Sizing a HVAC

Status
Not open for further replies.

goodoboy

Senior Member
Location
Houston
Hello,

We are ordering a two A/C condenser units and I need to size wire size and OPC for both units.

Can I size the wire and OPC now without the nameplate information (unit not delieverd to site yet) or do I need to wait to see the nameplate information? I have the specs for the unit.

What information do I use to size the wire and OPC (fuse with)? Is it the MOPD for fuse and MCA for the wire size?

Thank you kindly
 

kwired

Electron manager
Location
NE Nebraska
Hello,

We are ordering a two A/C condenser units and I need to size wire size and OPC for both units.

Can I size the wire and OPC now without the nameplate information (unit not delieverd to site yet) or do I need to wait to see the nameplate information? I have the specs for the unit.

What information do I use to size the wire and OPC (fuse with)? Is it the MOPD for fuse and MCA for the wire size?

Thank you kindly
Only to a certain extent can you do this without nameplate info.

If there are any doubts on min wire size you can always go larger with no code issues, but not smaller.

Do you have model numbers? If so you can probably find out what actual nameplate data is for the specific model before you see the actual equipment.
 

goodoboy

Senior Member
Location
Houston
Only to a certain extent can you do this without nameplate info.

If there are any doubts on min wire size you can always go larger with no code issues, but not smaller.

Do you have model numbers? If so you can probably find out what actual nameplate data is for the specific model before you see the actual equipment.

Thank you.

I am awaiting for model number now. I will return with number shortly.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
HVAC circuits

HVAC circuits

I will add that you are always better off going with the MAX breaker size as listed on the nameplate, especially when dealing with single phase units and 208V supply voltages.
 

goodoboy

Senior Member
Location
Houston
I will add that you are always better off going with the MAX breaker size as listed on the nameplate, especially when dealing with single phase units and 208V supply voltages.

Thanks,

So the MOPD listed on the nameplate will tell me what size fuse (or breaker) should be at the supply voltage?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Thanks,

So the MOPD listed on the nameplate will tell me what size fuse (or breaker) should be at the supply voltage?
It tells you that somewhere in that circuit you must have an OCP device that meets that requirement in both size and type.
It can be at the beginning of the branch circuit or at the unit.
It's rare, but watch for units the specify "fuse" with no mention of HACR breaker or other overcurrent device as they require "fuse" only protection.
 

goodoboy

Senior Member
Location
Houston
It tells you that somewhere in that circuit you must have an OCP device that meets that requirement in both size and type.
It can be at the beginning of the branch circuit or at the unit.
It's rare, but watch for units the specify "fuse" with no mention of HACR breaker or other overcurrent device as they require "fuse" only protection.

Make sense. I am still learning and struggle with terminology at times.

This unit has a breaker at the unit to protect the unit for over current.

Should another OPC be at the beginning of circuit, in this case, the MCC bucket? This OPC can protect the wiring from power supply to load.

It will be interesting to see what size breaker they have at the unit and how they adjust for start up current.

Thank you
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
All good info

All good info

I would directly question any OEM that specifies fuses and disallows breakers with regard to HVACR.
Fuses are major compressor killers in the 3 phase realm, whereas HACR breakers will often allow a Tech to correct whatever fault has arisen without single phasing the compressor to death. Many modern commercial-industrial grade machines have panel HACR breakers protecting major 3 phase loads within the machine, whereas older generation equipment often had 3 phase fuse blocks. The common trip is golden.


All the best
 

goodoboy

Senior Member
Location
Houston
Only to a certain extent can you do this without nameplate info.

If there are any doubts on min wire size you can always go larger with no code issues, but not smaller.

Do you have model numbers? If so you can probably find out what actual nameplate data is for the specific model before you see the actual equipment.

I was able to get some numbers today:

460V 3 phase
MCA - 15amps
MOPC - 15
LRA - 49

Do you know where to look in NEC to determine what to size wire and OPC to?

