Dakota Cold Motors Guide for Red Seal Exam (CEC 2024)
Red Seal Exam Question Guides
So let’s talk about “Cold” Motors aka Refrigerant Motor Compressors aka Hermetic Motors (28-700)!
What Makes Them Different?
Hermetic motors are sealed inside the same casing as the compressor — kind of like putting the motor in a snow globe full of refrigerant!
No external shaft, no open motor—everything is tucked away and cooled by the very stuff it’s compressing!
No Horsepower? No Problem!
Unlike regular motors rated by horsepower, Hermetic motors use RLC (Rated Load Current) to tell us how hard they work.
Cool by Design
These motors chill out while they work, thanks to the refrigerant.
This special cooling means we can work them harder, but it also means we size protection devices a bit differently.
The Mysterious Code Rules (With a Twist)
Code 28-700 to 28-714:
This is your rulebook for cold-hearted machines (literally — they run A/Cs, freezers, etc.)
RLC = FLA (basically, same value)
LRC = RLC × 6 – Locked rotor current is that huge surge when the motor first turns on. For “hot” motors we call this INRUSH current. (28-704(1))
If you can’t find the RLC on the motor, dig into Table 44 or 45(28-704(2))
Conductor Sizing: Rule 28-706
Sizing conductors for “cold” motors is exactly the same as with “hot” motors. Reference back to the Guide for Motor Conductor Sizing.
Question Example:
What is the size of Teck90 supplying a 30 HP, 600-volt, 3-phase squirrel cage motor with an overload relay that is controlling a roof-top cooling unit?
This motor is running a rooftop cooling unit — so yep, it’s a “cold” motor! Now here’s the thing: we treat cold motors just like hot motors when it comes to wiring ... we just give the numbers cooler names.
On cold motors, we call it RLC (Rated Load Current) instead of FLA. But it’s the exact same value. If the RLC isn’t on the nameplate, no worries — just grab the FLA from the tables.
Walkthrough
Table 44 says: 32A
And since this is continuous duty, we use our 125% multiplier: 32A * 125%= 40A
Now which Column do we use in Table 2?
We figure it out exactly the same way as with Hot motors. This is supply, so it is 75°C.
Table 2 (75°C): #8awg
Overcurrent Protection: Rule 28-708
Overcurrent = keeps wires from catching fire Use 50 or 65% of the LRC for your fuse or breaker
Question Example:
What is the size of Circuit Breaker Protecting a 30 HP, 600-volt, 3-phase squirrel cage motor with an overload relay that is controlling a roof-top cooling unit?
So this is where Cold Motors differ significantly from Hot Motors.
With Hot motors, we care about what TYPE of Over-Current.
Cold Motors, it doesn’t matter.
We calculate all Over-Current the exact same way-- based on LRC. The Locked Rotor Current.
Walkthrough:
LRC= RLC (or FLA) * 6
Over-Current Size=LRC/2
Table 13—GO DOWN
Table 44- 32A
LRC= 32A * 6
LRC= 192A
Over-Current= 192A/2 = 96A
Table 13—90A breaker (or TD Fuse, or NTD fuse. It depends on what it’s asking for).
Fun Fact: If 50% won’t allow the motor to start, then you use 65%!!
Overload Protection: Rule 28-710
Overload = protects the motor from long-term strain Use 140% of RLC for relays, or 125% for fuses
Question Example:
What is the size of Overload Relay protecting a 30 HP, 600-volt, 3-phase squirrel cage motor that is controlling a roof-top cooling unit?
This is where it gets pretty weird.
Hot motors the overload is sized based on Service Factor (SF).
Cold Motors don’t have that.
Instead we size based on WHAT KIND of Overload.
Our options are:
Relay: RLC * 140% (Calculate, then leave it)
Other than Relay (such as fuses): RLC * 125% (Then Table 13—Go DOWN)
Walkthrough:
Table 44- 32A
Since the example asked about Overload Relay:
32A * 140%= 44.8A and Leave it.
If the example had asked about “other” for Overload:
32A * 125%= 40A
Table 13: 40A Fuse
Disconnecting Means: Rule 28-714
Must be within 3m and in sight of the motor-compressor
Disconnects are sized at 115% of RLC
Walkthrough:
Disconnect = 32A× 1.15 = 36.8 A
So a 40 A switch (or 60 A fused)
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