Cable Ampacity Calculator

A cable's ampacity — the maximum current it can carry continuously without overheating — isn't a single fixed number. A tabulated ampacity is published for a specified installation arrangement, ambient/reference condition, conductor configuration and cable construction — the applicable standard or manufacturer's data should be used to obtain the starting ("base") ampacity for your exact cable. Real installations rarely match that reference condition exactly, which is why derating (correction) factors exist: to adjust the tabulated base ampacity down to the actual safe current for your real site conditions.

Ambient temperature correction formula: CF_temp = √[(T_max − T_ambient) ÷ (T_max − T_ref)], where T_max is the cable's maximum rated conductor temperature (70°C for standard PVC, 90°C for XLPE), T_ambient is your actual site ambient temperature, and T_ref is the standard reference ambient (30°C) the base ampacity table was built around. This is an approximation widely used to correct tabulated ampacity for a different ambient temperature — it follows from the physical relationship between resistive heating (proportional to current squared) and the temperature rise it produces, but the applicable standard's own correction table is the authoritative source; treat this as an illustrative approximation for preliminary work, not a substitute for that table.

Grouping (bunching) correction factor: when multiple loaded cables are installed close together, they collectively reduce each other's ability to shed heat to the surroundings, since each cable is now partly surrounded by other heat sources instead of cooler open space. Standard tables (from IS 3961, IEC 60364-5-52, or similar) provide grouping factors based on the number of circuits bunched together and their arrangement — more circuits grouped means a progressively lower factor, and therefore more derating.

Worked example: a cable has a base (tabulated) ampacity of 100 A at the standard 30°C reference, is PVC-insulated (70°C max), and will be installed at a site ambient of 40°C, grouped with 3 other loaded circuits. Temperature correction: CF_temp = √[(70−40) ÷ (70−30)] = √(30÷40) = √0.75 ≈ 0.866. Using this page's illustrative grouping factor for 4 circuits together: CF_group ≈ 0.65. Estimated derated ampacity = 100 × 0.866 × 0.65 ≈ 56.3 A — significantly below the 100 A tabulated figure, illustrating why real-world derating matters and can't be skipped for a hot, grouped installation.

Why this matters for protection coordination, not just the cable itself: the protective device must be selected so that the conductor remains adequately protected under the applicable wiring rules, taking the cable's actual installed (derated) ampacity and load requirements into account — a breaker rated above the cable's actual (derated) safe current provides no real overload protection for that cable under the actual installed conditions, even though it might look correctly sized against the cable's tabulated, unadjusted rating. This is one of the more common and consequential real-world cable sizing errors, particularly in retrofit or expansion projects where new circuits get added to an existing, already-grouped cable route.

Conductor size and material also drive base ampacity: beyond installation method and insulation type, base ampacity scales strongly with conductor cross-sectional area (larger conductors, more copper cross-section to carry current and dissipate heat, higher ampacity) and material (copper conductors have meaningfully higher ampacity than aluminum conductors of the identical cross-sectional area, since copper's lower resistivity means less resistive heating for the same current). This calculator works with whatever base ampacity you supply, so getting that starting figure right — matched to your actual conductor size, material, and insulation — is the single most important input, more consequential to the final answer than either correction factor alone. If you're starting from a required load current rather than an existing cable, the Cable Size Calculator can help identify a starting conductor size before you look up its base ampacity here.

Aluminum conductors, increasingly common for larger sizes due to cost, also generally need a larger cross-sectional area than copper to achieve an equivalent ampacity — a common rule of thumb is that aluminum needs roughly one to two standard sizes larger than copper for the same current rating, though the exact figure should always be confirmed against the applicable standard's table for the specific sizes involved rather than assumed from a general rule.

Buried (underground) cables use a different correction approach entirely: the temperature and grouping correction formulas and factors discussed here apply to cables in air (free air, conduit, tray). Directly buried cables dissipate heat very differently — through the surrounding soil rather than air — and are subject to their own separate set of correction factors for soil thermal resistivity, burial depth, and grouping/spacing of buried cables, published in different tables within the same standards. Don't apply air-installation correction factors to a buried cable calculation, and vice versa — always match your correction method to your actual installation environment.

Short-time and cyclic loading considerations: everything in this calculator addresses continuous (steady-state) current rating — the current a cable can carry indefinitely without exceeding its temperature limit. Some loads are cyclic or intermittent (motor starting currents, batch process equipment) and a cable's thermal mass allows it to tolerate a higher current for a short period than its continuous rating alone would suggest, since it takes time for the conductor to actually reach its maximum temperature. Short-time and cyclic rating calculations are a separate, more involved engineering exercise (typically requiring the cable's specific thermal time constant) and shouldn't be assumed from the continuous ampacity figure this calculator produces.