Thank you in advance.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I was able to get some numbers today:

460V 3 phase
MCA - 15amps
MOPC - 15
LRA - 49

Do you know where to look in NEC to determine what to size wire and OPC to?

Thank you in advance.
When the minimum circuit amps and the maximum overcurrent protection list the same number, you do not have a lot of choice. Fifteen amps OCP and a wire ampacity, including all derating factors (temperature and fill) of 15A or more. :)
 

kwired

Electron manager
Location
NE Nebraska
It tells you that somewhere in that circuit you must have an OCP device that meets that requirement in both size and type.
It can be at the beginning of the branch circuit or at the unit.
It's rare, but watch for units the specify "fuse" with no mention of HACR breaker or other overcurrent device as they require "fuse" only protection.

I have not seen one that specifies fuses and no mention of HACR breakers in a long time, I think it was done on older units, maybe not even intending the use of fuses only. Marking them with both was just progress with what was intended all along IMO.

I would directly question any OEM that specifies fuses and disallows breakers with regard to HVACR.
Fuses are major compressor killers in the 3 phase realm, whereas HACR breakers will often allow a Tech to correct whatever fault has arisen without single phasing the compressor to death. Many modern commercial-industrial grade machines have panel HACR breakers protecting major 3 phase loads within the machine, whereas older generation equipment often had 3 phase fuse blocks. The common trip is golden.


All the best
Newer equipment (other than maybe a stand alone condensing unit) usually has phase loss monitors incorporated in the controls and will not even attempt to run with a lost phase.
 

goodoboy

Senior Member
Location
Houston
I was able to get some numbers today:

460V 3 phase
MCA - 15amps
MOPC - 15
LRA - 49

Do you know where to look in NEC to determine what to size wire and OPC to?

Thank you in advance.

Hello,

Here is my results. Any confirmation is appreciated?

Voltage drop distance: 1500ft (voltage at load with this distance, 467V)

3 conductors size - #3

30amp Disconnect at power supply equipment

Fuses - 20 amp (15 x 125%)

Questions:

1. Per code, is the unit required to have OPC next to it?

Thanks
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Hello,

Here is my results. Any confirmation is appreciated?

Voltage drop distance: 1500ft (voltage at load with this distance, 467V)

3 conductors size - #3
Io
30amp Disconnect at power supply equipment

Fuses - 20 amp (15 x 125%)

Questions:

1. Per code, is the unit required to have OPC next to it?

Thanks
The MOCP figure on the nameplate already takes into account continuous load starting surge, etc. It is a MAXIMUM value so you are not allowed to increase it.

On the other hand, you are allowed to increase the wire size all you want as long as the EGC is increased proportionally as well. If you are using the raceway as your EGC, it is assumed to be large enough.
 
Last edited:

goodoboy

Senior Member
Location
Houston
The MOCP figure on the nameplate already takes into account continuous load starting surge, etc. It is a MAXIMUM value so you are not allowed to increase it.

The reason I chose 20amp fuses at the MCC bucket is because of 210.20(A) stating 125% of load for minimum overcurrent device. so 15 x 1.25 = 18.75amps, so this means I need 20amp fuses.

Or am I confused?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
The reason I chose 20amp fuses at the MCC bucket is because of 210.20(A) stating 125% of load for minimum overcurrent device. so 15 x 1.25 = 18.75amps, so this means I need 20amp fuses.

Or am I confused?

You do not need to apply 210.20(A). 210.2 directs you to Article 440 for Air-Conditioning and Refrigeration equipment. As Golddigger pointed, the MOCP on the nameplate is all that is needed.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
#3 would be correct for 2% loss which is what the HVAC manufactures require.

# 6 would be for 5% drop and will lead to compressor and component failures.

But isn't that 2% off of the rated equipment voltage of 460V, and not system voltage of 480V?

2% drop from 460V is approx 451V. You can have over 6% drop from the 480V and still be above the minimum of 451V.
 
Status
Not open for further replies.
Top