MCA & COOLING LOAD 2

[james64]

Hello,

I am evaluating the power consumption at steady load of a RTU(roof top unit) which operate at 600V an MCA of 8.2A. So that would come out kw = 600 x 8.2 x 1.73 = 7.1 kw. But cooling power input (w/o blower)load says only 2.51kW as per name plate, which load should I consider for load calculation ?

Many Thanks in advance !!

 

[waross]

Are you able to post a picture of the nameplate?

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Ohm's law
Not just a good idea;
It's the LAW!

 

[james64]

Yes, I can. Please see the attached.

 

[davidbeach]

Something in between, but the 8.2A number would be the better choice of the two. It includes all components plus 25% of the largest motor.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[edison123]

Missing power factor?

Muthu

http://www.edison.co.in

 

[davidbeach]

What's missing is the RLA - Rated Load Amps - number. Packaged HVAC equipment nameplate should provide load amps, wire sizing amps, and over current protection amps.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[james64]

Thank davidbeach,edison123 and waross for your comment!!

What is the relation between FLA and MCA ? As far as I know, MCA is for wiring sizing, it means 25% more than FLA. So FLA would be 0.8* MCA. Am I right ?

 

[davidbeach]

It's an extra 25% of the largest motor. If you have just a compressor and a single fan, MCA can be much larger as a percentage of the actual load than if you have a unit with 2 compressors, six condenser fans and 2 ventilation fans. It's hard to back into the real number.

But if getting it down to something much more precise than MCA is important, you might just have bigger problems. It probably means you're too close to your maximum capacity, never a good place to be.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Well, you are starting with the wrong formula:
kw = 600 x 8.2 x 1.73 = 7.1 kw
should be;
KVA = 600 x 8.2 x 1.73 = 7.1 KVA

With less than 0.5 HP from a 1.5 HP motor, your power factor will be fairly low. The KVA will be quite a lot more than your kW.

The manufacturer has taken the sum of the motor currents and added 25% of the largest motor;
He has given that to you as:
MINIMUM CIRCUIT AMPACITY, MCA, 8.2 Amps

If you want the load in kW, add 0.40 kW and 2.51 kW =2.91 kW.
That is the energy that you will pay for.
But don't use that for calculating panel loading.
For that you should use KVA, which, in this case, appears to be quite a bit higher.
Use your calculated 7.1 KVA for panel loading. You will be about 1 or 2 kW on the safe side.
As edison123 mentioned, the power factor is important.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[james64]

Thank you all again !!

 

[jraef]

MCA is only used for sizing the conductors going to the unit. It has no discernible relationship to any individual load inside, other than as part of the whole.

MCA = 1.25 x (Load 1 + Load 2 + Load 3 etc.), with allowances for non-coincidental loads that usually only the equipment manufacturer knows for sure.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden