Motor Protection Essentials: MCA, MOP & How to Size Them

Every HVAC unit, pump, and compressor on a building has a nameplate. On that nameplate, two critical values determine how you design the electrical circuit: MCA and MOP. Get them wrong, and you're either tripping breakers on startup or leaving equipment unprotected.

What is MCA (Minimum Circuit Ampacity)?

MCA stands for Minimum Circuit Ampacity. It defines the minimum current-carrying capacity that the wire and other circuit components must have. The wire you select must have an ampacity equal to or greater than the MCA value on the equipment nameplate.

How MCA is Calculated

For a single motor, the Canadian Electrical Code (CEC) Rule 28-106 defines:

MCA = 125% × FLC (Full Load Current) of the largest motor + 100% × FLC of all other motors + 100% of any heater loads

The 125% multiplier accounts for the continuous duty rating of motor circuits — motors are expected to run at full load for extended periods, and the wire must handle this thermal load safely.

Practical Example

Consider a rooftop unit (RTU) nameplate showing:

• Compressor: 28A FLC
• Supply fan motor: 8A FLC
• Electric heater: 15A

MCA = (125% × 28) + 8 + 15 = 35 + 8 + 15 = 58A

You must select a wire with ampacity ≥ 58A. Per CEC Table 2, this means #6 AWG copper (65A) in typical conditions.

What is MOP (Maximum Overcurrent Protection)?

MOP stands for Maximum Overcurrent Protection. It defines the maximum size of the circuit breaker or fuse protecting the circuit. You must select a breaker equal to or less than the MOP value.

How MOP is Calculated

Per CEC Rule 28-200, for motor circuits:

MOP = Maximum percentage of FLC (per CEC Table 29) for the largest motor + 100% × FLC of all other motors + 100% of heater loads

The maximum percentage depends on the protective device type and motor type:

Protection Device	Squirrel Cage Motor	Wound Rotor Motor
Non-time-delay fuse	300%	150%
Time-delay fuse	175%	150%
Inverse-time circuit breaker	250%	150%
Instantaneous-trip breaker	800%	800%

Continuing Our Example

Using an inverse-time circuit breaker (most common):

MOP = (250% × 28) + 8 + 15 = 70 + 8 + 15 = 93A

Round down to the next standard breaker size: 90A breaker.

Critical Rule: If the MCA requires #6 AWG wire (65A ampacity) but the MOP allows up to a 90A breaker, this is normal and correct. Motor circuits are the exception to the general rule that the breaker must not exceed wire ampacity. The motor's internal overload relay provides the running overcurrent protection.

Common Motor Protection Selection Table

The following table provides quick reference values for standard three-phase motors at 208V and 460V, using inverse-time circuit breakers:

Motor HP	FLC @ 208V	MCA @ 208V	Wire Size	MOP (Breaker)	FLC @ 460V	MCA @ 460V
1	4.6A	5.75A	#14	15A	2.1A	2.6A
2	6.8A	8.5A	#14	20A	3.0A	3.75A
3	9.6A	12A	#14	25A	4.2A	5.25A
5	16.7A	20.9A	#10	45A	7.6A	9.5A
7.5	24.2A	30.3A	#8	70A	11A	13.75A
10	30.8A	38.5A	#8	80A	14A	17.5A
15	46.2A	57.8A	#6	120A	21A	26.25A
20	59.4A	74.3A	#4	150A	27A	33.75A
25	74.8A	93.5A	#3	200A	34A	42.5A
30	88A	110A	#1	225A	40A	50A
40	114A	142.5A	#1/0	300A	52A	65A
50	143A	178.8A	#3/0	400A	65A	81.25A

Note: Values are approximate and based on CEC Table 44/45 FLC values for standard NEMA B design motors. Always verify with the actual equipment nameplate and local code edition.

The Role of the Overload Relay

The circuit breaker (MOP) protects against short circuits and ground faults. It does not protect the motor from overloads. That job belongs to the overload relay, which is typically:

• Built into the motor starter (contactor + overload)
• Set to 115–125% of the motor FLC
• Trips on sustained overcurrent, protecting against locked rotor and overload conditions

VFD Considerations

When a motor is fed through a Variable Frequency Drive (VFD), the circuit design changes:

• MCA/MOP are based on the VFD input current, not the motor FLC
• VFD provides electronic motor overload protection internally
• Requires VFD-rated cable on the output side to handle high-frequency switching
• Line reactor or input filter may be required to limit harmonic distortion