Minimum Disconnect Switch Size for HVAC RTU

[TXMaster]

Per UL 98, the standard that the horsepower rated disconnect is listed to, the disconnect capable of being opened under locked rotor conditions.

For switches rated 100 hp or less it must be able to do that 50 times without damage. For switches over 100 hp, it must be able to do that 10 times without damage.

The current used for this testing for a disconnect rated at 150 hp is 2170 amps at 240 volts, and 1085 amps at 480 volts.
Give the code full load currents of 360 amps and 180 amps for a 150 hp motor at 230 and 460 volts, the UL test currents equals the rule of thumb locked rotor current of six times full load.

Thats great info, Don. Thank you! I'll try to post a better reply with code references, but in looking at several examples....
LRA of compressors tends to be much higher than 6 times FLA. I bet thats the answer.... if I only can get that LRA data of the compressors, I bet it is over 150 HP equivalent and then this starts to make some sense.

 

[topgone]

I would see if an internal disc is supplied with the unit.
If your worried about HP.
Run the numbers backwards to get a close to HP.
If a spec job a disconnect type should be listed.
Looking at the numbers a HD 480 volt three phase 200 amp fuse able should work fine.
Maybe look at SCCR rating of the equipment if applicable.
However this is a disconnect question.

Good point. This device doesn't interrupt currents, it merely conveys whatever the HVAC pulls from the service. The code even requires a label saying "Do Not Operate Under Load".

 

[mtnelect]

That DSS does not seem to apply for what is being talked about here.

The world is changing so fast ... Not the "Knob & Tube" wiring anymore !

 

[don_resqcapt19]

The world is changing so fast ... Not the "Knob & Tube" wiring anymore !

No idea what you are talking about...the DSS in this thread is very clearly the equipment disconnect...nothing to do with the link you posted.

 

[TXMaster]

This might help to explain.

No, this is a large RTU... 50T. DSS definitely means minimum disconnect switch size. I've seen it spelled out in words in this context.

 

[mtnelect]

No, this is a large RTU... 50T. DSS definitely means minimum disconnect switch size. I've seen it spelled out in words in this context.

Since it doesn't appear on the name plate ... It's information that is not important.

 

[infinity]

No, this is a large RTU... 50T. DSS definitely means minimum disconnect switch size. I've seen it spelled out in words in this context.

So why would it ever need to be 250 amps when a 250 amp disconnect does not exist?

 

[masterinbama]

What are the odds that all motors involved would be in a locked rotor state at the same time? i would put a 200 amp heavy duty switch on it and move on.

 

[mtnelect]

I have specs for a RTU and I am confused how this DSS value was calculated and what it means... The unit is 3ph 460V and I am given this information:

Condenser Motor FLA= 2.2 A
4x condenser motors
Compressor 1 RLA= 20.5 A
Compressor 2 RLA= 31.7 A
Compressor 3 RLA= 31.7 A
Supply Fan FLA= 24.7 A
CCH FLA= 1.0 A

MCA= 127.0 A
MOP= 150.0 A
DSS= 250.0 A

I am not given the horsepower ratings of any of this, nor the locked rotor current or locked rotor code letters. The equivalent FLA of the unit comes out to 118.4 A, which when cross-referenced with NEC T430.250 gives equivalent HP of 100HP. The 200A HD disconnect I want to use is rated for 125HP at 480V. Does anyone know where this 250A DSS is coming from or what is the significance of this number? I feel like the 125HP rating of the 200A disconnect is adequate- am I wrong?

What is the manufacturer & model number ? CCH is a Crankcase Heater 1 amp at 120 volts. Also, sounds like a "Cascade" low temperature system.

 

[topgone]

What is the manufacturer & model number ? CCH is a Crankcase Heater 1 amp at 120 volts. Also, sounds like a "Cascade" low temperature system.

As you said, it depends on who makes your unit. FYI, Copeland has crankcase heaters requiring 240V/ 400V. LINK

 

[Fred B]

So why would it ever need to be 250 amps when a 250 amp disconnect does not exist?

If you are looking at a fusible disconnect, you can fuse to lower amperage than disconnects rating but not higher than the disconnects rating. My thought is that mfg is requiring a minimum switch rating but it can be above that rating. Similar to certain applications requiring a minimum Bus rating but you can exceed the minimum.