Lena looked at the old 25mm² cable, then at her clipboard. “But the spec sheet says this cable is rated for 100 amps. The press only draws 85 at full load. We were within the number.”
Marco did the math in his head. “Grouping factor for twelve cables? 0.5. Temperature correction for 45°C? About 0.8. Multiply those. 100 amps times 0.5 times 0.8 is… 40 amps. You were running 85. You weren’t ‘within the number.’ You were running more than double what that cable could handle. It didn’t trip the breaker because the breaker is also hot, and its own calibration drifted. But the cable? It cooked.” cable size current carrying capacity
Standard charts usually assume an ambient air temperature of $30^\circ C$. If you are installing cables in a boiler room or a hot climate where the temperature is $40^\circ C$, the cable starts "hotter," meaning it has less headroom before it overheats. This requires a correction factor, lowering the allowable current. Lena looked at the old 25mm² cable, then at her clipboard
Cables are rated based on a standard environment (usually 30°C). If a cable is installed in a hot attic or near a boiler, it cannot dissipate heat as effectively. In these cases, you must use a larger cable to compensate for the higher ambient temperature. B. Installation Method How a cable is "housed" affects its cooling: Best cooling; highest capacity. In Conduit/Trunking: Restricted airflow; lower capacity. We were within the number
“Current-carrying capacity isn’t just about the copper,” Marco said. “It’s about getting rid of the heat the copper makes. Resistance creates heat. Every electron squeezing through that wire is like a runner in a tunnel. The more runners, the more heat. The insulation can only take so much before it gives up—usually 70, 90, or 105 degrees Celsius, depending on the type.”
This guide breaks down the relationship between cable size and current carrying capacity (ampacity) and the factors that influence them. 1. What is Current Carrying Capacity (Ampacity)?
Heat needs to escape. A cable clipped directly to a wall or run through open air can dissipate heat easily, allowing for maximum current. However, if that same cable is buried in thermal insulation (like in a modern home wall) or run inside a conduit with other hot cables, the heat is trapped. The cable cannot carry as much current without overheating, requiring a larger size for the same load.