Short answer: the AC breaker between a solar inverter and the grid is sized on the inverter’s maximum continuous AC output current, not on the array’s DC rating, and it is chosen for the fault level at the board it feeds into — not at the inverter. In practice that means: take the inverter’s stated maximum AC current, apply the code multiplier used at the destination (the North American rule is 125 % of that current), check the derated current of the chosen device at panel ambient, verify its breaking capacity against the prospective fault current at its own terminals, and pick a magnetic trip that will not fire on the inverter’s start-up. The breaker does not do anti-islanding — that is the inverter’s job.
We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — so the ratings below are quoted per exact ordering code.
Where this breaker sits, and what it is for
A grid-tied string inverter has five things on its AC side, and buyers routinely collapse them into one: the inverter’s internal output protection, an AC isolator within sight of the inverter, the AC overcurrent device, the residual-current device, and the point of connection at the main board or a dedicated sub-board. Each exists for a different reason.
| Item | What it actually does | What it must be rated for |
|---|---|---|
| Inverter internal protection | Detects islanding, over/under voltage, over/under frequency, DC injection; shuts the output down | The grid code at the destination — the external breaker has no role here |
| AC isolator | Gives the service technician a local, lockable break so the inverter can be worked on | Load-break duty at the inverter’s maximum AC current |
| AC overcurrent breaker | Protects the AC cable and the connection against overload and short circuit, and clears a fault in the inverter’s output stage | Cable ampacity, prospective fault current at its terminals, ambient derating |
| Residual-current device | Protects people, and on some grid codes provides the required earth-fault detection path | Type and sensitivity per the local code; Type A is common, Type B where DC fault current is possible |
| Connection point | Sets the maximum current the installation may export to the busbar | The busbar backfeed allowance in the local wiring code |
The sizing sequence that works
One — start from the inverter datasheet, not the array. The number you need is the maximum continuous AC output current at the lowest grid voltage the inverter supports. A 10 kW inverter at 230 V does not deliver the same current as the same inverter at 200 V, and the datasheet’s headline rating is usually the optimistic end.
Two — apply the multiplier your code requires. North American practice sizes the overcurrent device and the AC disconnect at 125 % of the maximum continuous output current (NEC Article 690 and 705). IEC-based practice usually asks for a device rating at or above the inverter’s maximum output current with the local derating factors applied. Two countries, two answers — send the destination, not just the current.
Three — derate for the enclosure. A breaker rated 63 A at 30 °C is not a 63 A breaker inside a sealed outdoor cabinet at 50 °C. The derating curve in the datasheet applies, and the IEC 60898-1 range and the IEC 60947-2 range derate differently because they are rated under different conditions.
Four — check the breaking capacity at the right point. The prospective fault current at the inverter terminals is low; at the main board it can be many kiloamps. The breaker is installed at the board end, so that is the number it must survive. An MCB at 6 kA is fine on a suburban feeder and wrong on an industrial busbar.
Five — confirm the connection allowance. If the inverter feeds a busbar that already carries a load, the busbar’s backfeed allowance caps how much PV current may be added. The breaker size may be legal at the cable and still illegal at the busbar.
