Definition
Conduit fill is the NEC-defined limit on the percentage of conduit cross-sectional area occupied by conductors. Maximum fill is 40% for ≥3 conductors. More than 3 current-carrying conductors triggers ampacity derating per NEC Table 310.15(B)(3).
NEC 310.15(B)(3)(a) Derating
| Conductor count | Derate factor |
|---|---|
| 1–3 | 1.00 |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
| 41+ | 0.35 |
Applies to current-carrying conductors only (EGC and bonding doesn’t count).
Key Takeaways
- Conduit fill ≤ 40% for 3+ conductors (NEC Chapter 9 Table 1).
- Bundle of 4+ current-carrying conductors triggers ampacity derating.
- Plan solar conductor routing carefully to balance conduit count and ampacity.
- Document conduit fill calculation in the SLD or schedule.
Frequently Asked Questions
2 commonly searched questions about Conduit Fill.
What is conduit fill?
The percentage of a conduit's cross-section occupied by conductors. NEC 358.22 and Chapter 9 Table 1 specify maximums (typically 40% for 3+ conductors).
Conduit fill derating?
Per NEC 310.15(B)(3)(a): 4–6 conductors × 80%; 7–9 conductors × 70%; 10–20 × 50%. Solar projects often need careful conduit planning to maintain ampacity headroom.
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