Fundamentals calculator
Cable Ampacity Calculator: Thermal-Balance Estimate
Estimate steady-state cable current using a reduced thermal model. Enter AC resistance, thermal resistance, temperature limits and other cable losses.
BasicsBasis: Reduced steady-state heat-balance model
Results
- Enter values to calculate
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How to use this calculator
Cable ampacity is the current that reaches the allowed conductor temperature under stated installation conditions. This calculator demonstrates the heat balance using one effective thermal path. It is useful for sensitivity checks: higher ambient temperature, electrical resistance or thermal resistance leaves less room for current.
Neher–McGrath and IEC 60287 methods model the installation in greater detail. Their layers, loss factors and mutual-heating treatment cannot be replaced by a generic thermal resistance without validation. This page therefore identifies its output as a reduced-model estimate. It does not select an NEC or CSA conductor size.
Formula
Δθ = (I² R′ac + W′other) T′thermal
I = √{[(θmax−θambient)/T′thermal − W′other] / R′ac}
Convert R′ac (Ω/m) = Rac (Ω/km) / 1000. T′thermal is K·m/W; W′other is W/m. The expression inside the square root must be positive. Resistance is evaluated at the limiting conductor temperature.
Worked example
With θmax = 90°C, ambient = 25°C, Rac = 0.1 Ω/km, T′ = 1.5 K·m/W and W′other = 2 W/m, I = √[(65/1.5 − 2)/0.0001] = 642.91 A. This is the result of the stated lumped assumptions, not a cable manufacturer rating.
Assumptions and standards
- One equivalent thermal path; fixed other losses and constant ambient. Sheath/armour current-dependent losses are not automatically calculated.
- Installation geometry, soil drying, grouping, ducts, load factor and cable-layer thermal data must be established separately.
- Full Neher–McGrath / IEC 60287 compliance is not claimed.
Calculation model checked: November 4, 2026
Frequently asked questions
Is this a full Neher–McGrath calculator?
No. It is a single-path thermal-balance estimator. A full installation rating needs cable construction, separate thermal paths and losses, soil conditions and mutual heating.
Why does hot soil reduce ampacity?
Hotter soil reduces the permitted temperature rise between the conductor and its surroundings. With less heat-transfer capacity, less I²R heating can be tolerated.
Can I use these results for final design?
Use them for preliminary design and checking. Final designs must be verified by a qualified engineer against the code edition and standards adopted where the work is installed.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you type is uploaded or stored on our servers.
References and further reading
Power system calculations: formulas and model guide