ac units and supply fans

[legato]

Hi everyone,

I'm the electrical designer for a T/I which calls for new rooftop units...I found a few threads with similar multi-motor situations...And wanted to make sure I'm on the same page.

I'd like to know if I am correctly sizing a feeder to feed an rooftop AC unit and supply fan. Voltage is at 460V 3-phase.

The AC unit is a 5-Ton AAON that states FLA 13A, MCA 15A, MOCP 20A
The supply fan is a 1HP fan with FLA 2.1 per NEC.

So to properly size the feeder: 13 + 2.1 = 15.1A
The breaker feeding the two units would be 175% of 15.1 so 1.75x15.1=26.4 or a 25A breaker.

I intend to put a fused disconnect with 20A fuses for the AC and a fused disconnect with 3A fuses for the fan.

There are 3 of these going on top of an existing 1975 single-story wood construction building.

Thanks much!

 

[augie47]

Hi everyone,

I'm the electrical designer for a T/I which calls for new rooftop units...I found a few threads with similar multi-motor situations...And wanted to make sure I'm on the same page.

I'd like to know if I am correctly sizing a feeder to feed an rooftop AC unit and supply fan. Voltage is at 460V 3-phase.

The AC unit is a 5-Ton AAON that states FLA 13A, MCA 15A, MOCP 20A
The supply fan is a 1HP fan with FLA 2.1 per NEC.

So to properly size the feeder: 13 + 2.1 = 15.1A
The breaker feeding the two units would be 175% of 15.1 so 1.75x15.1=26.4 or a 25A breaker.

I intend to put a fused disconnect with 20A fuses for the AC and a fused disconnect with 3A fuses for the fan.

There are 3 of these going on top of an existing 1975 single-story wood construction building.

Thanks much!

I find this section a bit confusing. Perhaps someone will enlighten both of us.
I don't see where Art 440 addresses feeders which is what you have but
according to 440.33 for motor compressors with additional loads, you would use the greater of the rated full load or branch circuit selection current, so I believe you would use the MCA for the compressor (15) plus the sum of the other loads. But then it goes on to say plus 25% of the highest rating of motors, but you would think that is already added into the MCA.

 

[legato]

I find this section a bit confusing. Perhaps someone will enlighten both of us.
I don't see where Art 440 addresses feeders which is what you have but
according to 440.33 for motor compressors with additional loads, you would use the greater of the rated full load or branch circuit selection current, so I believe you would use the MCA for the compressor (15) plus the sum of the other loads. But then it goes on to say plus 25% of the highest rating of motors, but you would think that is already added into the MCA.

Hi Augie,

Yeah thats what I was saying. I'm assuming the AC unit's MCA is taking the 25% into consideration.

Well thanks for the further input

 

[augie47]

Be patient... there are a bunch of really knowledgable folks here...what one of us does not understand, another will....

 

[david luchini]

The supply fan is a separate unit from the AAON A/C unit?

I've generally seen AAON units where the fans and compressors are together as one unit.

 

[skeshesh]

I have the same question as david. If they are indeed two seperate units, I would not use the MCA. In that case I'd use 430.24, as in FLA of the largest unit x 1.25 + rest of the load to size the conductor. Assuming both units have integral motor overload protection, I would also size the OCPD for short circuit protection based on the larger unit using table 430.52. I may be wrong as far as the operation of the two units, but I would guess that both would not start in close enough an interval to require short circuit sizing for both. In addition I'm pretty sure if you look at the I-T curves for a breaker of that size (around 20A) would you find that even that the breaker would allow for a start up current several times the sum of both FLAs for a couple of seconds.

 

[defears]

All packaged roof top units I've ever seen are 1 unit, and the 125% is built in to the MCA. It's in the UL whitebook.

I think this whole section of the code was written when everyone was make shifting compressors before they were common. I've been in the trade for 15 years only, but have never seen a normal RTU in 2 pieces. Except small split system A/C's for server rooms and the like.

By the way I checked a running RTU with my amp meter and was way lower than I expected too.

