AS/NZS Safety
RCD protection and cable sizing are separate requirements, but they interact in ways worth understanding.
RCD (Residual Current Device) protection and cable sizing are governed by separate clauses in AS/NZS 3000, and it's worth understanding how — and how little — they actually interact.
An RCD monitors for current imbalance between active and neutral conductors, tripping quickly when it detects leakage to earth — protecting against electric shock. This is a fundamentally different protective function from a circuit breaker's overcurrent protection, which is what cable ampacity sizing is designed around.
Adding RCD protection to a circuit doesn't reduce the cable sizing requirements — the conductor still needs to be sized for its full ampacity and voltage drop requirements independent of whatever shock protection sits upstream. RCDs and correctly sized cable are complementary, addressing different hazards, not substitutes for each other.
RCD selection itself (Type AC, A, F, or B) does depend on the type of load and any DC components it might introduce — solar inverters, in particular, require at minimum a Type A RCD due to potential DC leakage current, a genuine interaction worth understanding but distinct from cable ampacity sizing itself.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks