HVAC Unit loads and demand factors

jjs

Member
Location
Puryear, TN, USA
So normally if we have equipment such as a natural gas rooftop unit, or other packaged equipment, I take the MCA of the nameplate, turn it into VA based on the voltage/phase and divide that by how many poles are feeding it. I put that value into each leg of the panelboard schedule. I take a 100% demand factor to size feeders, service, etc. The maximum overcurrent protection device is selected based on the value given on the nameplate.

Is this the correct way to do this?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
So normally if we have equipment such as a natural gas rooftop unit, or other packaged equipment, I take the MCA of the nameplate, turn it into VA based on the voltage/phase and divide that by how many poles are feeding it. I put that value into each leg of the panelboard schedule. I take a 100% demand factor to size feeders, service, etc. The maximum overcurrent protection device is selected based on the value given on the nameplate.

Is this the correct way to do this?
I would use the fla of the units and turn it into VA.

And then add 25% of the single largest motor.
 
I would use the fla of the units and turn it into VA.

And then add 25% of the single largest motor.
It is my understanding that the MCA given on the nameplate is exactly that.
But the issue is of you use the mca for multiple units, you are counting that extra 25% for every unit, vs if you use David's method, you don't "over count" and end up with a higher than needed feeder or service load.
 

JoeStillman

Senior Member
Location
West Chester, PA
Sounds like you're filling in the loads on a panel schedule. Using published MCA as the actual connected load is conservative, as it will include the extra 25% of the largest motor.

If you want to use the actual connected load, you have to dig deeper into the electrical data for the mechanical equipment. Be sure you account for all the motors. MCA is always the right way to size the wire and MOCP for the breaker, but neither of these can be solved for connected load.
 

jjs

Member
Location
Puryear, TN, USA
220.14(C) refers to 440.6 for service/feeder sizing
440.6(A) exception 1 seems to say use the MCA for sizing if there is a equipment nameplate
440.6(B) Seems to say to use the nameplate rating also.
I don't see the 125% in 440.6
In Mike Holt's book Understanding the NEC Volume 1 (2014) in the introduction it says "Thus, Article 440 requires you to use the nameplate circuit protection rather than what you develop from applying Article 430"

Then it says :
"Three key points to remember so you apply Article 440 correctly are:
1. Hermetic motors are different. ...
2. Use the NAMEPLATE information.
3. ..."

Lots of times I am only given MCA/MOCP and I don't have the individual motor values. But it sure seems to me like the MCA is the value to be used based on the wording in the code.
Can someone explain to me what I am missing?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
220.14(C) refers to 440.6 for service/feeder sizing
440.6(A) exception 1 seems to say use the MCA for sizing if there is a equipment nameplate
440.6(B) Seems to say to use the nameplate rating also.
I don't see the 125% in 440.6
In Mike Holt's book Understanding the NEC Volume 1 (2014) in the introduction it says "Thus, Article 440 requires you to use the nameplate circuit protection rather than what you develop from applying Article 430"

Then it says :
"Three key points to remember so you apply Article 440 correctly are:
1. Hermetic motors are different. ...
2. Use the NAMEPLATE information.
3. ..."

Lots of times I am only given MCA/MOCP and I don't have the individual motor values. But it sure seems to me like the MCA is the value to be used based on the wording in the code.
Can someone explain to me what I am missing?
There's nothing in Article 440 about feeders, only branch circuits.

430.24 talks about Several motors or a Motor(s) and Other Load(s)
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Sorry should have been 220.50 which says the same thing.
And 220.50 directs you to 430.24, which is for Several Motors or a Motor(s) and Other Load(s), which is what you have.

In addition, 440.1 Scope says
The provisions of this article apply to electric motor-driven air-conditioning and refrigerating equipment and to the BRANCH CIRCUITS and controller for such equipment.
In addition, 440.3(A) Article 430 says
These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in the Code, which apply except as modified in this article.
 

jjs

Member
Location
Puryear, TN, USA
Not saying you are wrong David. But that interpretation brings up lots of questions in my mind. I appreciate all the time you have already given this.

And 220.50 directs you to 430.24, which is for Several Motors or a Motor(s) and Other Load(s), which is what you have.
I don't believe that is true. I have a piece of refrigeration equipment. The multiple motors 430 is talking about are standalone motors that are being connected to the same circuit by the electrician. Not a packaged unit that is assembled by the manufacturer that then puts a nameplate on the entire unit.
In 430.1 there is a FPN#1 says ... A/C and refrigerating equipment are covered in Article 440.

In addition, 440.1 Scope says
The provisions of this article apply to electric motor-driven air-conditioning and refrigerating equipment and to the BRANCH CIRCUITS and controller for such equipment.
Yes it does. And 220.50 says to use those rules to size your feeders and services for A/C equipment
430.1 scope does not refer to services. But in 220.50 it directs us to 430 for our service sizing requirements when we have motors.
Why would the same not apply to 440.6 when 220.50 directs us there for feeders and services?

