Circuit Breaker Size Calculator

Breaker sizing sits at the intersection of safety, continuity of operations and contracted supply capacity. An MCB or MCCB undersized for the load will trip on legitimate demand; oversized, it will not protect the cable downstream when something does go wrong. The right answer flows from a measured load profile, a type rating that matches the equipment characteristics, and a discrimination scheme that isolates faults without taking the whole board down.

Distribution board upgrades and capacity uplifts are common triggers for breaker reviews, particularly on sites adding HV-to-LV transformers, additional EV charging, or new process equipment as part of a phased growth plan. Each new feed adds load and adds the question of whether the existing protection coordinates correctly with the upstream device. Done well, the review surfaces opportunities to tighten capacity bands and reduce Availability Charges as a side effect, often paying back the design fees on the bill alone.

These figures are indicative; final selection should be made by a competent electrical designer against BS 7671 and the manufacturer's curves. Purely Energy's part of the conversation is contractual: when a capacity uplift is on the table, we run the DNO process end-to-end, model the new MIC against tariff and standing charges, and make sure the engineering capex earns back the right way through the bill rather than landing as an unexplained step-change in monthly spend.