Safety Cable Sizing

Why Circuit Breaker Coordination Depends on Correct Cable Sizing

A breaker only protects a cable correctly if the two were sized together, not independently.

A circuit breaker's job includes protecting the cable it feeds from sustained overcurrent — but that protection only works correctly if the breaker and cable were sized as a coordinated pair, not selected independently of each other.

The Coordination Principle

The breaker's trip rating needs to sit at or below the cable's actual derated ampacity — a breaker rated higher than what the cable can safely carry allows sustained overcurrent to flow through the cable without tripping, defeating the breaker's protective purpose for that specific cable.

Where This Coordination Breaks Down in Practice

• A cable is upsized or downsized after the original design without revisiting the breaker rating to match
• A breaker is swapped during a later modification (perhaps to address nuisance tripping) without checking that the original cable is still adequately protected at the new rating
• Derating factors change the cable's real capacity after installation (added conduit fill, a hotter environment than originally assumed) without the breaker rating being reconsidered

The Practical Rule

Treat cable size and breaker rating as a coordinated pair whenever either one changes — a correctly rated breaker paired with an incorrectly sized cable (or vice versa) still leaves a real protection gap, even though each component looks individually fine on its own spec sheet.

Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks

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