So let’s talk about “Cold” Motors aka Refrigerant Motor Compressors (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.
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
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
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 = The Power Cutoff Button!: 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)
