AC Unit Circuit Size and OCPD Rating

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pdemapan

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Hello Forum,

I'm trying to size the OCPD and conductors for a direct expansion air conditioning unit. The mechanical engineer has told me the indoor unit is rated: 460V, 3phase, 60hz with FLA 66 A, MCA 74 A and MOP 90 A. There is also a reheat which is 22.5 kW. Here are my calcs.

Circuit Size:
(66 amps x 1.25) + (22.5kW/(480V x 1.732)) = 109.56 A
I've selected #2 conductor.

OCPD:

(66 A x 2.25) + (22.5kW/(480V x 1.732)) = 175.56 A
I selected a 3P175A circuit breaker.

I'm concerned because this seems breaker seems rather large for a branch circuit breaker. Can anyone verify if I'm doing this correctly? Any comments is greatly appreciated.
Thanks
 
Is the reheat load separate from the other data? If not, forget all of your calculations. The answers are right in the nameplate data provided in the engineer's document. MCA is used to size the conductors. MOPD is used to size the OCPD.
 
I was working on a reply but during the wording I accidentally returned to Rob's better answer.
If the two com pents, the A/C & Heat are a combination listed unit then the manufacturer should provide the MCA and MOCP for the combination per 440.4(B).

If they are not a listed combination, then each unit must be looked at independently with the OCP & conductors supplied per the appropriate
section. A feeder for the total could then be selected and the OCP sized for that feeder.
 
pdemapan said:
The mechanical engineer has told me the indoor unit is rated: 460V, 3phase, 60hz with FLA 66 A, MCA 74 A and MOP 90 A. There is also a reheat which is 22.5 kW. Here are my calcs.
Click to expand...

The given information does not jive. 66 FLA x 1.25 = 82.5 MCA
MOP = 66 a x 1.75 = 115 amps
22.5 kw/(0.46 x 1.73) = 28.3amps x 1.25 = 35.4 MCA
 
Yeah it didn't really sound correct but I'm taking this directly from the mechanical equipment schedule. In my case I've assumed that the A/C and reheat coil are separate but I put them on the same circuit. What puzzles me is the fact that the MOP calls 90Amps when I'm getting almost twice that for my OCPD rating. My questions still however is if i take the A/C and reheat as independent as I have but put them on the same circuit does the ocpd size sufficient to protect the equipment? Do you normally put them on separate circuits?
 
Because of Code requirements, unless they are part of ONE listed unit with a provision for single circuit feed, you are going to have the feed them with separate branch circuits, one sized no larger than 90 amp due to the A/C specs and the other sized by Art 424 (?) for the heater (which probably also has mfg MOCP).
So you will end up with a branch circuit for each.
Unlike with Art 430, I see no Art 440 Code for sizing a feeder to individual equipment. My opinion is you would need to size the feeder larger enough for the load with adequate OCP and size the wiring accordingly, but that is just an opinion. Someone else may see a route other than that.
 
I would contact the manufacturer and ask for documentation (fax, email, web-site, etc.) of exactly what I need to know.
 
pdemapan said:
Yeah it didn't really sound correct but I'm taking this directly from the mechanical equipment schedule.
Click to expand...

Mechanical equipment shcedules are only as good as the person developing them. You will not be able to accurately design your circuit(s) until you have actual manufacturer data sheets.

I'll bet that somewhere in the specs or a note on the drawings, it tells the Contractor to verify ratings with actual equipment furnished.
 
Why did the OP use 2.25 and Bob use 1.75 as multipliers on the FLA when calculating breaker size?

I have also seen A/C units have a LRA rating. What does this stand for?
 
bob said:
The given information does not jive. 66 FLA x 1.25 = 82.5 MCA
MOP = 66 a x 1.75 = 115 amps
22.5 kw/(0.46 x 1.73) = 28.3amps x 1.25 = 35.4 MCA
Click to expand...

The unit may have several fans. You have to look at its cut sheet to find out. Let's assume it has three fans/motors, FLA of the biggest one is 33A. Then the MCA=33x1.25+33(sum of FLAs of rest motors)=74.25A. MOP=33x1.75+33=90.75.

Most time the heater will need its own circuit unless the unit manufacturer can provide one circuit entry kit to combine the circuits for unit and heater.
 
pdemapan said:
OCPD:

(66 A x 2.25) + (22.5kW/(480V x 1.732)) = 175.56 A
I selected a 3P175A circuit breaker.

Thanks
Click to expand...


bob
The given information does not jive. 66 FLA x 1.25 = 82.5 MCA
MOP = 66 a x 1.75 = 115 amps
Click to expand...

Yes, mull982,

I have also seen many times for breaker and even for motor's breaker these factor ( 1.75 and 2.25 ) in some posts.

I dont know either why they take these factors. Is there any reference ?

Hope someone make it clear it.
 
Hameedulla-Ekhlas said:
Yes, mull982,

I have also seen many times for breaker and even for motor's breaker these factor ( 1.75 and 2.25 ) in some posts.

I dont know either why they take these factors. Is there any reference ?

Hope someone make it clear it.
Click to expand...

175% is for AC and refrigeration equipment motor-compressor. Please refer to NEC 440.22.
 
mull982 said:
Why did the OP use 2.25 and Bob use 1.75 as multipliers on the FLA when calculating breaker size?

I have also seen A/C units have a LRA rating. What does this stand for?
Click to expand...

I used 2.25 because this will give you the maximum size OCPD rating. 1.75 gives minimum OCPD size. LRA = locked rotor ampere basically the current drawn by the motor upon startup.
 
pdemapan said:
I used 2.25 because this will give you the maximum size OCPD rating. 1.75 gives minimum OCPD size. LRA = locked rotor ampere basically the current drawn by the motor upon startup.
Click to expand...

Patrick, 1.75 does NOT give the minimum OCPD size. This is the MAXIMUM OCPD size (but is allowed to be increased to 2.25 only if the 1.75 gives an ocpd size that is insufficient for the motor starting current.)
 
Last edited:
david luchini said:
Patrick, 1.75 does NOT give the minimum OCPD size. This is the MAXIMUM OCPD size (but is allowed to be increased to 2.25 only if the 1.75 gives an ocpd size that is insufficient for the motor starting current.)
Click to expand...

thanks, just took it straight from stallcup's book. "finding minimum and maximum size ocpd of the a/c unit". i understand though
 
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