Calculator
PV Cable Ampacity and Derating Calculator
A cable's catalogue rating assumes 30°C ambient, free air and no neighbours. A solar cable clipped in a bundle to a south-facing roof in August meets none of those conditions, and its real capacity can be barely half the printed figure.
This calculator applies the standard correction factors so the rating you design to is the rating the cable actually has on site.
Apply the correction factors
From the cable table for the installation method, at 30 °C or 60 °C reference.
Roof surfaces reach 60–70 °C in summer.
Ambient correction factor
0.707
90 °C conductor limit at 60 °C ambient
Grouping factor
0.65
4 circuits together
Installation method factor
1
Derated capacity
25.3 A
From a base of 55 A
Design current check
Pass
Design 17.4 A against 25.3 A derated
Protection coordination
Pass
Device 20 A must not exceed the derated capacity
Base rating needed for this device
43.5 A
Choose a cable whose tabulated rating exceeds this
How the calculation works
Ampacity is limited by conductor temperature. A cable can carry current until the heat it generates raises the conductor to its insulation limit, so anything that reduces the available temperature rise reduces the current.
The ambient factor is the square root of the ratio of remaining temperature headroom to the headroom at the reference condition. At a 90°C limit and 60°C ambient there is only 30°C of headroom instead of 60°C, so capacity falls by a factor of 0.707.
Grouping factors account for cables heating each other. Four circuits bundled together each lose roughly 35% of capacity. Installation method adds a further reduction where heat cannot escape, which is why conduit in an insulated wall is the worst case.
The final requirement from IEC 60364-4-43 is that the design current must not exceed the protective device rating, and the device rating must not exceed the derated cable capacity. All three inequalities must hold.
The formula
C_a = √((T_max − T_amb) / (T_max − T_ref)) I_z = I_base × C_a × C_g × C_m Requirement: I_b ≤ I_n ≤ I_z I_b design current · I_n protective device rating I_z derated capacity · C_g grouping · C_m installation method
Assumptions behind the numbers
- Base ratings come from the manufacturer's tables for the correct installation method and reference ambient.
- Grouping factors are typical values from IEC 60364-5-52 for bunched circuits in one layer; spaced arrangements are less severe.
- Solar cable to EN 50618 is rated at 120°C conductor temperature and 90°C ambient, which is why it survives roof conditions that would destroy general-purpose cable.
- Solar irradiance falling directly on cables adds heat that no standard factor covers; route DC cables in shade or in ventilated trays where possible.
- Fault-current withstand and earth-loop impedance are separate checks not performed here.
How to read the result
If the coordination check fails, the fix is a larger cable or fewer circuits in the group — not a smaller protective device, which would then fail to protect against the design current.
On DC string circuits the derating is usually comfortable because string current is low and EN 50618 cable has a high temperature limit. The tight cases are AC circuits, combiner-to-inverter runs and any cable passing through insulation.
Where several corrections stack — hot roof, bundled, in conduit — the combined factor can fall below 0.45. Designing from the catalogue figure in that situation understates the required cross-section by two or three sizes.
Worked example
A 6 mm² solar cable with a 55 A base rating in free air is used for four string circuits bundled on a roof tray, ambient 60°C, protected by a 20 A fuse, with a design current of 17.4 A.
The ambient factor at a 120°C limit would be √((120 − 60)/(120 − 30)) = 0.816, but if the designer specifies 90°C thermoset instead the factor drops to √(30/60) = 0.707. Grouping four circuits gives 0.65, and a ventilated tray gives 1.0.
Derated capacity with the 90°C cable is 55 × 0.707 × 0.65 = 25.3 A. Both the 17.4 A design current and the 20 A fuse fit under that, so the design passes — but with only 5 A of margin, and it would fail if a fifth circuit joined the bundle or the fuse were increased to 25 A.
Mistakes that skew the answer
- Using catalogue ampacity without any correction, the single most common error in DIY and low-cost installations.
- Applying 30°C ambient to a roof cable run.
- Counting only the cables in the same tray section and missing circuits added later.
- Using general-purpose building cable on the DC side, where ultraviolet exposure and temperature quickly degrade it.
- Reducing the protective device rating to make the coordination pass, leaving the circuit unable to carry its own design current.
Questions readers ask
- What size cable do I need for a solar string?
- Electrically, 4 mm² handles most residential strings comfortably after derating. The decision is usually driven by voltage drop over the run length rather than by ampacity, so size for drop first and then confirm the derated capacity.
- Why is solar cable rated so highly?
- EN 50618 H1Z2Z2-K is a cross-linked, halogen-free, double-insulated cable rated for 120°C conductor temperature, 90°C ambient and 25 years of ultraviolet exposure. That specification exists precisely because rooftop conditions destroy ordinary cable.
- Does grouping apply to DC strings?
- Yes. Six string circuits bundled in one tray heat each other exactly as AC circuits do. Space them or use a ventilated tray if the derating becomes binding.
- How hot does a roof cable actually get?
- Measurements on dark roofs in southern Europe regularly show 65 to 75°C in the cable tray in July, occasionally higher where cables sit against tiles in direct sun. Designing to 60°C ambient is a reasonable minimum in those climates.
- Can I use aluminium cable?
- On long AC feeders, yes, with terminations designed for aluminium and a scheduled torque check. On DC string circuits it is impractical: the connectors, crimps and small cross-sections are all built around copper.