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)

A diagram of overload

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