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Electrical Engineering

Power Cable Ampacity Simulator

Design the ampacity — the maximum continuous current a power cable can carry. Change the conductor material, cross-section, insulation type, ambient temperature and installation method to see, in real time, the safe current and the heat generated without exceeding the insulation temperature limit.

Parameters
Conductor material
Sets resistivity ρ₂₀ and temperature coefficient α
Conductor cross-section A
mm²
Insulation type
Sets the conductor temperature limit T_limit
Ambient temperature T_a
°C
Temperature of the air or soil around the cable
Installation method
Sets how easily heat escapes (effective heat-transfer coefficient)
Results
—
Ampacity I_max (A)
—
Conductor resistance (operating) (mΩ/m)
—
Allowable temp. rise ΔT (K)
—
Heat dissipated @I_max (W/m)
—
Current density @I_max (A/mm²)
—
Verdict
—
Cable cross-section — heat generation and dissipation

I²R heat is generated in the central metallic conductor and flows radially outward through the insulation to the surroundings. The conductor colour shows its temperature (blue = cool, orange-red = near the limit).

Ampacity vs conductor cross-section
Ampacity vs ambient temperature
Theory & Key Formulas

$$I_{max}=\sqrt{\dfrac{\Delta T}{R_{ac}\,R_{th}}},\qquad R_{ac}=\frac{\rho(T)}{A}$$

The ampacity I_max is the current at which I²R Joule heating just raises the conductor to its insulation temperature limit. ΔT: allowable temperature rise, R_ac: conductor resistance at the operating temperature, R_th: thermal resistance, A: conductor cross-section.

$$\rho(T)=\rho_{20}\,\bigl(1+\alpha\,(T_{limit}-20)\bigr)$$

Resistivity at the operating (limit) temperature. ρ₂₀: resistivity at 20 °C, α: temperature coefficient, T_limit: insulation temperature limit (XLPE 90 °C / PVC 70 °C).

$$R_{th}=\frac{1}{h_{eff}\,\pi\,d_{cond}},\qquad \Delta T=T_{limit}-T_{a}$$

Thermal resistance R_th from the conductor to the surroundings and the allowable temperature rise ΔT. h_eff: effective heat-transfer coefficient (set by the installation method), d_cond: equivalent conductor diameter, T_a: ambient temperature.

What is Cable Ampacity?

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The "ampacity" of a cable — is that the limiting current beyond which the cable would melt?
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That is a common misconception, but it is not the "melting current". Ampacity is set by temperature. When current flows, I²R Joule heat is produced in the conductor, and that heat escapes outward through the insulation. The ampacity is the current at which the heat generated exactly balances the heat that can be shed, so the conductor sits right at the maximum temperature the insulation can tolerate. The line is drawn at a temperature, far below the melting point.
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I see, so it's about temperature. Then what exactly is the "insulation temperature limit" — how many degrees?
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It is fixed by the insulation material. For ordinary vinyl (PVC) insulation the typical value is 70 °C, and for cross-linked polyethylene (XLPE) it is 90 °C. Try switching "Insulation type" on the left from XLPE to PVC. When the limit drops from 90 °C to 70 °C, the allowable temperature rise ΔT shrinks, so the ampacity falls right away. XLPE carries more current precisely because it tolerates a higher temperature.
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When I raise the ambient temperature slider, the ampacity keeps dropping. Is that also tied to the temperature limit?
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Exactly. The allowable rise is ΔT = (limit temperature − ambient temperature). The hotter the surroundings, the smaller ΔT, and the less heat margin you have to shed. Ampacity is proportional to √ΔT, so as the ambient approaches the limit temperature the ampacity falls toward zero. That is why a cable run through a hot attic in midsummer, or a machine room, is risky at the catalogue value. In practice the deviation from a reference temperature is discounted with a "temperature correction factor".
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You can also pick the installation method. Does conduit, in-air or direct burial really make that much difference?
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Quite a lot. It changes "how easily the heat escapes". Inside a conduit, the trapped air layer acts like insulation and the heat builds up, so the ampacity is on the low side. In air, convection sheds heat and you can carry a bit more. The surprise is direct burial — good, moist soil conducts heat fairly well and cools the cable nicely. "Buried is cooler" feels counter-intuitive, but it is true. And bundling several cables makes them warm one another, so that too needs a grouping correction.
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Switching the conductor from copper to aluminium dropped the ampacity. Is that because copper "conducts electricity better"?
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Yes — copper has lower resistivity. Aluminium is about 1.6× more resistive than copper, so at the same current it generates more I²R heat and reaches the temperature limit sooner. For the same cross-section, copper therefore carries more. Conversely, to carry the same current aluminium needs a larger cross-section. Even so, aluminium is light and cheap, so it is the workhorse for overhead lines and large distribution cables. Sizing comes down to the combination of "copper or aluminium", thickness, insulation and environment.

