Minimum Disconnect Switch Size for HVAC RTU

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I just realized how they got 127 amps.

The total of all the equipment is 118.4 amps plus 25% of the largest motor-- 31.7 x .25= 7.9 amps= 126.3 amps.... I guess they rounded it up
but 125% x 127= 158.75 for the conductor. I guess. lol
 
TXMaster said:
I can maybe get the internal disconnect option added if it's not too late with the factory. That's a good angle. Still kinda crazy to me that maximum OCPD is 150 but disconnect size has to be 250. How does that make any sense? Lol.
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Once you are over 200, the next size is 400, so saying DSS = 250 makes no sense.
 
Dennis Alwon said:
I just realized how they got 127 amps.

The total of all the equipment is 118.4 amps plus 25% of the largest motor-- 31.7 x .25= 7.9 amps= 126.3 amps.... I guess they rounded it up
but 125% x 127= 158.75 for the conductor. I guess. lol
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Yes, that is how that part is done, for sure! Now, if you can find me the calculation for their DSS.... that would be awesome. Lol.
 
mtnelect said:
That's an acronym I have never seen on a HVAC name plate, and not listed in the NEC Section 440.4, and you could use a HVAC rated breaker if stated on the name plate. You got all the information you need, but I would use a fuse to better protect the unit.
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You can use any type of OCPD that you want to unless the nameplate only shows a value for a fuse or a value for a breaker.
All listed breakers in the last 25's or so are HACR even if not marked on the breaker. Those requirements became part of the UL 489 product standard a long time ago and marking on the breaker is permitted, but not required.
 
don_resqcapt19 said:
Once you are over 200, the next size is 400, so saying DSS = 250 makes no sense.
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I think it's just saying that is minimum. You could technically use a 250 amp breaker-type switch. But, I think they are saying 200 is not good enough if de-energizing under load in some locked rotor condition.... I am thinking I will use a 200A disconnect and mark it "DO NOT OPERATE UNDER LOAD" and I should be covered no matter what happens.... I don't know I am still kicking it all around. Always overthinking. Such is my life. Ha
 
Dennis Alwon said:
I just realized how they got 127 amps.

The total of all the equipment is 118.4 amps plus 25% of the largest motor-- 31.7 x .25= 7.9 amps= 126.3 amps.... I guess they rounded it up
but 125% x 127= 158.75 for the conductor. I guess. lol
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The reason that the numbers don't add up is because the suction return cools the winding in the compressor(s) and it runs more efficiently than a standard motor. So, if the HVAC mechanic doesn't monitor the refrigerant charge, you can damage the unit.
 
TXMaster said:
I think it's just saying that is minimum. You could technically use a 250 amp breaker-type switch. But, I think they are saying 200 is not good enough if de-energizing under load in some locked rotor condition.... I am thinking I will use a 200A disconnect and mark it "DO NOT OPERATE UNDER LOAD" and I should be covered no matter what happens.... I don't know I am still kicking it all around. Always overthinking. Such is my life. Ha
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I believe you can go to 200 because max overcurrent protective device is 118.4 x 175% = 207
 
Here are some more acronyms for Dennis Alwon files:
Distributed Scroll System (DSS)
■ Compact Scroll System (CSS)
■ Distributed Scroll Pack (DSP)
 
infinity said:
That seems to be the prevalent opinion but IMO it makes no sense in the context of an AC nameplate.
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We know it isn't the department of social services but here is another

DSS or Directional Sequence Switch is a device used to control the movement of tapping switch in both the directions.
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Obviously I don't know what it is....lol
 
Scroll compressors have replaced "Reciprocal Hermetic Compressors". Now, we are in "Inverter Technology".
 
Dennis Alwon said:
I think it is a disconnect safety switch or something like that
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It is minimum disconnect switch size. I know that for a fact. :)
I just don't know how they came to that conclusion... software math for sure.
I'm assuming if I could get the LRA data on the compressors it would be over 150HP equivalent.... thats the only thing that makes sense. I will update if I can get to the bottom of it before the thread is closed. Thank you all for your bandwidth.
 
mtnelect said:
Scroll compressors have replaced "Reciprocal Hermetic Compressors". Now, we are in "Inverter Technology".
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This is true. Sure cuts down on the inrush factor of across-the-line starters. Makes things easier on the system in a lot of ways... not so much in other ways like distortion and harmonics. Its a new world. Lol.
 
TXMaster said:
This is true. Sure cuts down on the inrush factor of across-the-line starters. Makes things easier on the system in a lot of ways... not so much in other ways like distortion and harmonics. Its a new world. Lol.
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Looks like you are up on the new HVAC technology !
 
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.
 
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