Ampacity Derating Calculator: engineer's guide
Base cable ampacity tables assume a reference condition. Real installations rarely match it. This tool applies the product of C-factors to give the corrected ampacity you can actually use.
Open the Ampacity Derating calculator →
What this calculator does
The calculator multiplies the base table ampacity by ambient, grouping, installation-method, and (for buried cables) soil-resistivity factors, then compares the result against the design current.
Formulas used
- Corrected ampacity:
Iz = Itab × Ca × Cg × Ci × Cs - Design rule:
Ib ≤ In ≤ Iz
Variables
- Itab = base ampacity, from the code table
- Ca = ambient temperature factor
- Cg = grouping factor for multiple circuits
- Ci = installation-method factor (conduit vs tray vs free air vs buried)
- Cs = soil thermal resistivity factor (buried cables only)
Worked example
Scenario: 95 mm² XLPE cable, base ampacity 251 A on tray, ambient 45°C, 5 circuits grouped, PVC 70°C insulation.
- Ca at 45°C for 90°C XLPE ≈ 0.87
- Cg for 5 grouped circuits ≈ 0.75
- Iz = 251 × 0.87 × 0.75 ≈ 163.8 A
- If design current Ib is 150 A, corrected ampacity is sufficient with a 160 A protective device.
Common mistakes to avoid
- Multiplying instead of using published derating tables
- Forgetting soil resistivity for buried cables in dry regions
- Applying grouping factor for cables in separate ducts (they do not group)
- Using the same factor set for XLPE and PVC (they differ)
Frequently asked questions
Does a single circuit need a grouping factor?
No. Grouping applies only when multiple loaded circuits share the same enclosure or tray.
What ambient temperature should I use?
The maximum expected ambient at the cable route location during operation, not the room design temperature.
Do I derate for future spare circuits on the same tray?
Yes if they will be loaded. Unloaded spares can be excluded from grouping.
Standards and references
IEC 60364-5-52, BS 7671 Appendix 4, NEC 310.15
Related tools on this site
Open the calculator that pairs with this guide: Ampacity Derating Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.