Overview
When more than three current-carrying conductors are installed together in the same raceway, cable, or bundled group, their ability to dissipate heat decreases.
To prevent overheating, the ampacity (current-carrying capacity) of each conductor must be reduced according to the adjustment factors listed in Table 310.15(B)(3)(a).
Adjustment Factors
| Number of Conductors | Percent of Ampacity (from Table 310.16) |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
These percentages are applied after any temperature correction factors have been made.
What’s New
In recent NEC updates, the title and note under Table 310.15(B)(3)(a) were revised for clarity:
- Applies to Spare Conductors – Even if conductors are not currently energized, if they are installed as spares, they must be included in the conductor count because they may carry current in the future.
- Excludes Non-Simultaneous Conductors – Conductors that cannot be energized at the same time (such as travelers in a 3-way switch circuit) do not need to be counted.
- Bundled Cables Rule – When multiple cables are bundled together for more than 24 inches (600 mm) without spacing, the ampacity of each conductor must be derated using this table.
Practical Example
If you install nine current-carrying conductors in the same conduit, each conductor’s ampacity must be limited to 70% of the value listed in Table 310.16.
For example:
- A #12 AWG THHN copper wire rated at 30 amps becomes 21 amps after applying the 70% adjustment factor.
Why This Matters
Derating ensures that conductors do not overheat due to excessive grouping. Overheating can lead to insulation damage, fire hazards, and code violations.
Proper adjustment maintains safe operating temperatures and extends the lifespan of electrical wiring systems.
Reference Note
¹The number of conductors includes all current-carrying and spare conductors in the raceway or cable.
Conductors that cannot be energized simultaneously are not included in the count.