In addition, 440.3(A) Article 430 says
These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in the Code, which apply except as modified in this article.
Yes, and 440.6 specifically amends what 430 says, so they take precedence over article 430

If I have a dishwashing machine. I don't try and figure out the FLA of the motor for water pump, the Amps of the heating element, the fla of the possible garbage disposal motor, the control circuits, etc. It is an appliance and they give me a completed nameplate value for it. 440 seems to do the same for packaged hvac eqiupment.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Yes, and 440.6 specifically amends what 430 says, so they take precedence over article 430
There is nothing in 440.6 about feeders. There is nothing in Art 440 at all about feeders, so there is nothing to "take precedence" over the feeder requirements in Art 430.

Art 440 tells you in plain language that it is about equipment with hermetic refrigerant compressors and about their Branch Circuits. It also tells you that Art 430 also applies to that equipment. I don't know how it could be more clear that you would have to look to Art 430 when sizing feeders for that equipment.
 

jjs

Member
Location
Puryear, TN, USA
There is nothing in 440.6 about feeders. There is nothing in Art 440 at all about feeders, so there is nothing to "take precedence" over the feeder requirements in Art 430.

Art 440 tells you in plain language that it is about equipment with hermetic refrigerant compressors and about their Branch Circuits. It also tells you that Art 430 also applies to that equipment. I don't know how it could be more clear that you would have to look to Art 430 when sizing feeders for that equipment.
So you think that 220.50 which is about feeders and services tells you to look at 440.6, but 440.6 does not apply because it talks about branch circuits?
Seems odd that they would send me to a dead-end.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
So you think that 220.50 which is about feeders and services tells you to look at 440.6, but 440.6 does not apply because it talks about branch circuits?
Seems odd that they would send me to a dead-end.
440.6 tells you how to select the motor current for a hermetic refrigerant motor-compressor, which is different for how you select the current for a regular motor. As such, 220.50 directs you to 440.6 as well as 430.24.

Maybe this will help...look at 430.62 which tells you how to size the Motor Feeder scgf protection.

430.62 tells you that a feeder supplying motor loads and consisting of conductor sizes BASED ON 430.24 (ie, the load calculated in 430.24), shall have a protective device not greater than the largest rating or setting of of a branch circuit scgf protection, based on the maximum permitted value in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors, plus the sum of the full-load currents of the other motors of the group.

That is saying that the motor feeder overcurrent protection will be based on the largest branch overcurrent device for a motor (from Art 430) or a hermitic refrigerant motor compressor (from Art 440) plus the sum of the full load currents of all of the other motors. It further tells you that the feeder conductor size for those motors and hermetic refrigerant motor-compressors (that is to say, the load on the feeder) is sized per 430.24, which is based on 125% of the largest motor plus the sum of the full load currents of all of the other motors.
 

jjs

Member
Location
Puryear, TN, USA
440.35 is referring to the equipment I originally asked about, packaged rooftop units.
It says use the nameplate MCA.

I have never seen someone provide SCGF to a packaged rooftop unit? That stuff is all built-in to the unit. They want us to provide the MOCP - maximum OVERCURRENT protection device (sometimes called MOPD)

We generally have a circuit breaker in the panelboard for the overcurrent device and then a general non-fused disconnect at the unit. I have never seen a set of drawings with SCGF specified for a rooftop unit per the sections you cite. And I have never seen them installed in the field per that section. They exist as part of the equipment as provided by the manufacturer.

Wouldn't it ruin the labeling and/or listing if you go and add SCGF inside a rooftop unit?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
440.35 is referring to the equipment I originally asked about, packaged rooftop units.
It says use the nameplate MCA.

I have never seen someone provide SCGF to a packaged rooftop unit? That stuff is all built-in to the unit. They want us to provide the MOCP - maximum OVERCURRENT protection device (sometimes called MOPD)

We generally have a circuit breaker in the panelboard for the overcurrent device and then a general non-fused disconnect at the unit. I have never seen a set of drawings with SCGF specified for a rooftop unit per the sections you cite. And I have never seen them installed in the field per that section. They exist as part of the equipment as provided by the manufacturer.

Wouldn't it ruin the labeling and/or listing if you go and add SCGF inside a rooftop unit?
The mocp is the SCGF protection. Every packaged rooftop gets a branch circuit SCGF protection per 440.22. A feeder supplying several packaged rooftop units would get feeder SCGF protection per 430.62.
 

kwired

Electron manager
Location
NE Nebraska
440.35 is referring to the equipment I originally asked about, packaged rooftop units.
It says use the nameplate MCA.

I have never seen someone provide SCGF to a packaged rooftop unit? That stuff is all built-in to the unit. They want us to provide the MOCP - maximum OVERCURRENT protection device (sometimes called MOPD)

We generally have a circuit breaker in the panelboard for the overcurrent device and then a general non-fused disconnect at the unit. I have never seen a set of drawings with SCGF specified for a rooftop unit per the sections you cite. And I have never seen them installed in the field per that section. They exist as part of the equipment as provided by the manufacturer.

Wouldn't it ruin the labeling and/or listing if you go and add SCGF inside a rooftop unit?
You seen it on every unit installed unless for some reason you supplied the unit directly with service conductors. The main supply to the unit still has short circuit and ground fault protection on it, more so for the supply conductors than for the unit itself, overload protection is inherently provided because of overload protection on the motors/compressors in the unit.
 
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