FAQ What is minimum circuit ampacity?

Minimum circuit ampacity (MCA) is a calculated value that specifies the smallest current-carrying capacity, in amps, that the wiring supplying a piece of equipment must have. It is most commonly seen on the nameplate of motor-driven or HVAC equipment, such as air conditioning units, heat pumps, and compressors, and tells the installer the minimum size of conductor needed to safely carry the equipment's full operating load without exceeding the operating temperature of the cable's insulation. 

MCA is not the same as the equipment's normal running current. It is calculated by taking the full load current of the largest motor in the circuit, adding a safety margin (typically 125%), and adding the current of any other loads on the same circuit, such as heaters. This ensures the current carrying capacity of the wiring is sufficient under all expected operating conditions, including motor start-up and ageing of components, and not just during steady-state running. 

MCA is typically referenced alongside Maximum Overcurrent Protection (MOCP), which sets the largest breaker or fuse size permitted to protect the same circuit. Where MCA determines the minimum wire size, MOCP determines the maximum protective device size — the two values work together to size a circuit correctly. 

It is worth noting that MCA is a term defined under the US National Electrical Code (NEC 440.32) and is most often encountered on equipment manufactured to US standards. For cable selection in the UK, the equivalent process is carried out in accordance with BS 7671, using the equipment's design current alongside the current carrying capacity tables for the relevant cable type and installation method, rather than a single nameplate MCA figure.