Electrical
Adjustment Factors for Conductor Count
Ampacity multipliers applied when more than three current-carrying conductors share a raceway or cable.
Source of these values
Values as commonly published for NFPA 70 Table 310.15(C)(1). Verify against your adopted edition.
| Current-carrying conductors | Adjustment factor | Percent |
|---|---|---|
| 1 to 3 | 1.00 | 100% |
| 4 to 6 | 0.80 | 80% |
| 7 to 9 | 0.70 | 70% |
| 10 to 20 | 0.50 | 50% |
| 21 to 30 | 0.45 | 45% |
| 31 to 40 | 0.40 | 40% |
| 41 and above | 0.35 | 35% |
In short
When more than three current-carrying conductors share a raceway or cable, mutual heating reduces the ampacity of each. Four to six conductors are multiplied by 80%, seven to nine by 70%, ten to twenty by 50%, and the factor continues to fall as the count rises.
Notes and limits
- Ungrounded conductors always count. Equipment grounding conductors do not.
- A neutral carrying only the unbalanced current of a multiwire branch circuit generally does not count, but a neutral in a system with significant harmonic content does.
- This factor and the ambient temperature correction factor both apply, multiplied together.
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