 

[legato]

Clarifications

Clarifications

Hi all,

Thanks for the responses. There is a need for the additional supply fan as the intent of the mechanical design as described to me by the M.E. is that the additional supply fan would ramp up as the filter in the AC unit would get dirty.

There is an electronic interlock that will tie into the BMS.

The additional supply fan is to be provided with a VFD.

 

[gndrod]

One Motor?

One Motor?

Need a little more data. What is the model No. and is this a single package RTU with a scrolled setup?

 

[legato]

Design based on AAON

I can't seem to upload the file, but the model # that we were provided

RQ-005-3-V-EA09-212:L000-E00-QKD-0BB-C-G000D-00-0000000AB

Here's some further electrical info:
Compressor: 460V3P, 8.1RLA
Condenser Fan: 230V1P, 1100RPM, 2.8FLA
Supply Fan: 2HP, 460V3P, 1760RPM, 3.4FLA
Combustion: 0.09HP, 460V1P, 3000RPM, 0.7FLA

For the additional Supply Fan:
Cook
150SQN-B
1725RPM 460V3P
1HP
Belt drive

 

[ptonsparky]

Been a long time since I've dealt with RTUs but why is there a 230v motor on the condensing fan when everything else is 460?

 

[Mgraw]

The Cook Suppy Fan is an inline duct "booster" fan. It is a seperate unit from the RTU. These are usually used because of long duct runs or the duct is smaller than needed or to move alot of air to an area when needed. These are usually installed indoors. The model posted has an external motor.

Edit: I agree with you ptonsparky 230 v motor does seem strange.

 

[legato]

Hi Guys, I've inquired the mechanical engineer on that 230V motor.

Hopefully I get a response today.

I'll post an update once I get it!

Thanks again for your review on this

 

[legato]

Hi All,

Mechanical noted they are going with a Carrier 5-Ton in lieu of the AAON. The cook supply fan info is still good.

Carrier Unit 48PG 06 series
460V-3-60
Compressor RLA @ 7.7A, LRA @ 50A
OFM FLA @ 0.8A
Combustion Fan FLA @ 0.8
IFM @ 2.6A
No power exhaust
MCA @ 15.8A
Fuse/HACR Breaker @ 20A
Disconnect Size FLA 16A, LRA 70A

So with this info...I am looking at:
MCA:1.25x 7.7 + 0.8 + 0.8 + 2.6 + 2.1 (1HP Supply Fan) = 15.925A
MOCP: 2.5x15.925 = 39.8125A so 35A breaker at the panel and a heavy duty 60A disconnect switch with 35A fuses?

 

[defears]

If the above info is correct, there is nothing to do. You can use #14, if no voltage drop, and 20 amp breaker and fuses. And a 30 amp disco, why waste money on a 60 amp disco? Then you would need fuse reducers.

You don't add the 125%. It's built in. If you put an amp meter on it, it will probably draw around 12 amps.

The inline fan should be on a separate circuit weather it's 230 or 480.

 

[legato]

^ Thanks for your thoughts.

So even after doing the calcs, and adding the 1HP load to the Rooftop Unit, it looks like I'm good to go with what you've noted.

 

[defears]

MCA:1.25x 7.7 + 0.8 + 0.8 + 2.6 + 2.1 (1HP Supply Fan) = 15.925A
You don't need to do it. What you did there is already done for you with,
MCA @ 15.8A.

Fuse/HACR Breaker @ 20A
Tells us the maximum fuse size is 20. If you put a bigger fuse in than a 20 amp, you will void the warranty and the listing.

All of this
Carrier Unit 48PG 06 series
460V-3-60
Compressor RLA @ 7.7A, LRA @ 50A
OFM FLA @ 0.8A
Combustion Fan FLA @ 0.8
IFM @ 2.6A
No power exhaust

is just information about the parts.

MCA @ 15.8A and Fuse/HACR Breaker @ 20A is all the electrician needs

You are thinking too hard, relax.

 

[mtfallsmikey]

The official Stupid Question:

The official Stupid Question:

Why do they neeed a duct booster? ductwork not sized correctly/Is this a new install, or replacement unit?

 

[LarryFine]

Non sequitur.