HVAC disconnect size

Status
Not open for further replies.
M

mporterf

Member
Location
Athens, Da
I want to see what the min. size disconnect required would be for the HVAC system below. Not the unit has (2) 70HP compressors and (6) fans. I'm hoping a 200amp will work.


The nameplate on the unit states:
480v 3 phase 60 HZ
(2) COMPRESSOR MOTORS
Compressor RLA = 56.2
Compressor LRA = 283

(6) FANS
FLA= 2.1

Minimum Circuit Amps = 169.2
Maximum over current protection Amps = 200

I plan on using a 200amp I-line breaker, 3/0 copper THHN wire in 2" conduit and use a 200amp non-fused disconnect at the unit. Please advise if this falls within the code. I was looking over 440-11 but it's a little confusing.

Thanks!
 
mporterf said:
I want to see what the min. size disconnect required would be for the HVAC system below. Not the unit has (2) 70HP compressors and (6) fans. I'm hoping a 200amp will work.


The nameplate on the unit states:
480v 3 phase 60 HZ
(2) COMPRESSOR MOTORS
Compressor RLA = 56.2
Compressor LRA = 283

(6) FANS
FLA= 2.1

Minimum Circuit Amps = 169.2
Maximum over current protection Amps = 200

I plan on using a 200amp I-line breaker, 3/0 copper THHN wire in 2" conduit and use a 200amp non-fused disconnect at the unit. Please advise if this falls within the code. I was looking over 440-11 but it's a little confusing.

Thanks!
Click to expand...

Size your wires based on MCA(Minimum circuit amps) and OCPD on MOP(Maximum over protection Amps)....Why are you thinking of using non-fused disconnect?
 
mporterf said:
I want to see what the min. size disconnect required would be for the HVAC system below. Not the unit has (2) 70HP compressors and (6) fans. I'm hoping a 200amp will work.


The nameplate on the unit states:
480v 3 phase 60 HZ
(2) COMPRESSOR MOTORS
Compressor RLA = 56.2
Compressor LRA = 283

(6) FANS
FLA= 2.1

Minimum Circuit Amps = 169.2
Maximum over current protection Amps = 200

I plan on using a 200amp I-line breaker, 3/0 copper THHN wire in 2" conduit and use a 200amp non-fused disconnect at the unit. Please advise if this falls within the code. I was looking over 440-11 but it's a little confusing.

Thanks!
Click to expand...

Everything is good, you could get away with #2/0 Cu THHN conductors to save a few bucks.
 
shoaib10 said:
Can you use a disconnect switch without a fuse?How can that provide overload or short circuit protection for the whole unit?
Click to expand...

You have a 200 amp circuit breaker to provide the ground fault and short circuit protection so no fuse in the disco is needed. The overload protection is provided by the device with the motor.
 
Disconnect sizing

Disconnect sizing

My main question is about the disconnect sizing. Looking at section 440 in the code it's not clear to me how I should size the disconnect. Tad far as wire size it's a pretty long run so I will do a voltage drop calc. and see what I come up with. The 200amp breaker will serve as my short circuit protection and the unit already has a motor starter with overloads to protect the motor. Thanks for the input.
 
mporterf said:
My main question is about the disconnect sizing. Looking at section 440 in the code it's not clear to me how I should size the disconnect. Tad far as wire size it's a pretty long run so I will do a voltage drop calc. and see what I come up with. The 200amp breaker will serve as my short circuit protection and the unit already has a motor starter with overloads to protect the motor. Thanks for the input.
Click to expand...
Determined per 440.6...

(B) Multimotor Equipment. For multimotor equipment
employing a shaded-pole or permanent split-capacitor-type
fan or blower motor, the full-load current for such motor
marked on the nameplate of the equipment in which the fan
or blower motor is employed shall be used instead of the
horsepower rating to determine the ampacity or rating of
the disconnecting means, the branch-circuit conductors, the
controller, the branch-circuit short-circuit and ground-fault
protection, and the separate overload protection. This marking
on the equipment nameplate shall not be less than the
current marked on the fan or blower motor nameplate.
Click to expand...


Side query: I never understood why this section uses the qualifier "employing a shaded-pole or permanent split-capacitor-type fan or blower motor". What if the equipment is multimotor and does not employ such fan or blower motor type? Is that possible? The qualifier seems to indicate it is... but then what do we do for such multimotor equipment? We only have 440.6(A) and (B) to size with.
 
Status
Not open for further replies.
Top