Motors are all around us. They are in every building, moving the air to keep us warm and pumping water to keep us clean. When it comes to powering motors, there are special rules in the Canadian Electrical Code to ensure they run reliably and safely. And that means the engineering of the electrical distribution system needs to be designed for MCA and FLA. So what is the difference between MCA vs FLA?
MCA (Minimum Current Ampacity) is used for sizing conductors to ensure that the wiring does not overheat under normal operating conditions. The conductors may be larger to compensate for voltage drop but should never be sized smaller. MCA is the minimum safe loading that conductors should carry.
FLA (Full Load Amps) is the amount of continuous current that a running motor will consume and what we use in our load calculations.
MCA = (FLA + Heater Current) * 1.25 or put another way FLA = 0.80 * MCA
As the calculation show, the goal is to ensure that the conductors are only loaded to a maximum of 80% of allowable ampacity for a conductor (CEC Table 1-4 with the correction factors in Tables 5A-D). This is to avoid an overheating hazard in the building's electrical system which can lead to fire hazards.
Other Motor Specifications shown on Motor Nameplates
MOCP (Maximum Overcurrent Protection Device) is used to size the circuit breaker or fuse and is usually specified by the motor manufacturer. MOCP will always exceed the MCA.
As electrical engineers, we verify that the overcurrent protection device has a lower trip rating than the maximum allowable ampacity of the conductor. We want the protection device to trip or blow before the conductor is damaged.
On one project, the opposite was shown on the data sheet and the manufacturer was asked to re-specify. Thankfully, the motor nameplate was correct. To be cautious, we did oversize the conductor just be to safe.
(Incidentally, the data sheet also said it was a 2-phase motor. There's no such thing as a 2-phase motor! It's one phase using 2 hot conductors but it is still 1 phase... but I digress.)
Sizing Conductors for Motors
As a general rule, electrical consultants avoid a voltage drop below 85% on motor circuits. (For all other circuits, the voltage drop is limited to 3% on feeder and branch circuits; 5% from the supply side of the service to the point of utilization - Canadian Electrical Code 8-102.)
For design purposes, we use a maximum voltage drop for motors of 85%. While some engineers use a value as high as 92%, from years of experience, we have found that 85% to be a good balance between voltage drop and the size and cost of the conductors to achieve reliable starting and safe operation.
If a motor can not get enough power during startup, it will experience:
- A stall - the motor will remain stationary, unable to overcome inertia. There is
insufficient torque to overcome the load torque. The motor will consume a lot of current as it:
- struggles to start,
- may make horrible clunking sounds and
- possibly generate self-destructive vibrations.
The windings or branch conductor may overheat and eventually the circuit breaker or fuse will trip to prevent a serious hazard. We verify that a fire alarm system is installed in most industrial settings.
On a personal note, we were designing a communication system in Calgary and the control system kept failing. We were in a testing facility, costing $5k a day and the system would not start up properly. Random failures were happening throughout the entire system making it hard to diagnosis without a repeatable and definitive cause. It took an entire day to determine the root-cause was a failing, and silent, bearing in the air-exchanger fan unit! That was not a fun day.
Don't forget the Motor Service Factor
It's important to take into account the motor service factor. Since these motor circuits are going to be in use for decades, we must design for the occasion when these machines are going to be running continuously at 1.15 - 1.25 times the rated horse power.
During these extreme conditions, we want to make sure your motors continue to run safely and reliably and the conductors do not overheat and lead to a building fire.
MCA vs FLA are important motor specifications that engineers use to properly power motors. Once we create a safe and reliable electrical design, these circuits continue to work dependably in the background with minimal supervision and only occasional maintenance.