Frequently Asked Questions

Ampacity is not the current that melts the cable like a fuse — it is set by a steady-state temperature. Current produces I²R Joule heating in the conductor, and that heat flows out through the insulation to the surroundings. The ampacity is the current at which the heat generated exactly balances the heat that can be carried away when the conductor sits at the insulation temperature limit (70 °C for PVC, 90 °C for XLPE). The formula is I_max = √(ΔT/(R_ac·R_th)), where ΔT is the allowable temperature rise, R_ac the conductor resistance at the operating temperature and R_th the thermal resistance.
Ampacity depends on the temperature headroom ΔT = (limit temperature − ambient temperature) available to raise the conductor to its limit. A hotter ambient leaves less headroom and less heat can be shed, so the ampacity falls. Since ampacity is proportional to √ΔT, halving ΔT cuts the ampacity to about 0.71×. As the ambient approaches the insulation limit, ΔT tends to zero and the ampacity falls toward zero. In practice the deviation from a reference temperature (often 30 °C or 40 °C) is handled with a temperature correction factor.
The installation method changes how easily heat escapes from the conductor — the effective heat-transfer coefficient h. In air, convection sheds heat and h is moderately large; inside a conduit, the trapped air layer insulates the cable, h is small and the ampacity drops. Perhaps surprisingly, direct burial in good, moist soil cools well because soil conducts heat reasonably, so h is large and the ampacity rises. Bundling several cables or running them close together makes them heat one another, so a grouping factor reduces the ampacity further.
For the same cross-section, copper carries more current. The 20 °C resistivity of copper is about 1.72e-8 Ω·m and that of aluminium about 2.82e-8 Ω·m — aluminium is roughly 1.6× more resistive. Higher resistance means more I²R heating at the same current, so the temperature limit is reached sooner and the ampacity is lower. Conversely, to carry the same current, aluminium needs a larger cross-section. Aluminium is light and low-cost, so it is widely used for overhead lines and large distribution cables.

Real-World Applications

Feeders and branch circuits in building services: Sizing cables from the incoming switchboard to distribution boards, and from distribution boards to outlets and equipment, is the most basic use of an ampacity calculation. You choose a cable whose ampacity exceeds the load current, then check ambient-temperature and grouping corrections and the voltage drop. Insufficient ampacity leads to insulation ageing and burnout, so this is the foundation of wiring design.

Solar and renewable-energy installations: The DC and AC cables from solar panels to the inverter and the grid connection point are exposed to high outdoor ambient temperatures and rooftop radiant heat. In environments hotter than the reference temperature, the temperature correction sharply reduces the ampacity, so checking the ambient effect directly — as this tool does — helps prevent overheating problems.

Underground transmission and distribution lines: Urban power cables are laid in ducts or directly buried. Ampacity is set considering the soil thermal resistance, burial depth, cable spacing and mutual heating from other circuits. Direct burial lets the soil help shed heat, but in dry soil the thermal resistance rises sharply, so assessing the soil condition is critical.

Troubleshooting electrical installations: Faults such as "the cable is abnormally hot" or "the insulation jacket has discoloured and hardened" are often caused by exceeding the ampacity, an unexpected ambient temperature or bundled wiring. A heat-balance calculation like this tool lets you check whether the conductor temperature exceeds the limit at the present current and environment, and decide whether to add cables, increase the size or change the installation method.

Common Misconceptions and Pitfalls

The biggest misconception is "ampacity = the current that melts the cable". The ampacity is set far below the fusing current, by a steady-state temperature limit. Every ampere of current produces I²R heating, and that heat flows outward through the insulation into the surroundings. The ampacity is simply the current at which the heat generated exactly equals the heat that can be carried away when the conductor sits at the maximum temperature its insulation can tolerate — 70 °C for ordinary PVC, 90 °C for cross-linked polyethylene (XLPE). Anything that makes it harder to shed that heat reduces the ampacity: a hotter ambient (less temperature headroom), installation inside a conduit or bundled with other cables (poorer cooling), or running in a hot location. Conditions that improve cooling raise it — and direct burial in good, moist soil actually cools quite well. Copper, having lower resistivity, carries more current than aluminium of the same cross-section. Engineers start from a base ampacity and apply derating factors for ambient temperature and grouping; exceeding the ampacity cooks the insulation, ages it prematurely and eventually causes failure.

Next, the complacency of "the catalogue ampacity can be used as-is". A manufacturer's ampacity value is defined for specific reference conditions (reference ambient temperature, single-cable installation, a standard installation method). When the real site differs from the reference, you must reduce it by multiplying by a temperature correction factor and a grouping correction factor. Bundle several circuits in a hot midsummer attic and the corrected ampacity can easily fall to less than half the catalogue value. Designing with the catalogue value without checking the reference conditions invites overheating and insulation degradation.

Finally, the pitfall of "only the steady-state ampacity matters". This tool deals with the steady-state heat balance for continuous use. In reality you must separately check the short-time withstand current against large transient currents such as short-circuit current (set by heat capacity), and the transient heating from intermittent loads or starting current. Voltage drop is also a separate constraint from ampacity — on long runs, the size is often dictated by voltage drop even when the ampacity has plenty of margin. Decide the final cable size by considering all three together: ampacity, short-time withstand and voltage drop.

How to Use

  1. Select conductor material (copper or aluminum) and enter cross-sectional area in mm² using condAreaNum or condAreaRange slider (typical range: 1–400 mm² for industrial cables)
  2. Set ambient temperature in °C via ambientTempNum or ambientTempRange; typical values 20°C (indoor) to 40°C (outdoor/tropical)
  3. The simulator calculates maximum ampacity I_max in amperes, conductor resistance in mΩ/m at operating temperature, allowable temperature rise in K, heat dissipation in W/m, current density in A/mm², and a pass/fail verdict against IEC 60364 or NEC standards

Worked Example

A 70 mm² copper cable in a 30°C ambient environment: conductor resistance at 20°C is approximately 0.257 mΩ/m; accounting for temperature coefficient, at 70°C operating temperature it rises to ~0.31 mΩ/m. With an allowable temperature rise of 40 K (insulation limit typically 70°C for PVC), ampacity reaches approximately 285 A. Heat dissipation at maximum current: 285² × 0.00031 ≈ 25.2 W/m. Current density: 285 ÷ 70 ≈ 4.07 A/mm². The verdict confirms compliance if density remains below 5.5 A/mm² for continuous duty.

Practical Notes

  1. For buried cables in high soil temperatures (50°C+), derate ampacity by 10–15%; the simulator accounts for ambient but derating factors depend on installation method (conduit, tray, free air)
  2. Aluminum cables (conductivity ~61% of copper) require larger cross-section for equivalent ampacity; a 120 mm² aluminum matches ~70 mm² copper at the same temperature rise
  3. In DC systems or high-frequency AC (>10 kHz), skin effect and harmonics increase effective resistance; verify with detailed thermal models before oversizing
  4. Cable bundling reduces surface dissipation; group three single-phase cables and derate by ~15–20% unless spaced ≥2×cable diameter