Minimum Disconnect Switch Size for HVAC RTU

[TXMaster]

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?

 

[don_resqcapt19]

No idea of what DDS is, but assume is the disconnect switch.
Where is this information found? If it is on the equipment nameplate, it is a 110.3(B) instruction and must be complied with.

 

[TXMaster]

This information is from a printout of the specifications for the unit. DSS means minimum disconnect switch size and I assume it will be on the nameplate, but the unit is not finished being built at the factory yet. I mean, I can put in a 400 amp disconnect switch with fuse reducers for 150A fuses, but I am just wondering where this comes from and if the equivalent horsepower rating could be applied. It seems a little arbitrary without some more supporting information...

 

[don_resqcapt19]

This information is from a printout of the specifications for the unit. DSS means minimum disconnect switch size and I assume it will be on the nameplate, but the unit is not finished being built at the factory yet. I mean, I can put in a 400 amp disconnect switch with fuse reducers for 150A fuses, but I am just wondering where this comes from and if the equivalent horsepower rating could be applied. It seems a little arbitrary without some more supporting information...

I have never seen a disconnect size specified on a nameplate. I see no need for a disconnect larger than what is needed for the fuses at 150 amps.

 

[infinity]

I see no reason for the disconnect to be any larger than the MCA.

 

[augie47]

Is there such an animal as a 150 amp disconnect switch ? Once you exceed 100 amp would it not be a 200 amp switch ?

 

[TXMaster]

Do you guys think this could come from 440.12(B)(1) ? NEC 2020.

"The horsepower rating of the disconnecting means shall be determined from the sum of all currents, including resistance loads, at the rated-load condition and also at the locked-rotor condition. The combined rated-load current and the combined locked-rotor current so-obtained shall be considered as a single motor for the purpose of this requirement as required by 440.12(B)(1)(a) and (B)(1)(b)."

This section confuses me.... and why wouldn't the specification be listed in terms of HP? Frustrating.

I am trying to think this through.... they may have calculated the locked rotor current and FLA of all possible concurrent loads and that came out to 250 amps. If someone energized the unit in locked rotor condition, realized something was wrong, and then used the disconnect to quickly de-energize the equipment.... it could potentially have that large amount of current flowing through it at that moment.

 

[mtnelect]

Should it be under 440.4 if it is hermetic refrigerant.

 

[augie47]

I may be totally off base but I feel you might be over-thinking it. With a MCA of 127 amps you would need a switch with that rati8ng and the next standard size switch is a 200 amp. Depending on the rest of the nameplate data it could be NF or, if required, fused at 150 amps but it would still be a 200 amp DSS.

 

[TXMaster]

Ok, I just talked with an engineer and he said that, yes, it is probably based on the maximum concurrent full load/locked rotor amps that the unit could possibly pull. I think I could apply 440.12(E), which allows me to apply 430.109(E) since the equivalent HP is over 100HP.

NEC 430.109(E)
"Isolating Switches. For stationary motors rated at more than 40 hp dc or 100 hp ac, the disconnecting means shall be permitted to be a general-use or isolating switch where plainly marked "Do not operate under load.""

Anyone else know more about this that can shed some light? Thank you all for your responses and helping talk it out.

 

[TXMaster]

I may be totally off base but I feel you might be over-thinking it. With a MCA of 127 amps you would need a switch with that rati8ng and the next standard size switch is a 200 amp. Depending on the rest of the nameplate data it could be NF or, if required, fused at 150 amps but it would still be a 200 amp DSS.

I have the same thoughts, but feel I am missing something there with the locked rotor amps. This is a confusing topic. Lol.

 

[TXMaster]

Should it be under 440.4 if it is hermetic refrigerant.

I don't see a requirement that DSS is marked on the nameplate in that section, but it very well could be there when the unit arrives. As previously stated, if it's on the nameplate it has to be followed. Maybe I can use a main breaker enclosure for this with a 250A "switch"... you know the kind that looks like a breaker but only provides short-circuit protection, not overload. Thinking of best options...

 

[Tulsa Electrician]

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.

 

[mtnelect]

I don't see a requirement that DSS is marked on the nameplate in that section, but it very well could be there when the unit arrives. As previously stated, if it's on the nameplate it has to be followed. Maybe I can use a main breaker enclosure for this with a 250A "switch"... you know the kind that looks like a breaker but only provides short-circuit protection, not overload. Thinking of best options...

The name plate makes it simple:
1) Minimum wire size: 127 amps (If on a roof, adjust for temperature)
2) Maximum fuse size: 150 amps
3) Disconnect size: 200 amps

 

[TXMaster]

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.

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.

 

[TXMaster]

The name plate makes it simple:
1) Minimum wire size: 127 amps (If on a roof, adjust for temperature)
2) Maximum fuse size: 150 amps
3) Disconnect size: 200 amps

DSS is listed as 250A though.... that's what is killing me.

 

[mtnelect]

DSS is listed as 250A though.... that's what is killing me.

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.

 

[TXMaster]

Probably, as augie47 stated.... overthinking it. Any other situation and I would have no problem putting a 200A disconnect with 150A fuses.... just that pesky DSS spec. Thanks for the replies everyone.

 

[augie47]

Foolishly I misread your OP as the DSS being "200".
I see your delimma ... personally I would not let it concern me,,,as o0thers have stated, your nameplate data calls for 150 amp MOCP... so that's waht I would provide.

 

[TXMaster]

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.

Definitely. Current-limiting fuses every time. Class RK1. Thanks for your input; I appreciate you taking the time.