sizing per MCA

Status
Not open for further replies.
M

malachi constant

Senior Member
Location
Minneapolis
1) Given the voltage/phase and MCA of a motor, how do you determine kVA (for panel schedule)?

2) How do you determine wire size?

3) How do you determing OCP? Per NEC 430.52 or just use MCA to mean "minimum circuit overcurrent protection ampacity"?

I'll try my hand at answering my own question, let me know how I do...
Voltage: 208/1.
MCA: 40

1) kVA = 40A*208V*sqrt(1)/1000 = 8.32 kVA

2) wire size is based on Table 310.16, 75C rated = #8 AWG Cu

3) OCP is 40A fuse in 60A switch. But what would breaker be sized? 40A?
 
No, you need to determine the horsepower rating of the motor and use the FLC rating as specified in Tables 430.248 - .250.

Size the conductors per 430.21, and size the ground-fault and short-circuit device from 430.51.

Are you sure you are referring to a motor and not an A/C unit?
 
bphgravity sent his post before I could. Malachi, it looks like you're kinda going about this the wrong way. If you read through Art. 430 you'll have a better grasp of what these components are based on.

I'm not familiar with the term MCA either. It doesn't seem to correspond with a 208V motor full load current on Table 430.248. Most of the components are sized based on the Table value, the motor overloads are the only ones sized by the FLC on the motor name plate.

I think I'll wait for the answer to bph's question before saying any more. I could be way off base. :oops:

edit: bph-Maybe you did recognize the MCA and that's why you asked the question :?: :!: :?
 
bphgravity,

Yes it is a condensing unit. So how does that change things? Do I treat it as a set of motors? I can't get everything from MCA (minimum circuit ampacity)?

Here is what I know on a specific unit, can I size it using this information?:

Compressors:
RLA: 14.1 (run load amps?)
LRA: 66.0 (locked rotor amps)

Condenser:
Motor HP/RPM: 1/8 / 825

Evaporator:
Nominal Tons: 2.5
Total MBH: 30
High/low CFM: 875 / 585

Single Point Power:
Volt/phase: 208 / 1
MCA: 18.4

I sized the wire per the 18.4 amps at #12.
I sized the breaker at 20A and the fused disconnect at 30A with fusing to match manufacturer's nameplate data.
I got the kVA from 18.4A * 208V = 3.83kVA.

Any suggestions for how to this differently? Do I need to prod the ME a little more? (I'd prefer to work with what he's given me, if at all possible.)

Thanks.
 
Yes, you treat it differently. You use Article 440 not 430.

Size the conductors per 440.31 and use 440.21 for the ground-fault and short circuit protection.

(The minimum circuit ampacity is the RLC + 25%)
 
Good catch!

Thanks bhp, now I know.

". . .and knowing is half the battle." :wink: :lol:

(Some of you will get the quote, some won't.) :roll:

Thanks again.
 
the MCA should be the RLA (rated load amps) of the compressor motor of the unit plus any other loads (control, condenser, ect)multiplied by 1.25. The MOCP is a little tricker. I like to use the Manufacturer's suggested MOCP whenever possible. The MOCP is 1.75 * compressor motor RLA then add any other loads to that number. But, thats not the whole story. If the OCP wont let the motor start at 175% you can go up to 225%. Thats why I like to use the manufacturer's MOCP info because they already have that figured out. I suggest you look at article 440. specifically 440.22 (2002).
 
new_ee said:
the MCA should be the RLA (rated load amps) of the compressor motor of the unit plus any other loads (control, condenser, ect)multiplied by 1.25. The MOCP is a little tricker. I like to use the Manufacturer's suggested MOCP whenever possible. The MOCP is 1.75 * compressor motor RLA then add any other loads to that number. But, thats not the whole story. If the OCP wont let the motor start at 175% you can go up to 225%. Thats why I like to use the manufacturer's MOCP info because they already have that figured out. I suggest you look at article 440. specifically 440.22 (2002).
Click to expand...


The MCA should be the largest motor, the compressor FLA X 125%+evaporator fan FLA. Not 125% of both. For AC equipment covered under article 440 you must you the nameplate ratings for conductor sizing and OCPD. Using the manufacturers nameplate data is not optional.
 
infinity said:
The MCA should be the largest motor, the compressor FLA X 125%+evaporator fan FLA. Not 125% of both. For AC equipment covered under article 440 you must you the nameplate ratings for conductor sizing and OCPD. Using the manufacturers nameplate data is not optional.
Click to expand...


correct. thats what was in my head, but it didnt come out that way when i was typing. when i said i like to use the nameplate data i should have said have to use the name plate data.
 
Speaking of air conditioners, I was looking at the wiring diagram for one the other day, and in the portion of the diagram showing the connection to the premises wiring, there was a note: "Copper Conductors Only"

Would this be just for the lugs where I make my connection to the condenser, or would it be the branch circuit itself?

Secondly, does the 25% added in (resulting in the minimum circuit ampacity) cover continuous duty, or would I need to add 25% again for a continuous load? I ask, because I did run across a system which ran continuously for over three hours the other day, with this gorgeous weather we've been having. :)
 
georgestolz said:
there was a note: "Copper Conductors Only"

Would this be just for the lugs where I make my connection to the condenser, or would it be the branch circuit itself?
Click to expand...

No you must run copper all the way back to the POCOs generator.;)

George it is just for the actual connection, you could run AL to the unit splice on copper and connect that to the unit.

Secondly, does the 25% added in (resulting in the minimum circuit ampacity) cover continuous duty, or would I need to add 25% again for a continuous load?
Click to expand...

IMO yes you are already covered for the continuous load.

If you add the 25% 2 times you may be above the allowable circuit size.
 
iwire said:
IMO yes you are already covered for the continuous load.
Click to expand...
Do you have a reference to look at for that?

I'm not doubting, I'm just trying to have something I can lock in my head as gospel, so I can forget about it. :D

If you add the 25% 2 times you may be above the allowable circuit size.
Click to expand...
As in, the number we'd get would be above the MCA?

Thanks for the reply.
 
440.6(A) deals with this. It means that article 440 does not supercede article 100 (which applies to all sections of the code.)

Find a nameplate, add up the blowers and 125% compressor. Compare that to what the company has listed with UL and you'll see that they agree pretty closely. In any case, the AHJ should accept what's on the plate or there's a problem.
 
MCA is an abbreviation used by the hvac industry that stands for "minimum circuit ampacity". It represents the unit motor full load amperage X 1.25 or in the case of a unit with mutiple motors the "FLA" of the largezt motor x 1.25 plus the total combined FLA of all remaining motors x 1.00. MCA is used to determine minimum wire size requirements
 
MCA and MFA calculations for units with electrical heat from Trane cataloge.

MCA=1.25x(heater amps+all motor FLA's)
MFA or HACR type Circuit Breaker=(2.25xLargest Motor FLA)+Second Motor FLA+Heater Amps (If Applicable).

Normally manufactures will give the MFA (MOCP) and MCA for their units.

David
 
Status
Not open for further replies.
Top