Why MCB Selection Matters
A miniature circuit breaker (MCB) is the first line of overcurrent and short-circuit protection in any low voltage distribution board. Pick the wrong rating and you either get nuisance tripping that frustrates the end-user, or worse — a breaker that fails to trip during a fault and exposes downstream equipment and people to risk.
Step 1 — Match the Rated Current (In)
The rated current is the continuous current the MCB will carry without tripping. It must be larger than the calculated load current but smaller than the cable ampacity behind it. Standard ratings on TPKELE AC MCBs include 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A and 63A.
A common rule for branch circuits: pick the next standard rating above the load, then verify the cable rated for that current at the install temperature.
Step 2 — Choose the Right Breaking Capacity (Icn)
Breaking capacity is the highest short-circuit current the MCB can interrupt safely. For residential and small commercial boards, 4.5 kA is usually adequate. Industrial boards close to a transformer often need 6 kA or 10 kA. Always check the prospective short-circuit current at the installation point before specifying.
Step 3 — Pick a Trip Curve (B, C, or D)
The trip curve defines how fast the magnetic element responds to fault currents above the rated value:
- B curve (3–5x In) — resistive loads, lighting circuits, residential.
- C curve (5–10x In) — general purpose with mixed inductive loads (most commercial).
- D curve (10–20x In) — high inrush loads such as motors, transformers, X-ray equipment.
Step 4 — Get the Pole Count Right
1P for single-phase line protection, 2P when you need to switch the neutral too, 3P for three-phase three-wire systems, and 4P for three-phase plus switched neutral. For solar DC strings, 2P MCBs disconnect both the positive and negative conductors — never use a single-pole AC MCB on a DC circuit.
AC vs DC: Don't Confuse Them
DC arcs are sustained — they do not extinguish naturally at zero crossings the way AC arcs do. A DC MCB uses arc-quenching chambers and magnetic blow-out designed for the energy in a PV string. Substituting an AC MCB on a DC circuit is one of the most dangerous mistakes in solar installation.
TPKELE DC MCBs cover up to 1500V DC for utility-scale solar DC protection.
Quick Selection Checklist
- Load current calculated and 1.25x safety factor applied.
- Cable ampacity ≥ MCB rating at install temperature.
- Short-circuit current at panel < MCB breaking capacity.
- Trip curve matches load type (B/C/D).
- Pole count matches system topology.
- AC or DC variant matches circuit type.
- Certifications (CE, IEC 60898-1) verified for tender.
Frequently Asked Questions
- Can I use an AC MCB on a DC solar circuit?
- No. AC MCBs are not designed to interrupt DC arcs and may fail to break a fault current safely. Always use a dedicated DC MCB rated for the system voltage.
- What breaking capacity is enough for residential boards?
- 4.5 kA is generally sufficient for residential distribution boards far from the transformer. Verify the prospective short-circuit current at your specific installation point.
- Do I need C-curve or D-curve for a motor circuit?
- D-curve handles the higher inrush of motors, transformers and welding equipment without nuisance tripping. C-curve is acceptable for small motors with soft-start drives.