By Alvin Wong, CEO of Innovative Green Power Sdn. Bhd.
For home EV charger installations in Malaysia, one of the most common technical misunderstandings involves MCB (Miniature Circuit Breaker) selection.
A frequent assumption is:
“A 7 kW charger draws about 32 A, so a 40 A MCB is more than enough.”
From a standards and regulatory perspective, this assumption is incomplete and potentially unsafe.
This article explains why EV charging circuits are treated differently, how MCBs behave under continuous load, and how international standards—adopted in Malaysia—define correct practice.
1. MCBs Are Protection Devices, Not Power Ratings
An MCB is designed to:
– Protect conductors against overcurrent
– Disconnect supply under overload or short-circuit
– Operate within defined thermal and magnetic characteristics
Under IEC 60898-1 (MCBs for household and similar installations), an MCB’s rating:
– Is not a guarantee of indefinite operation at that current
– Is defined under reference conditions
– Assumes diversity and intermittent loading
This distinction becomes critical for EV charging.
2. EV Chargers Are Continuous Loads by Definition
As established in Week 1 and reinforced by IEC 61851-1, EV chargers are designed to:
– Operate at near-rated current
– For extended periods (several hours)
– With minimal load cycling
Under MS IEC 60364, this classifies EV charging as a continuous load.
Why this matters
Continuous loads place sustained thermal stress on:
– Conductors
– Terminations
– Protective devices
Standards require these components to be selected with additional margin, not just nominal current matching.
3. How IEC Standards Define MCB Behaviour Under Load
Under IEC 60898-1, MCBs are tested and characterised using time–current curves, which show:
– How long an MCB can carry a given current before tripping
– How temperature affects tripping behaviour
– That prolonged operation near rated current causes thermal buildup
In simple terms:
– An MCB can carry its rated current
– But doing so continuously brings it close to its thermal limit
Standards therefore do not assume continuous full-load operation unless specifically designed for it.
4. Why Continuous Loads Require Derating Consideration
IEC 60364 requires that protective devices be coordinated so that:
– Conductors are protected under all operating conditions
– Thermal limits are not exceeded during normal use
– Long-term degradation is avoided
For continuous loads:
– Operating too close to the MCB’s thermal limit reduces reliability
– Repeated heating accelerates contact wear and calibration drift
– Nuisance tripping becomes more likely over time
This is why EV charging circuits are treated differently from:
– Kettles
– Ovens
– Air-conditioners
which cycle on and off.
5. Why “40 A for 32 A” Is Often Misused
A 32 A EV charger running for several hours can keep a 40 A MCB at an elevated temperature for extended periods.
Under real Malaysian installation conditions:
– MCBs are installed in enclosed distribution boards
– Ambient temperatures are higher
– Other circuits contribute heat
When these factors are combined, a 40 A MCB may:
– Operate continuously near its thermal threshold
– Experience calibration drift
– Trip unpredictably after months or years
This does not violate the MCB’s datasheet — but it violates the intent of the installation standards.
6. Coordination Between Cable Size and MCB Rating
Under IEC 60364, protective devices must be coordinated with conductor size so that:
– The MCB protects the cable
– The cable is not operated at its thermal limit continuously
– Fault disconnection occurs before insulation damage
This is why, in compliant EV charger installations:
– Cable sizing (e.g. 6 mm²)
– MCB rating
– Continuous load characteristics
are considered together, not independently.
An undersized or marginal MCB can undermine an otherwise correctly sized cable.
7. Malaysian Regulatory Intent on Protection Devices
While Suruhanjaya Tenaga guidelines do not publish specific MCB ratings for EV chargers, they require that:
– Installations comply with MS IEC 60364
– Protection devices are suitable for the load characteristics
– Electrical systems remain safe over their service life
This places responsibility on the competent person to apply the standards correctly, rather than selecting devices purely by arithmetic current matching.
8. What Good Practice Looks Like for EV Charging Circuits
Good standards-aligned practice for EV charging circuits involves:
– Recognising EV charging as a continuous load
– Selecting MCBs with adequate thermal margin
– Ensuring proper coordination with cable size and installation method
– Avoiding designs that rely on operation at protection limits
The objective is not to oversize indiscriminately, but to respect how protective devices actually behave over time.
9. How Readers Can Verify This Themselves
You can independently refer to:
– IEC 60898-1 – MCB operating characteristics
– MS IEC 60364 – Protection coordination and continuous load treatment
– IEC 61851-1 – EV charging operational behaviour
– Suruhanjaya Tenaga – Guidelines for Electrical Wiring in Residential Buildings
These documents form the technical basis for compliant EV charger installations in Malaysia.
Final Takeaway
The reason EV charging guidelines treat MCB selection differently is simple:
EV chargers impose sustained electrical stress, and protective devices must be selected for long-term safety—not short-term operation.
A circuit that “works today” is not necessarily compliant with the intent of electrical standards.
Correct MCB selection ensures that:
– Cables remain protected
– Protection devices remain reliable
– EV charging does not quietly degrade the home electrical system
Safe and Reliable EV Charging Systems, one at a time.
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