HVAC Design Load

[electrofelon]

if the MCA is used to determine the actual load of the HVAC, in this case the MCA 38 amps, a 40 amp breaker will not suffice, unless i am missing something here.

The MCA figure already includes the 125% of largest motor safety factor. Also the motors are very unlikely to draw their FLA. Thus you will usually see actual load to be roughly 2/3 of the MCA.

 

[MyCleveland]

220.3 references Art 440, section IV, which tells you the minimum ampacity (mca) required for branch circuit conductors.

In other words, the branch circuit load is calculated by the required branch circuit minimum ampacity.

David
I was so hoping you were wrong on this, but unfortunately for me you are not.
I am still not 100% onboard...like that matters...but I reached out to someone at NFPA and they took the time for telephone conversation.
If I am understanding what you stated and they confirmed if I have say 50 HVAC units all the same that have 80amps FLA and 100A MCA I need to account for 5000amps in my service size rather 4000 based on the FLA values.
He recognizes this builds in a 25% buffer even though this is NOT continuous load.

This same approach needs to apply to ALL individually fed motors which builds in another 25% buffer for total motor load.
Are you in agreement with these statements ?

The last MH book I have on "Understanding the Code" is a 2005 version. In discussion on 220 it stated motor load TOTAL on a service is 25% of largest + balance of all others.
Is this correct ? or as stated above ?
If this is correct, it seems the same logic could be applied to art 440.

 

[david luchini]

I was so hoping you were wrong on this, but unfortunately for me you are not.
I am still not 100% onboard...like that matters...but I reached out to someone at NFPA and they took the time for telephone conversation.
If I am understanding what you stated and they confirmed if I have say 50 HVAC units all the same that have 80amps FLA and 100A MCA I need to account for 5000amps in my service size rather 4000 based on the FLA values.
He recognizes this builds in a 25% buffer even though this is NOT continuous load.

This same approach needs to apply to ALL individually fed motors which builds in another 25% buffer for total motor load.
Are you in agreement with these statements ?

That does not sound correct. For those 50 HVAC units, you would need to account for 4020A in your service size. This would fall under Article 430.24.

 

[Eddie702]

what is the proper method when doing load calculations for HVAC in a commercial application. i have a plan reviewer that wants to see the justification for the loads. it is not as easy as just looking at the nameplate on the unit, where you have MOP and MCA. i typically use 440.22(B)1. i find these motor values on the data sheet of the unit, and also include heat strip if applicable, and plug that KW number into the total design load, i am using 220.18(A) for motor loads and 220.51 for heat load, as a basis for design, but do you or should you consider the AC compressor/motors a continuous load ? and therefore add the 1.25,. i cannot find a specific code reference in article 220 or 440 to state requirements

Just my opinion but I usually consider all HVAC equipment to be a continuous load. The branch circuit must be sized to the MCA on the equipment nameplate taking into consideration that you must use the higher of the load during heating or cooling if they do not run at the same time. If your doing a Lab or other critical temp controlled space the heating and cooling may run at the same time to control the space temp (depends on the unit design)

The feeder over current device must be rated for 125% of the continuous load. To me this is the running amps of all equipment x 1.25. Since the feeder wire must match the breaker size you can calculate that.

As far as the service goes I would calculate that the normal way. If the equipment is oversize and will not be continuous there is nothing you can do about that. JMHO

As far as calculating the running amps off the MCA that is difficult. Older units used larger compressors. A 30 ton unit in the "old days" would probably have 2 15 ton compressors or maybe 1 20 ton and 1 10 ton.

Newer units that size will probably have 4 or 5 compressors so the MCA will change based on the largest compressor.

Also, some day those units will be changed out so some fudge should be included however that isn't necessary if you know the exact unit they are using.

 

[drktmplr12]

1.25 of the compressor load plus fans

or

kW of heating element plus the fans plus 0.25 of largest fan

whichever is greater.

 

[JoeStillman]

This is not right. I have a cutsheet of a unit with motors of 11A, 9.7A, 6.2A, 2.1A, 2.1A, 1.6A, 1A and 0.29A.

The cutsheet lists the unit FLA as 33.99A (the sum of the currents above) and the unit MCA as 36.7 (the FLA plus 25% of the largest motor.)

Taking the MCA divided by 1.4 is 26.21A which is 7.78A lower than the actual unit FLA. That would lead to the service being undersized.

That's the problem with the Rule Of Thumb. Sometimes you need a bigger thumb.

 

[electrofelon]

Just my opinion but I usually consider all HVAC equipment to be a continuous load. The branch circuit must be sized to the MCA on the equipment nameplate taking into consideration that you must use the higher of the load during heating or cooling if they do not run at the same time. If your doing a Lab or other critical temp controlled space the heating and cooling may run at the same time to control the space temp (depends on the unit design)

The feeder over current device must be rated for 125% of the continuous load. To me this is the running amps of all equipment x 1.25. Since the feeder wire must match the breaker size you can calculate that.

As far as the service goes I would calculate that the normal way. If the equipment is oversize and will not be continuous there is nothing you can do about that. JMHO

As far as calculating the running amps off the MCA that is difficult. Older units used larger compressors. A 30 ton unit in the "old days" would probably have 2 15 ton compressors or maybe 1 20 ton and 1 10 ton.

Newer units that size will probably have 4 or 5 compressors so the MCA will change based on the largest compressor.

Also, some day those units will be changed out so some fudge should be included however that isn't necessary if you know the exact unit they are using.

If I may nitpick a bit: Motors have their own rules and are not continuous loads like may other things might be. There is a 25% factor commonly used in article 430 for calculating ampacities and sizing overloads, so on paper it may the same effect, but it is "different"

 

[topgone]

I have attached a cut sheet( I think) from a typical Bard wall mount HVAC, i just don't understand how you use MCA to calculate a circuit load, much less the breaker size,in the example highlighted, you have MCA 38 amps and a MOP 40, just does not add up.

There you go! Your posted info tells your unit is a Bard WL5S2-B09, a 5-ton split-type airconditioner! LINK