HVAC calcs

[alixenos]

Hello,

I'm trying to learn how to size conductors for different HVAC equipment and I'm confused as to what to use as far as the following:


1- MCA/MFA = 1.9/15 @ 208/1phase/60 HZ. What current should be used to calculate the KVA? and what should be used to size the conductor?

2- MCA/MOCP = 58/70 @ 208/3phase/60HZ. what current should be used to calculate the KVA? and what should be used to size the conductor?

3- FLA/MCA/MOCP = 2.4/20,37/25 @ 208/3phase/60HZ. same question as above.

4- RLA/MCA/MOCP = 7.1/9/15 @ 208/3phase/60HZ. same question as above.

Thank you so much.

 

[infinity]

The MCA is fairly useless because it has 125% added to the largest load unless there is only a single load then you can multiply by .8. IMO you should use the provided FLA and RLA.

 

[augie47]

+ you use the MCA to size the conductor and MAX to size the largest over current device allowed

 

[wwhitney]

For conductors/OCPD for a single HVAC unit, you just need the MCA and MOCP. [MFA = Max Fuse Allowable? Same idea as MOCP.] The conductors needs to have an ampacity of at least MCA. The OCPD needs to me no more than MOCP. The load in VA is based on MCA: MCA * V for 2-wire motors, MCA * V_L-L * sqrt(3) for 3-wire 3 phase motors.

Once you have conductors supplying multiple motors or HVAC units, you need to "unpack" how MCA and MOCP are calculated. MCA for a single unit is 125% * the largest motor's FLA (or RLA for compressors) plus 100% of all the other loads. MOCP is MCA + another 125% * largest FLA/RLA, rounded down to a standard breaker size. [Equivalently, the MCA formula with 250% replacing 125%, then rounded down.]

The point is that with multiple pieces of equipment, you still only include the 125% (or 250% for MOCP) of the single largest FLA/RLA supplied. So let's take a feeder that supplies just two of your #4 units. 125% * RLA = 8.9A, so there's 0.1A of other loads for an MCA of 9.0. And 250% * RLA = 17.8A, which rounds down to 15A.

So for two of the units, the computations are MCA = 125% * 7.1 + 7.1 + 0.1 + 0.1 = 16.2A. And the MOCP is 250% * 7.1 + 7.1 + 0.1 + 0.1 = 25.2A, rounded down to 25A OCP.

Once you have this combined MCA calculation, the load in VA is calculated the same as for a single unit. Although I guess the discussion so far has been for all motors single phase or all motors 3 phase; some further thought would be required for the case of some 3 phase motors and some single phase motors.

[I'm using HVAC and motor interchangeably here, as for motor driven HVAC equipment, the only difference in the formulas is that a motor driving a compressor may have an RLA rating (rated load amps) instead of a FLA (full load amps) rating.]

Cheers, Wayne

 

[alixenos]

For conductors/OCPD for a single HVAC unit, you just need the MCA and MOCP. [MFA = Max Fuse Allowable? Same idea as MOCP.] The conductors needs to have an ampacity of at least MCA. The OCPD needs to me no more than MOCP. The load in VA is based on MCA: MCA * V for 2-wire motors, MCA * V_L-L * sqrt(3) for 3-wire 3 phase motors.

Once you have conductors supplying multiple motors or HVAC units, you need to "unpack" how MCA and MOCP are calculated. MCA for a single unit is 125% * the largest motor's FLA (or RLA for compressors) plus 100% of all the other loads. MOCP is MCA + another 125% * largest FLA/RLA, rounded down to a standard breaker size. [Equivalently, the MCA formula with 250% replacing 125%, then rounded down.]

The point is that with multiple pieces of equipment, you still only include the 125% (or 250% for MOCP) of the single largest FLA/RLA supplied. So let's take a feeder that supplies just two of your #4 units. 125% * RLA = 8.9A, so there's 0.1A of other loads for an MCA of 9.0. And 250% * RLA = 17.8A, which rounds down to 15A.

So for two of the units, the computations are MCA = 125% * 7.1 + 7.1 + 0.1 + 0.1 = 16.2A. And the MOCP is 250% * 7.1 + 7.1 + 0.1 + 0.1 = 25.2A, rounded down to 25A OCP.

Once you have this combined MCA calculation, the load in VA is calculated the same as for a single unit. Although I guess the discussion so far has been for all motors single phase or all motors 3 phase; some further thought would be required for the case of some 3 phase motors and some single phase motors.

[I'm using HVAC and motor interchangeably here, as for motor driven HVAC equipment, the only difference in the formulas is that a motor driving a compressor may have an RLA rating (rated load amps) instead of a FLA (full load amps) rating.]

Cheers, Wayne

Thank you very, very much. this is very helpful.