Cable Ampacity Derating Calculator
Corrected current-carrying capacity of a cable, and the table value a design current needs.
Results
What this tool does
The current printed in a cable table assumes one circuit, in free air, at 30 °C. Every departure from that costs capacity, and the factors multiply rather than add — four circuits bunched at 40 °C together take almost forty percent off. The fourth result works backwards: it gives the table ampacity you must look for so that the cable still carries your design current after derating.
Formula
Iz = Iref × k1 × k2 × k3 × k4
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ir | Table ampacity | A |
k1 | Ambient temperature factor | — |
k2 | Grouping factor | — |
k3 | Insulation factor | — |
k4 | Soil resistivity factor | — |
ib | Design current | A |
IZ | Corrected ampacity | A |
KT | Overall derating factor | — |
UT | Utilisation | % |
IR | Table ampacity | A |
MG | Current | A |
Worked example
- Table ampacity32 A
- Ambient temperature factor0.87
- Grouping factor0.7
- Insulation factor1
- Soil resistivity factor1
- Design current20 A
- Corrected ampacity19.488 A
- Overall derating factor0.60900
- Utilisation102.63 %
- Table ampacity32.84 A
- Current-0.512 A
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
Where do the correction factors come from?
From tables in the wiring standard that applies where you are — IEC 60364-5-52 and the national versions of it, or the NEC in North America. They cover ambient temperature, how many circuits share the containment, soil thermal resistivity for buried runs, and the insulation type. The defaults here show a common case: 40 °C ambient on PVC cable with four circuits bunched together, which together cost nearly forty percent of the capacity. Always take the numbers from the table, never from memory.