Underground Cable Derating Calculator
Work out how much current a buried cable can safely carry once real-world install conditions are taken into account. Starting from the cable's tabulated (base) ampacity, the tool applies grouping, soil thermal resistivity, burial depth and ambient temperature correction factors to give the derated capacity.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the cable's base ampacity from the AS/NZS 3008 tables for its size and installation method.
- Enter the four correction factors (grouping, soil resistivity, burial depth, ambient temperature) for your actual conditions, or leave them at the illustrative defaults.
- Read the overall derating factor and the derated ampacity — compare that against your design load current.
How it works
Derated capacity = base ampacity × (grouping × soil × depth × temperature). Each factor is a multiplier ≤ 1 (occasionally >1 for favourable conditions) taken from the wiring-rules tables. Multiplying them together gives one overall factor, and multiplying the base rating by that factor gives the current the cable can actually carry in that trench.
Worked example
Worked example. A cable tabulated at 200 A, grouped with others (0.8), in average soil (0.9), at standard depth (0.95) and rated ground temperature (1.0): overall factor = 0.8 × 0.9 × 0.95 × 1.0 = 0.684, so derated ampacity = 200 × 0.684 = 136.8 A — a loss of 63.2 A versus the table value.
Common mistakes
- Ignoring grouping — several circuits sharing a trench heat each other and this is often the single biggest derating factor.
- Guessing the factors instead of reading them from the AS/NZS 3008 tables for the exact number of circuits, soil condition, depth and temperature.
- Forgetting to compare the derated ampacity against the design load current and the protective device rating.
Frequently asked questions
Where do the correction factors come from?
From the derating tables in AS/NZS 3008 (or the equivalent IEC/NEC tables). Each addresses one install condition — number of grouped circuits, soil thermal resistivity, burial depth and ground/ambient temperature.
Can a factor be greater than 1?
Yes — very favourable conditions (e.g. a cooler-than-rated ground temperature) can give a factor above 1, increasing the allowable current. The tool accepts any positive value for each factor.
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