Moulded Case Circuit Breaker (MCCB) vs Thermal Overload Relay: Which One Does Your Manufacturing Plant Need? It's a question that comes up on almost every panel build we quote for UK manufacturing sites: do you need an MCCB, a thermal overload relay, or both? The short answer is that they usually protect against different things and end up working together in the same motor circuit — but knowing which one is doing which job (and which one you're actually short of) saves a wasted order and a second trip to the supplier. This guide breaks down what each device does, how they compare, and which one your plant needs for a given circuit. Not sure which one your circuit needs? Talk to Our Expert Team ZF Corporation Supplies ABB & Allen Bradley MCCBs and Thermal Overload Relays — In Stock for UK Motor Circuits What Is a Moulded Case Circuit Breaker (MCCB)? An MCCB is a higher-current circuit breaker, typically rated from around 63A up to several thousand amps, housed in a moulded plastic case. Like an MCB it combines thermal and magnetic tripping to protect a circuit from overload and short-circuit current, but it's built for distribution boards, motor control centers and main incomers where an MCB's frame size simply isn't rated for the current involved. Many MCCBs also allow the trip settings to be adjusted, which a fixed MCB does not. What Is a Thermal Overload Relay? A thermal overload relay is a different kind of device entirely — it doesn't protect the cable or the circuit from a short-circuit fault, it protects the motor from sustained overload. Mounted directly onto a contactor, it monitors current on all three phases using bimetallic strips and trips the contactor if the motor draws more current than its full-load rating for too long — the classic cause being a stalled or overloaded motor, or a lost phase. Quick Tip These two aren't rivals — most motor starter panels use both: an MCCB (or MCB) upstream for fault protection, and a thermal overload relay on the contactor for motor overload protection. Head-to-Head Comparison Factor MCCB Thermal Overload Relay Protects against Short-circuit & general overload on the circuit Sustained motor overload & phase loss Typical current range 63A to several thousand amps Sized to motor FLC, typically under 100A Mounting Standalone, DIN rail or panel-mounted Direct-mounted onto a contactor Trip settings Fixed or adjustable depending on frame Adjustable dial matched to motor FLC Typical location Distribution board, MCC incomer Motor starter, direct-on-line or star-delta panel Which Should You Choose for Your Manufacturing Plant? If you're protecting a distribution board, sub-main or an incomer feeding multiple circuits, you need an MCCB. If you're protecting an individual motor from overload or single-phasing on a starter panel, you need a thermal overload relay sized to that motor's full-load current. Most motor circuits on a manufacturing line will actually use both, in series — the MCCB (or MCB) handling fault protection upstream, and the overload relay handling motor protection at the contactor. Most Common Mistake Assuming the MCCB upstream will protect the motor from overload. It won't — an MCCB is sized for the whole circuit, not the individual motor's full-load current, so a stalled motor can overheat and fail long before the MCCB ever trips. Selection Checklist Identify whether you're protecting a distribution circuit (MCCB) or an individual motor (thermal overload relay). Size the overload relay to the motor's actual full-load current, not the contactor rating. Confirm MCCB breaking capacity against prospective fault current at the board. Check that the overload relay physically mounts to the contactor frame you're using. Brands We Supply Both product lines are genuinely on the shelf across two manufacturers, which is useful when one part is on back-order: Brand What We Stock Brand Page ABB Full SACE Tmax MCCB range plus T16/TF42 thermal overload relays View ABB → Allen Bradley E100 and Bulletin series thermal overload relays for contactor mounting View Allen Bradley → Related Categories Category What's Inside Category Page Molded Case Circuit Breakers 249 MCCBs from 63A upward View Category → Thermal Overload Relays 118 relays across ABB and Allen Bradley View Category → Contactors 626 contactors to mount the overload relay onto View Category → Motor Protection Circuit Breakers Combined breaker + overload protection in one device View Category → Miniature Circuit Breakers Lower-current alternative for smaller circuits View Category → These pair naturally with our MCB Buying Guide for smaller circuits, and with our Electrical Enclosures Guide for housing the finished panel. Frequently Asked Questions Can a thermal overload relay replace an MCCB? No. They protect against different faults — an MCCB clears short-circuit and general overload on the whole circuit, while a thermal overload relay only protects the individual motor from sustained overload. Do I need …