2 conductors each = 20 conductors.

Analysis:

The total number of current-carrying conductors is within the 20-conductor limit. The provision under 310.15(C)(1)(d) can be applied, and no adjustment factors are needed.

Scenario 2: Mixed 12 AWG and 10 AWG Conductors

Bundle Composition:

• 10 MC cables with 12 AWG copper conductors (2 current-carrying conductors per cable).
• 4 MC cables with 10 AWG copper conductors (2 current-carrying conductors per cable).

Total Current-Carrying Conductors:

• From 12 AWG cables: 10 cables × 2 conductors each = 20 conductors.
• From 10 AWG cables: 4 cables × 2 conductors each = 8 conductors.
• Total conductors in the bundle: 28 current-carrying conductors.

Analysis:

Because the bundle includes 10 AWG cables that do not meet the criteria of 310.15(C)(1)(d), the provision cannot be applied to any part of the bundle. The entire bundle must be evaluated as a single unit, and adjustment factors based on the total number of current-carrying conductors (28) must be applied. According to Table 310.15(C)(1), the adjustment factor for 28 conductors is 45%.

Additional NEC Guidance: Non-Simultaneously Energized Conductors

NEC 310.15(C)(1) clarifies that conductors connected to components that cannot be simultaneously energized are excluded from the conductor count for adjustment factors.

Example: HVAC System

Scenario:

An HVAC system has:

• 8 continuously energized conductors.
• 6 conductors serving a furnace and air conditioner that are mutually exclusive (controlled by a thermostat).

Analysis:

Only 3 of the 6 HVAC conductors are energized simultaneously (maximum for either heating or cooling). Total current-carrying conductor count = 8 (continuous) + 3 (HVAC) = 11. Adjustment factor for 11 conductors = 70% (Table 310.15(C)(1)).

Key Takeaways

• Exclusive to AC and MC Cables: The provision under 310.15(C)(1)(d) applies only to Type AC and Type MC cables meeting specific conditions.
• Mixed Bundles: In mixed bundles containing both compliant and non-compliant cables (e.g., 12 AWG and 10 AWG MC), the provision under 310.15(C)(1)(d) cannot be selectively applied. The entire bundle must be treated as a single unit, and adjustment factors must be applied to all conductors based on the total count.
• Exceeding 20 Conductors: When the total number of current-carrying conductors exceeds 20 but meets all other criteria, a 60% adjustment factor applies.
• Non-Simultaneously Energized Conductors: Properly account for mutually exclusive conductors to avoid overestimating heat buildup.
• Cannot Use Provision with Non-12 AWG Conductors: If the total number of current-carrying conductors exceeds 20 and they are not all 12 AWG, the provision cannot be applied, and standard adjustment factors must be used.

By understanding and applying these rules, electrical professionals can ensure safe, compliant installations while optimizing cable sizing and costs. Always consult the Authority Having Jurisdiction (AHJ) for final approval of complex installations.