Which device class for which current
| AC side current | Usual device | Why |
|---|---|---|
| Small residential, up to the 63 A modular range | Modular MCB — CHINT NXB-63 (Icn 6 kA, H version 10 kA) or NB1-63 / NB1-63H (Icn 6 kA / 10 kA), 1P to 4P | Fits the same DIN rail as the rest of the board, cheap, wide availability of accessories |
| Commercial three-phase, above the modular range | Moulded-case breaker — CHINT NM1 or NXM, adjustable thermal setting | Higher ratings, adjustable Ir, and a breaking capacity that matches an industrial board |
| Where discrimination with the main incomer matters | MCCB with adjustable Ir and a published discrimination table | An MCB cannot be tuned to sit under the incomer |
| DC side of the same installation | CHINT NB1-63DC — DC 250 V on one pole, 500 V on two, 1000 V on four, Icu 6 kA | DC has no zero crossing; an AC breaker must not be used on the string |
| Surge protection at the AC entry | CHINT NU6-I+II or NU6-II | Class I+II where a lightning protection system or overhead line exists, Class II otherwise |
| Export metering | CHINT DTSU666-CT (MID B+D) or DDSU666 | Bidirectional metering with RS485 Modbus-RTU |
Trip curve: the nuisance-trip decision
Inverters are not motors, but their output stage is not a clean resistive load either. A C curve is the usual default because it tolerates the short inrush of the output filter and the switching transients without tripping on start-up. A B curve is tempting for cable protection on long runs but will trip on the inverter’s own start-up current on many installations — that is one of the most common “the breaker is faulty” call-outs, and the breaker is fine.
A D curve is for loads with genuinely high inrush; on a PV AC circuit it usually buys nothing except slower fault clearance. The better tool is an MCCB with an adjustable magnetic setting, because it lets you sit above the start-up transient and still clear a real fault quickly.
Three-phase, neutral and the pole count
Single-phase inverters on a 230 V supply take a 1P+N or 2P device; three-phase units take 3P or 4P depending on whether the neutral is switched. Where the inverter is three-phase four-wire, a 4P device lets the neutral be broken for service. Where the inverter is three-phase three-wire, a 3P device is correct and a fourth pole is dead copper.
On 110–127 V / 60 Hz markets the same logic holds with different numbers: the current is higher for the same kW, the modular range fills up faster, and the breaker must be rated for the actual supply voltage rather than a 230 V assumption. At 127 V the current rating becomes the limit, not the voltage.
What the AC breaker must not be expected to do
It does not stop the inverter exporting during a grid outage — that is the inverter’s anti-islanding function, tested to the grid code. It does not isolate the DC side; the DC isolator and the string breakers do that. It does not provide rapid shutdown at the array. And it does not replace the meter: export and import measurement is a separate device, and on many grid codes a bidirectional meter is mandatory before commissioning.
What to send us for a quotation
Inverter model and its maximum continuous AC output current · grid voltage, frequency, single or three phase · the board it connects to and the prospective fault current there · cable size and length · ambient inside the enclosure · whether the destination requires a 125 % multiplier · required breaking capacity and certificate · quantity. You get unit price, packing data and the certificate document for the exact ordering code. Request a quotation.
FAQ
Q: Do I size the AC breaker on the inverter’s kW rating?
A: No. Use the maximum continuous AC output current from the datasheet, at the lowest grid voltage the inverter supports. kW divided by nominal voltage understates the current on a weak grid.
Q: Is a 6 kA MCB enough on the AC side of a solar installation?
A: It depends entirely on the prospective fault current at the board the inverter feeds. A 6 kA device is normal on a domestic feeder and inadequate next to a transformer. Measure or ask the utility; do not assume.
Q: Why does my inverter trip the AC breaker at start-up?
A: Almost always the magnetic trip, not the thermal. A B-curve breaker or a breaker sized too close to the inverter’s maximum current will fire on the start-up transient. Move to a C curve, or to an MCCB with an adjustable magnetic setting.
Q: Do I need a Type B RCD on the inverter output?
A: Only where the local code requires it — typically where smooth DC fault current can reach the device. Many transformerless inverters already include an internal Type A detection path, which changes the answer. Check the grid code at the destination and the inverter’s declaration.
Q: Can I use the same breaker type on the DC and AC sides?
A: No. DC arcs do not self-extinguish at a zero crossing, and the DC voltage rating depends on how many poles are in series. Use a DC-rated device such as the NB1-63DC on the string side.
Related: MCB range · MCCB range · DC MCB for solar, 250–1000 V · PV combiner box components · DC SPD for solar PV · 6 kA vs 10 kA breaking capacity · MCB trip curve B vs C vs D · Voltage and frequency by country