MCB Selection for Solar PV and EV Charger Circuits
What MCB setup does a solar PV inverter or an EV charge point need? Both use a standard AC-rated MCB — typically C-curve, 16-40 A depending on inverter or charger output — wired to IEC 60898-1 and paired with an RCD or RCBO sized for the load's leakage behavior. Pick the wrong curve, undersize the breaker against the inverter's inrush, or fit a Type AC RCD where the EV charger injects DC leakage, and the circuit either nuisance-trips or stops clearing the fault it was installed to clear. This guide covers AC-side MCB sizing against inverter output current, why a standard AC MCB is forbidden on the PV DC array side, curve selection for inverter and EV inrush, RCD/RCBO type rules for vehicle chargers, and a comparison of typical PV and EV circuit protection.
AC-Side Protection for a Grid-Tied PV Inverter
A grid-tied inverter has two very different circuits either side of it, and only one of them looks like a normal electrical installation. The AC output — the run from the inverter to the distribution board or a dedicated AC isolator — carries a sinusoidal 50/60 Hz waveform at grid voltage, and it gets protected exactly like any other final circuit: a standard AC MCB to IEC 60898-1, sized to the inverter's rated output current, coordinated with the cable's current-carrying capacity, and backed by an RCD or RCBO for earth-fault protection. Nothing exotic about it. The complication comes from the other side of the inverter, which is not AC at all.
Most installers land on a dedicated MCB per inverter rather than sharing a way with other loads. That keeps fault-finding simple — trip the inverter breaker, and only the PV feed goes dark — and it means the breaker's rating only has to track one inverter's output current, not a diversified mix of loads.
Why a Standard AC MCB Must Never Sit on the PV DC Side
This is the point worth being blunt about: a standard AC-rated MCB must not be used to protect or switch the DC array side of a PV system. An AC MCB's breaking capacity rating (6 kA, 10 kA, and so on) is proven on a test circuit with an alternating waveform that crosses zero twice per cycle. That zero-crossing is what lets the arc inside the breaker self-extinguish. DC current has no zero-crossing. Once an arc forms across contacts that were never designed to interrupt it, it can sustain itself, generate intense localized heat, and in the worst documented rooftop PV fires, ignite the surrounding enclosure.
The DC array side needs devices rated and tested for DC interruption: DC-rated circuit breakers or fuses, DC-rated isolating switches, and — depending on the jurisdiction and array voltage — arc-fault detection at the string or combiner level. None of these are interchangeable with the AC MCB you fit on the inverter's output. If a job requires DC-side overcurrent or isolation devices, source parts explicitly rated for the array's DC voltage and fault current, not an AC breaker with a higher current or voltage number stamped on it.
Sizing the AC-Side MCB to Inverter Output Current
Once you're on the AC side, sizing follows the same cable-coordination logic used across low-voltage distribution: the Ib ≤ In ≤ Iz coordination rule. The inverter's rated AC output current sets the design current (Ib). The MCB's nominal rating (In) has to be equal to or above that, and the cable feeding the point of grid connection has to carry at least In continuously (Iz).
Formula: AC-Side MCB Rating for a PV Inverter — Source: IEC 60364-5-52, cable/device coordination clause
Ib ≤ In ≤ Iz
| Symbol | Description | Unit |
|---|---|---|
| Ib | Inverter's rated AC output current (the circuit's design current) | A |
| In | Nominal rating of the selected MCB | A |
| Iz | Continuous current-carrying capacity of the AC cable to the grid connection point | A |
A worked example: a 5 kW single-phase inverter at 230 V draws close to 21.7 A rated output current. Rounding up to the next standard commercial size gives a 25 A MCB, provided the cable run has an Iz of at least 25 A after any derating for containment method, ambient temperature, or grouping. Undersize the cable instead and add a bigger breaker to compensate, and the breaker stops protecting the cable — it will let through more current than the conductor can dissipate as heat before it trips.
Curve Selection for Inverter and EV Charger Inrush
Both circuit types see a short current pulse when they switch on, and that's what drives curve choice among the MCB tripping curves. An inverter's internal contactor closes onto the grid at connection, and any internal transformer stage draws a brief inrush; an EV charge point's contactor closes when the vehicle authorizes a charging session, and the charger's power electronics can add a short current spike on top of that. Neither is in the same league as a DOL motor start or a welding transformer, but it's enough that a B-curve (3-5x In) will occasionally nuisance-trip on inverter start-up or the first second of a charging session.
C-curve (5-10x In) is the default for both circuit types for that reason, and it's what most manufacturers spec in their installation manuals. D-curve has no real place here — it's reserved for genuinely high-inrush loads like large transformers or DOL motors, and fitting it to a PV or EV circuit just delays a genuine fault clearing. Some electricians default to C-curve on every circuit in the board out of habit; on a PV or EV circuit that habit happens to be correct, but it's worth knowing why rather than treating it as a rule of thumb. For a fuller walk-through of the trade-offs, see choosing the right tripping curve.
