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MCB Selector
An MCB protects the cable, not the appliance, so its rating must never exceed what the wire can safely carry. Choose the circuit type and cable size and this will give you a rating and a curve, and warn you if the combination is unsafe.
Frequently asked questions
What MCB rating do I need for a geyser?
A 2000W storage geyser on 2.5 sq mm cable takes a 16A Curve C MCB, and a 3000W unit on 4.0 sq mm takes 20A. An instant geyser around 4500W needs 4.0 sq mm with a 25A breaker. On all of them an RCBO is preferable to a plain MCB, because a geyser element is the highest shock-risk item in a house.
What is the difference between MCB Curve B and Curve C?
Curve B trips faster and suits resistive loads such as lights, heaters and general sockets. Curve C tolerates the brief inrush current drawn by motors and compressors, which is why air conditioners, pumps and refrigerators belong on Curve C. Nuisance tripping when a motor starts is usually a curve problem.
Can I fit a bigger MCB to stop nuisance tripping?
No. The MCB protects the cable, so its rating must not exceed what the wire can carry. Fitting a larger breaker means the cable will overheat before the breaker reacts. If a circuit trips repeatedly, the correct fix is a larger cable or a separate dedicated circuit.
What MCB does an air conditioner need?
A 1.5 ton unit on 4.0 sq mm cable takes a 20A Curve C MCB, and a 2 ton unit takes 20A or 25A depending on the rated input. Each air conditioner should be on its own dedicated circuit from the distribution board rather than sharing a room socket circuit.
Do I need an RCCB as well as MCBs?
Yes. MCBs protect the wiring against overload and short circuit; an RCCB protects people by tripping on earth leakage. A 40A or 63A, 30mA RCCB is standard for a home, and splitting the house across two RCCBs makes faults easier to isolate.
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