HVAC Unit loads and demand factors

[jjs]

That is what I get for posting when I only had two hours of sleep.
But at least it prompted someone else to jump into the discussion.

 

[jjs]

440.3(A) ... in addition to, or Amendatory of ... 430
440.3(B) The rules of Articles ... 430 as applicable shall apply to air-conditioning and refrigerant equipment that does NOT incorporate a hermetic refrigerant motor-compressor.

440.6 The required ampacity of conductors and rating of equipment shall be determined according to 440.6(A) and (B). Then 220.50 refers you to 440.6 for services and feeders, which is conductors and equipment.

220.50 refers you to section 430 for service sizing information. 430 does not mention service sizing information, at least it is not in the scope of 430.1. So according to the logic earlier that we don't use 440.6 for services and feeders because only branch circuits are mentioned in 440.6, then wouldn't it seem we have no way to size services that have motors or hermetic compressors installed?

 

[david luchini]

then wouldn't it seem we have no way to size services that have motors or hermetic compressors installed?

No, it doesn't seem that way at all.

 

[kwired]

440.3(A) ... in addition to, or Amendatory of ... 430
440.3(B) The rules of Articles ... 430 as applicable shall apply to air-conditioning and refrigerant equipment that does NOT incorporate a hermetic refrigerant motor-compressor.

440.6 The required ampacity of conductors and rating of equipment shall be determined according to 440.6(A) and (B). Then 220.50 refers you to 440.6 for services and feeders, which is conductors and equipment.

220.50 refers you to section 430 for service sizing information. 430 does not mention service sizing information, at least it is not in the scope of 430.1. So according to the logic earlier that we don't use 440.6 for services and feeders because only branch circuits are mentioned in 440.6, then wouldn't it seem we have no way to size services that have motors or hermetic compressors installed?

Because the 220 section refers you there, you are using it for service/feeder purposes. What 220 has that 430 doesn't have is potential demand factors that may be applied, though the potential to apply demand factors still isn't that great for motors. Few items get to apply demand factors to branch circuit level, ranges and welders are a couple items where you can.

 

[jjs]

No, it doesn't seem that way at all.

So what load do we use for motors for services?

 

[kwired]

So what load do we use for motors for services?

The sections in 430 that 220 referred you to.

 

[david luchini]

So what load do we use for motors for services?

..

220.50 directs you to 430.24, which is for Several Motors or a Motor(s) and Other Load(s)

 

[jjs]

The sections in 430 that 220 referred you to.

But David said we could not use 440.6 for sizing feeders and services for hermetic compressor motors, even tho the code in 220.50 told us to go there, because it did not mention feeders or services in section 440.6.
430 does not mention services, so by the same logic, we cannot use 430 for sizing services, in regards to motors, even tho 220.50 told us to, because 430 does not mention services.

 

[david luchini]

But David said we could not use 440.6 for sizing feeders and services for hermetic compressor motors, even tho the code in 220.50 told us to go there, because it did not mention feeders or services in section 440.6.
430 does not mention services, so by the same logic, we cannot use 430 for sizing services, in regards to motors, even tho 220.50 told us to, because 430 does not mention services.

What do you see in 440.6 that tells you how to size a feeder for multiple motors? There is nothing there. It is in 430.24. The load calculation for multiple motors on a feeder is the same for multiple motors on a service.

There's nothing really complicated about this.

 

[jjs]

What do you see in 440.6 that tells you how to size a feeder for multiple motors? There is nothing there. It is in 430.24. The load calculation for multiple motors on a feeder is the same for multiple motors on a service.

There's nothing really complicated about this.

Why is there an article 440 at all since it seems in your estimation there is no difference between equipment that utilizes a hermetic compressor and regular motors?

Why would 220.50 say go to 440.6 when you say 440.6 has nothing to do with services and feeders?

that is the complicated part in my mind. The code does not just have random articles that you should never use, or at least it should not after 20 or more years of revisions.

 

[jjs]

The sections in 430 that 220 referred you to.

so why don't we use the section in 440 that 220 referred us to for services and feeders for hermetic compressors?

 

[david luchini]

Why is there an article 440 at all since it seems in your estimation there is no difference between equipment that utilizes a hermetic compressor and regular motors?

Why would 220.50 say go to 440.6 when you say 440.6 has nothing to do with services and feeders?

that is the complicated part in my mind. The code does not just have random articles that you should never use, or at least it should not after 20 or more years of revisions.

I don't think you're reading what 440.6 actually tells you. Compare 430.6 to 440.6 and see if makes any sense to you.

 

[jjs]

What do you see in 440.6 that tells you how to size a feeder for multiple motors?

I guess the biggest difference is that when I have a rooftop unit with a nameplate from the manufacturer, I have a piece of equipment with a hermetic motor in it, along with a bunch of other stuff, and that is what I am sizing for, not multiple motors, the type of which are covered in 430.

The other difference I see is that 430.24 refers you to 430.6 which applies to motors that are not hermetic compressors, and then refers you to a table that has you look up the HP of the motor and translate it into FLC to do your calcs. Hermetic compressor motors have rated load current on the nameplate and are not required to have HP marked. A packaged rooftop equipment could have redundant loads in it. Without the sequence of operations, you would not know that. All of these reasons are why 440 seems to tells us to use the equipment nameplate data for this type of equipment.

 

[david luchini]

I guess the biggest difference is that when I have a rooftop unit with a nameplate from the manufacturer, I have a piece of equipment with a hermetic motor in it, along with a bunch of other stuff, and that is what I am sizing for, not multiple motors, the type of which are covered in 430.

The other difference I see is that 430.24 refers you to 430.6 which applies to motors that are not hermetic compressors, and then refers you to a table that has you look up the HP of the motor and translate it into FLC to do your calcs. Hermetic compressor motors have rated load current on the nameplate and are not required to have HP marked. A packaged rooftop equipment could have redundant loads in it. Without the sequence of operations, you would not know that. All of these reasons are why 440 seems to tells us to use the equipment nameplate data for this type of equipment.

220.50 directs you to 430.24 for the calculation of motor loads for feeders and services.

430.24 tells you to take 125% of the largest motor (as determined by 430.6(A)) plus the sum of the full load currents of the other motors (as determined by 430.6(A)) as the load for several motors.

430.6(A) tells you that the current rating of the motor shall be determined by the Tables 430.247 thru 430.250.

440.6 MODIFIES the provisions of 430.6 when dealing with Air-Conditioning and Refrigerating Equipment. (If you recall we talked about how Art 430 applies to Art 440 equipment, except as modified by Art 440.)

440.6(A) says that the Rated Load Current of the compressor shall be used as the current rating (instead of the current rating from Tables 430.247 thru 430.250.)

440.6(B) says that the marked full load current of fans or blowers in Art 440 Equipment shall be used as the current rating (instead of the current rating from Tables 430.247 thru 430.250.)


So, imagine you had a 480V feeder supplying a 5HP pump, a 1HP fan, and a piece of Air Conditioning Equipment that had a 5HP compressor with an RLA of 6.5A and a 1HP blower with an FLA of 1.8A.

Per 220.50 and 430.24 (and 440.6) the load on the feeder would be:

5HP pump = 7.6A*1.25 = 9.5A
1HP fan ......................= 2.1A
5HP Comp ..................= 6.5A
1HP blower .................= 1.8A
Total.......................... = 19.9A

The MCA of the Air Conditioning Equipment (which would be 9.93A) does not figure into the load calculation for the feeder at all.

 

[jjs]

David,
You are assuming all those motors are expected to run at the same time. You don't know based on the individual nameplate data of each motor. So your load could be significantly higher than what is required for the actual load. That is why in 440.4(B)Multimotor and combination load equipment shall be provided with a visible nameplate ... the ampacity shall be calculated ... counting all motors and other loads that will be operated at the same time.

 

[jjs]

David, I really appreciate all your time in trying to convince me of your interpretation. I am not saying in the end I won't agree with you, but it just still doesn't make sense that way to me at this point, but I am keeping an open mind.

 

[david luchini]

David,
You are assuming all those motors are expected to run at the same time. You don't know based on the individual nameplate data of each motor. So your load could be significantly higher than what is required for the actual load. That is why in 440.4(B)Multimotor and combination load equipment shall be provided with a visible nameplate ... the ampacity shall be calculated ... counting all motors and other loads that will be operated at the same time.

The minimum ampacity for the multimotor equipment is calculated by adding the currents of all the motors. See 440.33.

And if the equipment has interlocks which prevent all of the motors from running at the same time, then 430.26 lets you use a feeder conductor with an ampacity less than required for 430.24.

 

[kwired]

If you had a RTU unit with heat pump and electric backup heat - MCA would generally be based on the heat rating plus indoor blower, as that would be the highest load that can run at one time, unless it is capable of running heat pump and backup heat at same time - some may do this for short time during defrost but otherwise usually don't run at same time.

Reality is when it is cold outside and it is in heat mode, those compressors don't run anywhere close to RLA, they are not moving enough heat to load them that much.

 

[david luchini]

Let's say a feeder is supplying 3 rooftop units, each with a compressor with an RLA of 18A, a condenser fan with an FLA of 2.5A, and a blower with an FLA of 11A.

Each unit has a nameplate that says the MCA is 36.

With all of the motors running at the same time, the current on the feeder will be 94.5A.

According to 430.24, the load on the feeder is 99A, and the feeder would have to be at least #3AWG conductors.

If I just added the nameplate MCA of the units, I would calculate the load at 108A and the feeder conductors would have to be at least #2 AWG.

I'd rather use the #3 feeder conductors for an actual current of 94.5A.

 

[kwired]

Let's say a feeder is supplying 3 rooftop units, each with a compressor with an RLA of 18A, a condenser fan with an FLA of 2.5A, and a blower with an FLA of 11A.

Each unit has a nameplate that says the MCA is 36.

With all of the motors running at the same time, the current on the feeder will be 94.5A.

According to 430.24, the load on the feeder is 99A, and the feeder would have to be at least #3AWG conductors.

If I just added the nameplate MCA of the units, I would calculate the load at 108A and the feeder conductors would have to be at least #2 AWG.

I'd rather use the #3 feeder conductors for an actual current of 94.5A.

Yes, and also keep in mind those units will never draw full rated amps, unless they are about worn out maybe. On a hot day with the condenser plugged with cottonwood seeds - it may draw hard, it won't cool inside space very well either and the HVAC guy is getting called out to see what is wrong.