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

Under the Canadian Electrical Code (CEC), Rule 28-106 sets 125% of FLC for a single motor and Rule 28-108 covers several motors; nameplate MCA follows the same logic:

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.

The same nameplate values set the circuit for a residential heat pump's outdoor unit; the heat pump sizing guide shows where they fit in the selection.

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 Rules 28-200 and 28-204 (Table 29), 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:

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.

From the nameplate to the wire and the breaker

The rooftop unit of this article’s worked example, not a specific product. Not to scale.

Detail A: unit nameplate

Fields from the worked example; no brand

Parts

1. 1

   Nameplate: compressor 28 A FLC, supply fan 8 A FLC, electric heater 15 A. Detail A

2. 2

   MCA = 125 % × 28 + 8 + 15 = 35 + 8 + 15 = 58 A. Rules 28-106, 28-108

3. 3

   Conductors: ampacity of at least the MCA: #6 AWG copper (65 A) in typical conditions. Table 2

4. 4

   MOP = 250 % × 28 + 8 + 15 = 70 + 8 + 15 = 93 A for an inverse-time breaker; rounded down to the standard size: 90 A. Rules 28-200, 28-204, Table 29

5. 5

   A 90 A breaker on 65 A wire is normal for a motor circuit: the breaker protects against short circuits and ground faults.

6. 6

   Overload relay: built into the starter (contactor + overload), set to 115–125 % of the motor FLC; it provides the running overload protection.

With a VFD, MCA and MOP are based on the VFD input current, and the VFD provides the motor overload protection.

• Branch-circuit breaker (MOP)
• Conductors in EMT (MCA)
• Liquid-tight flexible conduit
• Electric heater elements
• Gas line

Figure 1. From the nameplate to the wire and the breaker

Common Motor Protection Selection Table

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

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

Reading a mechanical equipment schedule

An example schedule built from the values in this article. Read every row of every schedule against the nameplate.

Mechanical equipment schedule: tag, service, horsepower or load, voltage and phase, MCA, MOP, starter or VFD, remarks.

1TAG	SERVICE	2HP / LOAD	3V / PH	4MCA	5MOP	6STARTER / VFD	7REMARKS
RTU-1	Rooftop unit	Compressor 28 A FLC, fan 8 A, heater 15 A	Nameplate	58 A	90 A	Unit-mounted contactor + overload relay	#6 AWG Cu (65 A), 90 A inverse-time breaker
M-1	Motor	10 HP, FLC 30.8 A (table, 208 V)	208 / 3	38.5 A	80 A	Starter: overload set to 115–125 % of FLC	#8 AWG (selection table)
M-2	Motor on a VFD	—	—	VFD input	VFD input	VFD: electronic overload protection built in	VFD-rated cable on the output side

Quick check for each row: the same tag on the M and E drawings · conductors from the MCA · breaker no larger than the MOP · overloads in the starter or the VFD.

Electrical points

1. 1

   TAG: find the same tag on the mechanical schedule and on the electrical drawings; every tag needs a circuit.

2. 2

   HP / LOAD: the size of the load. For a simple motor, the FLC comes from CEC Table 44/45 at the system voltage (10 HP at 208 V: 30.8 A). Tables 44, 45

3. 3

   V / PH: the voltage and phase the values are for; the selection table in this article is for three-phase motors at 208 V.

4. 4

   MCA: the conductors need an ampacity of at least the MCA: RTU-1, 58 A → #6 AWG copper (65 A). Rules 28-106, 28-108 · Table 2

5. 5

   MOP: the breaker or fuse may not be larger than the MOP: RTU-1, 93 A calculated → a 90 A inverse-time breaker. Rules 28-200, 28-204 · Table 29

6. 6

   STARTER / VFD: the overload relay in the starter is set to 115–125 % of the motor FLC; with a VFD, MCA and MOP come from the VFD input current and the VFD provides the overload protection.

7. 7

   REMARKS: wire size and breaker; a breaker larger than the wire ampacity is normal and correct on a motor circuit.

Selection-table values are approximate (standard NEMA B motors); always verify with the actual nameplate and the local code edition.

• Column the electrical design reads
• 58 AMCA (conductors)
• 90 AMOP (largest breaker)

Figure 2. Reading a mechanical equipment schedule

Frequently Asked Questions