Safety Derating
A cable that runs hot but doesn’t trip a breaker isn’t necessarily fine — sustained heat degrades insulation gradually over years.
Not every consequence of cable undersizing is immediate. A cable operating at the edge of its real (derated) capacity, without ever tripping a protective device, can still be experiencing a slower, cumulative form of damage: gradual insulation degradation from sustained elevated temperature.
Cable insulation has a rated maximum operating temperature, but its actual service life is also affected by how consistently it operates near that limit over time — insulation materials generally follow a pattern where sustained operation closer to their thermal limit shortens expected service life, even without any single catastrophic overheating event.
A marginally undersized cable that never trips its breaker and never becomes hot enough to notice by touch can appear completely fine for years — the actual consequence (reduced insulation life, increased long-term failure risk) doesn't announce itself the way an immediate overload trip would.
Circuits that run near-continuous, near-full-load operation for years at a stretch — commercial refrigeration, continuous industrial processes, solar PV output circuits during peak production seasons — are exactly where this slow degradation mechanism has the most time and opportunity to actually matter.
Genuine derating margin, correctly applied at design time, is the actual protection against this slow-failure mode — not just avoiding an immediate breaker trip, but avoiding sustained operation near the conductor's real thermal limit at all.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks