Cable current capacity & derating
Why the same cable can safely carry less current in some installations than others — it all comes down to heat.
It's a heat limit
A cable's current rating isn't about how much current it can pass — it's about how much it can pass without its insulation getting too hot. Current through the conductor's resistance produces heat; the cable can only carry the current at which that heat balances with how fast it escapes to the surroundings.
So the same cable has different safe ratings depending on how easily it loses heat. Anything that traps heat reduces what it can carry; anything that helps it escape increases it.
What derates a cable
The big factors are how the cable is installed (clipped in free air loses heat well; buried in insulation barely at all), how many other loaded cables are bunched with it (each adds heat to its neighbours), and the ambient temperature it sits in (a hot plant room or roof space leaves less headroom).
Each of these applies a derating factor that cuts the cable's capacity. Stack a few together — grouped cables in a hot space surrounded by thermal insulation — and a cable's real-world rating can be a fraction of its free-air figure. Sizing on the headline rating alone is how cables end up running hot.
Voltage drop too
Heat sets the minimum size; on long runs, voltage drop often forces a bigger one still. A cable can be thermally fine yet leave too little voltage at the load, so both are checked and the larger size wins.
Safety first
An overloaded or over-derated cable runs hot and degrades its insulation over time — a fire risk that can be invisible until it fails. Size for the real installation conditions, not the headline rating.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
Voltage drop on a long cable run
Equipment at the end of a long run misbehaves — dim lights, a contactor that won't hold, a motor struggling — because volt-drop along the cable leaves too little voltage at the load.
Related definitions
Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.