Help with HVAC load calc

[electrofelon]

So If I am doing a service calculation for a building that has multiple Ac units, Am I correct that I just use the sum of all the "minimum circuit ampacity" on the nameplates, and do NOT need a continuous load or largest motor adder?
Thanks.

 

[electrofelon]

Just saw "HVAC feeder size" a few threads down. That helps, but still a little uncertain about the calc side of it. Didnt see anything specific mentioned in 220 or 440 on what numbers to use for the calc. Thanks.

 

[topgone]

So If I am doing a service calculation for a building that has multiple Ac units, Am I correct that I just use the sum of all the "minimum circuit ampacity" on the nameplates, and do NOT need a continuous load or largest motor adder?
Thanks.

You'll have a very big number by doing that. It's best that you secure the cut sheets of all the HVAC units and know each FLA's. Then find the largest load and so on.

 

[electrofelon]

You'll have a very big number by doing that. It's best that you secure the cut sheets of all the HVAC units and know each FLA's. Then find the largest load and so on.

I had a feeling that would include a bunch of fudge multiple times. I have a real hard time following 440 its super confusing. So I start with 440.6? But then what? It is not clear to me what to put together for the service load. There are all the 125% and 115% adjustments but those are all under "disconnecting means" and "branch circuit conductors"....Isnt there a "largest motor" adder, where is that? Do I have to shoot over to 430? Thanks for the help.

 

[topgone]

I had a feeling that would include a bunch of fudge multiple times. I have a real hard time following 440 its super confusing. So I start with 440.6? But then what? It is not clear to me what to put together for the service load. There are all the 125% and 115% adjustments but those are all under "disconnecting means" and "branch circuit conductors"....Isnt there a "largest motor" adder, where is that? Do I have to shoot over to 430? Thanks for the help.

Here's one nameplate I found:

As you can see, the MCA is basically 125% of largest motor plus load amps of fans (1.25 x 29.3 + 1.9 + 2.3 = 40.9A).

 

[david luchini]

I had a feeling that would include a bunch of fudge multiple times. I have a real hard time following 440 its super confusing. So I start with 440.6? But then what? It is not clear to me what to put together for the service load. There are all the 125% and 115% adjustments but those are all under "disconnecting means" and "branch circuit conductors"....Isnt there a "largest motor" adder, where is that? Do I have to shoot over to 430? Thanks for the help.

Article 440 doesn't address feeders, only branch circuits.

You are correct that you would have to use Art 430 for that.

 

[kwired]

I had a feeling that would include a bunch of fudge multiple times. I have a real hard time following 440 its super confusing. So I start with 440.6? But then what? It is not clear to me what to put together for the service load. There are all the 125% and 115% adjustments but those are all under "disconnecting means" and "branch circuit conductors"....Isnt there a "largest motor" adder, where is that? Do I have to shoot over to 430? Thanks for the help.

Not that much fudging multiple times unless every unit is a different size.

Say you have four of the units that is pictured by topgone. If you simply add the MCA of all four units you come up with 162.8 amps.

If you add RLA of compressors, FLA of fans and add 25% of one compressor you come up with 141.25 amps.

Get a higher number of units and those two methods get a wider gap. You are not really making anything unsafe by doing it wrong, but code wise can get away with the smaller supply.

 

[electrofelon]

Article 440 doesn't address feeders, only branch circuits.

You are correct that you would have to use Art 430 for that.

Ok now its making more sense

Say you have four of the units that is pictured by topgone. If you simply add the MCA of all four units you come up with 162.8 amps.

If you add RLA of compressors, FLA of fans and add 25% of one compressor you come up with 141.25 amps.

Get a higher number of units and those two methods get a wider gap. You are not really making anything unsafe by doing it wrong, but code wise can get away with the smaller supply.

Ill brush up on 430 later, have to run to work now, but in a nutshell its just the sum of all the RLA's, plus 25% of the largest motor served by the conductors? In other words a feeder or service with 5 units would just get ONE 25% of the largest motor of all 5?


The specs on this unit (its a 40 ton dehumidifier) state the following:

Largest load (compressor 1) 17.9
Other compressors and motors 66.5
Unit MCA 89 amps.

So it seems a bit strange at first but I guess it uses several compressors. 17.9 X 1.25 +66.5 gives the 89 figure. So in this case because the largest load is so small, I dont pick up much fudge by just adding the MCA's, only 27 amps for 6 units.

 

[JoeStillman]

It's no felony to add up the MCA's. But feeders only require 125% of the largest motor. Thats built into the MCA of each unit. You need all the nameplate data to NOT include it for each.

Sent from my SM-G900V using Tapatalk

 

[electrofelon]

So one final thing. Where does the continuous load factor come in? I always heard it was build into the MCA already, but going through the nameplate data and RLA's, I dont see it anywhere. Is it already included in the RLA's?

 

[topgone]

So one final thing. Where does the continuous load factor come in? I always heard it was build into the MCA already, but going through the nameplate data and RLA's, I dont see it anywhere. Is it already included in the RLA's?

Please see the post with a sample nameplate.

 

[electrofelon]

Please see the post with a sample nameplate.

I still dont understand. The 125% for largest motor and the 125% for continuous loads are different things no?

 

[GoldDigger]

There are three types of loads:
Continuous loads, non- continuous loads, and motor loads.
There is no 125% continuous factor for circuits serving motor loads.
A good question is whether the MCA for a unit containing a blower and strip heaters includes a 125% multiplier for the heaters, as well as whether the heaters would be classified as continuous loads in the first place.

 

[electrofelon]

There is no 125% continuous factor for circuits serving motor loads.

Ahh ok that explains it. I guess I never made that distinction.

A good question is whether the MCA for a unit containing a blower and strip heaters includes a 125% multiplier for the heaters, as well as whether the heaters would be classified as continuous loads in the first place.

Good question. I think every heat strip/duct heater I have come across has its own branch circuit.

 

[kwired]

Good question. I think every heat strip/duct heater I have come across has its own branch circuit.

But they are often on the same service or feeder as the compressor/blowers and when calculating service/feeders you only need 125% factored into largest motor and may or may not need 125% factored into the heat strip depending on if it is actually considered a continuous load.

 

[ramsy]

There is no 125% continuous factor for circuits serving motor loads.

If the Op's equipment is all HVAC, 440.6 nameplates prevail, and no separate motor adjustment applies, per 220.18(A).

For other 430.22 motor loads "conductors shall not be less than 125 percent of motor full-load current", per 220.18(A).

Rather than consider motors as "continuous" loads, I consider them "Inductive" loads, along with inductive lighting.
220.18(A-B) clarifies only Nameplates override 430.22, or 125% must be added for inductive loads before 310.15 De-rating adjustments.

 

[ramsy]

There are three types of loads:
Continuous loads, non- continuous loads, and motor loads.

Something like a GoldDigger's law is needed to prevent wire undersizing, when NEC-DeRating Tables are used with Motors (125% 430.22 is typically forgotten).

Perhaps there should be 4 types of loads:
Continuous loads, non-continuous loads, motor loads, and only after these 3 are load adjustments DeRated.

Adding more motor loads to GoldDigger's law may also help.
There are 4-types of Motor loads:
Nameplate loads, Inductive loads, Combinations loads, and only after these 3 are load adjustments DeRated.