I need help with this HVAC label.......

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dionysius

Senior Member
Location
WA
I can clearly see MOPD is 40A for this 460V 3 phase compressor.

Next I see Minimum Circuit Ampacity is 31.9A

Two or more people told me that this is the load. I disagree.

The load is Fan Motor FLA(1.0*2) + Compressor Motor RLA (7.3+10.3) =19.6A

Who is correct???? HVAC Compressor Label.jpg
 
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nickelec

Senior Member
Location
US
The math is already factored in there use a 40a breaker or fuse with conductors suitable for 31.9 amps

Sent from my SM-G950U using Tapatalk
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The MCA is the largest component (usually the compressor) *125% + the other loads. Curious as to why do you need to know the actual load?
 
kwired

kwired

Electron manager
Location
NE Nebraska
dionysius said:
I can clearly see MOPD is 40A for this 460V 3 phase compressor.

Next I see Minimum Circuit Ampacity is 31.9A

Two or more people told me that this is the load. I disagree.

The load is Fan Motor FLA(1.0*2) + Compressor Motor RLA (7.3+10.3) =19.6A

Who is correct???? View attachment 20729
Click to expand...
19.6 is the maximum load it is rated for. Conditions of operation will determine actual load, but should not exceed that. Not sure just how they came up with MCA of 31.9, unless there is other load not on the nameplate for some reason. It is usually 125% of largest load plus all other loads. Even taking both compressor figures x 1.25 still doesn't get us all that close to 31.9.
 
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dionysius

Senior Member
Location
WA
nickelec said:
The math is already factored in there use a 40a breaker or fuse with conductors suitable for 31.9 amps

Sent from my SM-G950U using Tapatalk
Click to expand...
Elaborate on why you believe 31.9A is the running load. It cannot be that since it would heat up the wire.
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Running load is 19.6, the nameplate says wire it with conductors suitable for 31.9, not that the running load is 31.9, and use a maximum of a 40A breaker. So if you could find a 35A three pole you could use it.

Apparently they are designing for the max starting draw and voltage drop.

-Hal
 
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Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
texie said:
I wonder if this a VRF system with multiple evaporator units. Note the label says the electrical characteristics are for the outdoor unit only. That might explain the discrepancy.
Click to expand...

Yes it is.

Also the compressor rating is in RLA which is a useless number for calculating MCA. RLA is only used for calculating the compressors overload.
 
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Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
electrofelon said:
FWIW, I usually find the actual load to be about 66% of MCA. I am not saying there are not exceptions, but most I have measured have been right about there.
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These VRF units usually draw less than that but manufacturers are required to calculate worst case.


I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.
 
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topgone

Senior Member
Mgraw said:
These VRF units usually draw less than that but manufacturers are required to calculate worst case.


I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.
Click to expand...

RLA is a computed figure by manufacturers, it is the maximum continuous current (or MCA) of the compressor divided by the overload factor of the unit. Since refrigeration compressors are internally protected, the factors per code are 1.70 for loads up to 9 amperes; 1.56 for loads 9.1A to 20, and 1.40 for loads over 20A.
 
kwired

kwired

Electron manager
Location
NE Nebraska
electrofelon said:
FWIW, I usually find the actual load to be about 66% of MCA. I am not saying there are not exceptions, but most I have measured have been right about there.
Click to expand...
Load on a compressor is effected by how much heat is being moved by the refrigerant, and conditions of the heat exchanger coils can have an impact. Plugged condenser coil can't get rid of heat - leaving the system running at higher pressure and loading the compressor even more then usual.

Mgraw said:
These VRF units usually draw less than that but manufacturers are required to calculate worst case.


I should also note in my previous post I should have said RLA is just a percentage of compressor overload values.
Click to expand...
VRF needs to be running at 100% before it will reach RLA. When you first start system up you might get 100% speed but still depends on actual refrigerant loading as to what the motors will draw. In heating mode you may see 100% speed more often then during cooling mode - but there isn't as much heat being moved during extreme cold conditions and it won't be as much load on the system.


RLA is sort of same thing as FLA in a regular motor - it is what the thing will draw when running at it's maximum design output. If it isn't loaded to it's maximum design it will draw less current. Just because a unit is rated for a certain BTU output doesn't mean it puts out that amount of BTU anytime it is running - other factors go into that load.
 
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topgone

Senior Member
Here are my calculations, if you will:
RLA TOTAL = 7.3 + 10.3 = 17.6
MAX. CONT. CURRENT = 17.6X 1.56 = 27.5
FLA OF COMPRESSORS = 27.5/1.15 = 23.9 (115% OVERLOAD APPLIED)
MCA = 1.25X23.9 + 2 = 31.9!
MOP = 2.25 X 23.9 + 2 = 43.34, rounded to the nearest available protection = 40A
 
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