300A DC Circuit Breaker MCCB for DIY Battery Boxes | 2-Pole Battery Isolator, EU Stock

A 300A DC circuit breaker does one job your BMS cannot do on its own: it gives you a physical break in the circuit. Your BMS protects the cells from over-current and short circuits, but when you need to work on the pack, isolate the inverter, or cut the power in an emergency, you need something you can actually switch off. This 2-pole DC breaker sits between your battery and your inverter, adds short-circuit protection on top of the BMS, and gives you a visible disconnect point.

Why a 300A DC Circuit Breaker Beats Relying on the BMS Alone

A BMS is a protection device, not a switch. It will cut the current when something goes wrong, but it will not let you isolate the pack on demand — and it will not clear a hard short circuit on the main DC feed the way a breaker does. Adding a breaker gives you three things a BMS alone cannot:

• A manual disconnect for maintenance, upgrades and emergencies
• Short-circuit clearing rated at 7.5kA
• A fault you can see — a tripped breaker shows you something happened, instead of leaving you guessing

AC breakers do not belong on a battery circuit

This is the mistake that matters most. Alternating current passes through zero 50-60 times a second, and that zero crossing is what lets an AC breaker extinguish the arc when it trips. Direct current has no zero crossing, so the arc keeps burning. An AC breaker can fail to clear a fault on a battery system, which is exactly the situation a breaker is supposed to solve.

Choosing a 300A DC Circuit Breaker for High-Output Systems

300A is the top of this series. Choose it when your system genuinely runs at that level:

• You run a 300A BMS
• You have an inverter drawing close to the BMS limit on a continuous basis
• You are running more than one inverter, or plan to add one
• Your load profile is high and sustained rather than short bursts
• You want no margin compromise — the breaker stays above everything downstream

One warning that matters more than the table: at 300A the cable becomes the bottleneck. A 300A breaker does not make an undersized cable safe — size the cable for the current first, and make sure your terminals and busbars are rated for it too.

Reading the Label: DC Rating and Breaking Capacity

Fitting a DC Circuit Breaker Into a DIY Battery Box

At 78 × 38 × 63mm and 225g, this breaker fits inside a battery enclosure or a tight cabinet instead of needing its own panel.

1. Mount it where you can reach it. A breaker you cannot access is decoration.
2. Follow the LINE and LOAD markings, and confirm the wiring diagram in the manual before connecting.
3. Torque the terminals to spec and re-check after the first thermal cycle.
4. Keep AC and DC separate. This device goes on the DC side between battery and inverter; the AC output side needs its own AC breakers.
5. Check heat at full load. High-current builds warm up; leave airflow around the breaker and check terminal temperature after the first hour at full load.

Specifications

Not included: the battery box, the BMS and the battery cells. This listing is the circuit breaker only.

FAQ

Which systems need a 300A DC circuit breaker?
300A BMS builds, systems with inverter loads running close to the BMS limit, and installations with more than one inverter.

Can I use a smaller breaker to save money?
No. A breaker rated below your system’s continuous current will nuisance-trip. Size at or just above your continuous current.

Can I use an AC breaker on a battery circuit instead?
No. DC arcs do not self-extinguish, so an AC breaker may fail to clear a fault. This device is DC-rated at 125/250V DC.

How much short-circuit current can it clear?
7.5kA (Icu/Ics).

Will it fit inside my battery box?
At 78 × 38 × 63mm and 225g it fits most DIY enclosures. Send us the mounting space if you are unsure.

Do you make other ratings?
Yes — 220A (BCPS2DM220) and 250A (BCPS2DM250) in the same series.

Does it come with the box, BMS or cells?
No. This listing is the breaker only.

Related: Battery Box Parts and Accessories

Building a complete pack? Pair the breaker with a DIY battery box kit, a battery box base, a set of battery box wheels, and a JK BMS heating pad if you charge in cold weather. Cells in 280Ah, 314Ah and 320Ah are available from the same warehouse.