EV Charge Point Circuits: Dedicated Supply and RCD/RCBO Rules
An EV charge point gets its own dedicated circuit back to the board — not a shared spur, not an extension off an existing socket circuit. Typical single-phase installs run a 32 A C-curve MCB for a 7.4 kW charge point (7400 W / 230 V ≈ 32.2 A), while a three-phase 22 kW unit runs closer to 32 A per phase on a 400 V supply. Confirm the exact figure against the charge point's nameplate current rather than backing it out from the kW rating alone, since efficiency and power factor shift the number slightly.
The part that catches installers out is the RCD, not the MCB. Many onboard vehicle chargers rectify AC to DC internally, and a fault downstream of that rectifier can produce a smooth DC residual current rather than the AC residual current a standard Type AC RCD is designed to detect. A smooth DC component above roughly 6 mA can saturate a standard RCD's current transformer and blind it to a genuine AC earth fault occurring elsewhere on the same circuit. IEC 61851-1 addresses this directly: an EV charge point either needs an RCD Type A EV (or Type B) upstream, or it needs its own internal residual current monitoring device to IEC 62955 that disconnects the charger before its DC leakage can blind the upstream RCD.
Whether the installation uses an RCBO in place of a separate MCB and RCD is mostly a board-space and flexibility decision — the protection outcome is the same either way as long as the RCD portion is the correct type for the load.
PV AC-Side vs EV Charge Point: Side-by-Side Comparison
| Criteria | PV Inverter (AC side) | EV Charge Point |
|---|---|---|
| Typical MCB curve | C-curve | C-curve |
| Typical rating | 16-40 A (matches inverter output) | 16-32 A single-phase, ~32 A/phase three-phase |
| Standard | IEC 60898-1 | IEC 60898-1 (MCB) + IEC 61851-1 (charge point) |
| RCD/RCBO requirement | Standard Type A or AC RCD, per local wiring rules | Type A EV or Type B, unless charger has documented IEC 62955 DC monitoring |
| Circuit basis | Dedicated circuit per inverter | Dedicated circuit per charge point |
| DC-side devices | Separate DC-rated breakers/fuses/isolators on the array — never an AC MCB | Not applicable |
Both circuits sit on a standard miniature circuit breaker foundation, and the differences between them come down to the leakage current profile downstream, not the MCB itself.
Frequently Asked Questions
Can the same MCB be used on both the PV DC and AC sides?
No. A standard AC MCB is rated and tested for an alternating current waveform with a natural zero-crossing that helps extinguish the arc. DC current has no zero-crossing, so the arc can sustain and the breaker may fail to interrupt the fault. The DC array side needs DC-rated breakers, DC-rated disconnects, and fuses specified for DC arc voltage.
What size MCB does a 5 kW single-phase PV inverter need on the AC side?
A 5 kW inverter at 230 V draws roughly 21.7 A rated output current. The next standard MCB size up in most commercial ranges is 25 A, provided the AC cable's continuous current-carrying capacity (Iz) is equal to or greater than 25 A. Always confirm against the inverter's actual nameplate current, not just its kW rating.
Does an EV charge point need an RCBO or is a separate RCD and MCB enough?
Either works electrically — an RCBO combines both functions in one module and saves DIN-rail space, while a separate MCB plus RCD gives more flexibility if the RCD needs a different sensitivity or type than the rest of the board. What matters more is the RCD type: many EV chargers need Type A EV or Type B protection, not standard Type AC.
What tripping curve should an EV charging circuit use?
C-curve is the common choice. The charge point's internal contactor and any onboard transformer draw a brief inrush current when charging starts, similar in character to other contactor-switched loads, and C-curve's 5-10x In magnetic band tolerates that without nuisance tripping. B-curve is used only where the installer has verified the specific charge point has a low, well-documented inrush.
Do all EV chargers need a Type B RCD?
Not necessarily. Many EV charge points have an internal residual current monitoring device (per IEC 62955) that detects smooth DC leakage above roughly 6 mA and disconnects the charger before it can blind an upstream Type A RCD. Where that internal protection is confirmed and documented, Type A is acceptable; where it is not, IEC 61851-1 calls for Type B, or Type A plus an external 6 mA DC monitor.
Conclusion
The AC side of a PV inverter and an EV charge point circuit both reduce to a familiar problem: pick a C-curve AC MCB to IEC 60898-1 sized against the load's rated current and the cable's Iz, then get the RCD type right for what's downstream. The one rule that overrides everything else is the DC boundary — a standard AC MCB never goes on the PV array's DC side, no matter how the current or voltage rating looks on the label. For the broader sizing and curve logic behind these decisions, see the MCB engineering guide, and for the difference between RCD types referenced here, see MCB vs RCBO differences.