Two corrections that are easy to forget: a cable buried in loft, wall or roof insulation loses its sideways heat path, and even a short enclosed length costs real ampacity — a fully surrounded run can be down to around half its free-air figure. Separately, triplen harmonics from non-linear loads add rather than cancel in the neutral of a four-wire circuit, so a cable feeding drives, LED drivers or switched-mode supplies runs hotter than its sinusoidal rating assumes. Both are selectable above and both multiply into the same chain as temperature and grouping.

Parallel runs and the load check: where one cable cannot carry the load, two or more identical cables in parallel on the same phase share it, and total capacity is the per-run derated figure times the number of runs — but those runs are themselves grouped circuits, so they must also be counted in the grouping field if they share a route. Entering your design load current turns the calculation around: as well as checking whether the derated capacity covers the load, the calculator works back to the minimum base ampacity you need per run, which is the number to take into your cable table when selecting a size.

Summary: start with the correct base ampacity for your exact conductor size, insulation type, and installation method from the applicable standard's table, apply CF_temp = √[(T_max−T_ambient)÷(T_max−30)] for ambient temperature, apply the grouping factor matching your actual number of bunched circuits and arrangement, multiply all three together for the true derated ampacity, and size the upstream protective device against that derated figure, not the tabulated base value.

Why this calculation matters beyond just avoiding overheating: correctly derated cable sizing sits at the intersection of safety (avoiding a fire risk from an overheated, degrading cable insulation), reliability (avoiding nuisance tripping or premature cable failure from chronic overload), and cost (avoiding unnecessarily oversizing every cable "just to be safe," which adds real material and installation cost across a large facility). Getting the derating calculation right, rather than guessing conservatively high or optimistically low, is genuinely valuable engineering work, not just a code-compliance formality.

Working with an electrical contractor or consulting engineer: for anything beyond a straightforward single-circuit installation, sharing your actual site conditions (measured or design ambient temperature, planned cable routing and grouping, installation method) with whoever is doing the detailed cable sizing ensures the correction factors applied match your real installation, not generic assumptions. This calculator is a useful tool for understanding and cross-checking that calculation, but a full electrical design should also confirm voltage drop, short-circuit withstand rating, and compliance with the specific edition of the applicable wiring standard in force for your jurisdiction.

1. Sizing the protective device against tabulated ampacity instead of derated ampacity. A breaker or fuse should be rated at or below the cable's actual derated ampacity for the real installed conditions, not its unadjusted catalog rating — using the tabulated figure for protection sizing leaves the cable under-protected against overload in a hot or grouped installation.

2. Applying only one correction factor when both temperature and grouping apply. Ambient temperature and grouping derating are independent effects that both apply simultaneously when both conditions are present — multiply both factors together with the base ampacity, don't apply just the larger (more severe) one alone.

3. Using the wrong maximum conductor temperature for the actual insulation type. PVC (70°C) and XLPE (90°C) have different maximum operating temperatures, which changes both the temperature correction factor and the base ampacity itself — using PVC assumptions for an XLPE cable (or vice versa) gives an incorrect derating calculation in both directions.

4. Forgetting to re-derate when new circuits are added to an existing bundle. Adding cables to an already-installed group (a common retrofit scenario) changes the grouping factor for every cable in that group, not just the new one — existing circuits that were adequately sized for the original grouping may become under-derated once additional circuits are added alongside them.

5. Using a base ampacity from the wrong installation method. Tabulated ampacity differs substantially between installation methods (free air, enclosed conduit, buried directly, on a ventilated tray) for the identical cable — pulling a base ampacity figure from the wrong table (e.g., free-air figures for a cable that's actually in conduit) produces a meaningfully wrong starting point regardless of how correctly the temperature and grouping factors are then applied.

6. Not accounting for solar/radiant heating on cables run in direct sunlight. Outdoor cable trays or conduits exposed to direct sun can experience a significantly higher effective ambient temperature than the shaded air temperature alone would suggest — standards typically provide an additional correction or guidance for this condition, which is easy to overlook when only using a general site ambient temperature figure.

7. Applying air-installation correction factors to a buried cable. Buried cables dissipate heat through soil rather than air and require entirely different correction factors (soil thermal resistivity, burial depth, buried-cable grouping/spacing) from a separate table — using free-air or conduit correction methods for a directly buried cable gives an incorrect result.

8. Assuming continuous ampacity applies unchanged to short-time or cyclic loads. A cable's thermal mass allows it to briefly exceed its continuous rating for intermittent loads (like motor starting) without immediately overheating — but confirming a safe short-time or cyclic rating requires its own separate calculation using the cable's thermal time constant, not just the continuous ampacity figure treated as an absolute ceiling for every load pattern. Do not use short-time thermal behaviour to justify routinely exceeding the applicable continuous ampacity unless a documented engineering calculation specifically permits it for your actual load profile.