Roof Top Units : Calcs

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starbright28

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Minnesota
Our Mechanical department specifies roof top units for commerical aplications. My job as an electrical drafter/designer is to make sure and show the correct load the roof top unit will be having.

My question is this: How do you determine what fuse dize the roof top unit is going to be on? How you get the VA for that roof top unit? What information from the roof top specifications do I use for these calculations. Is there anything in the code book where I can find this calculation information?

Here is one roof top that I am dealing with:
4 ton unit.
Trane YSC048A3EMA0000
0.6 HP
Volts: 208
3 Phase
Amps: 23.9
Max Fuse: 35


btw, this is kind of top priority for me to learn since I have a project due tomorrow and it inovles the roof top changed unit size and I have to re-figure how to calculate it.

Thanks for the help in advance.
 
If you're lucky, the rooftop nameplate will have two things:
Minimum Circuit Ampacity (MCA)
Max Breaker or Fuse

The wires must be sized to the MCA or larger. The fuse must be > MCA and no larger than the maximum fuse size specified. If the nameplate says fuses without also saying breaker or HACR breaker, then you can only use fuses. You also need a disconnect located within sight of the rooftop unit. Its ampacity needs to be >= to the MCA. In your example, #12 copper wire could be used because its ampacity is 25A (assuming that amp value is MCA).

If MCA was not specified, then it gets more complicated. You need the FLA of the fan motor, the FLA of the compressor, and the FLA of anything else in it (like control electronics unless powered by low voltage controls that are elsewhere). Add all of those together and then add 25% to the compressor FLA. You've now basically done what the factory would have done to create MCA.

The exact fuse size to use is not set in stone. For your example, a 30A fuse would probably work (assuming the amp value is really MCA). If the MCA was 28, then a 30A fuse will probably nuisance blow on startup, especially on hot days or when the unit gets some age on it. Sizing to max fuse avoids that, but reduces any margin you may wish to have in giving that unit additional protection via the branch circuit. Picking this fuse value is expanding you from a drafter to a designer unless you just specify the max size every time.
 
I might also add:

If the nameplate says "Max Fuse Size" then as suemarkp pointed out, you must use a fuse, but it doesn't have to be in the disconnect switch that is mounted on or near the rooftop unit. If you are feeding the unit from a fusible distribution panel down below that counts as the fuse.

If it just says "Max Overcurrent Protective Device" then you can again use a non-fused disconnect at the rooftop unit and a CB at the source.

Just a little "value engineering" to save $$, allthough some would still insist on a fusible switch.
 
On that example Trane unit, the VA would be 208V X 23.9A = 4,971.2 VA and then multiply that X 125% OR 1.25. 4,971.2 X 1.25 = 6,214VA or 6.2KW. The fuse size would be 23.9 X 125% or 1.25 = 29.875A or rounded off a 30Amp fuse. Minimum size wire for 30A is 10AWG copper or 8AWG aluminum. And depending on the distance to the roof, you may also have to increase the size of the wire to keep the voltage drop at or below 3%.
 
I would assume that the ampacity listed in the OP as 23.9 amps is the MCA. In that case the OCPD would be any size between that value and the maximum value listed as 35 amps. Article 440 requires that you use the information provided by the manufacturer on the nameplate. As Suemark said if the MCA is 23.9 than #12 tHHN conductors are suitable for this load since their ampacity at 75 degrees C is 25 amps.
 
Adrian,
The fuse size would be 23.9 X 125% or 1.25 = 29.875A or rounded off a 30Amp fuse. Minimum size wire for 30A is 10AWG copper or 8AWG aluminum.
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The maximum fuse size was listed on the nameplate per the original post...no calculations are required. The conductors are sized to the minimum circuit amps...in this case 23.9 amps which requires a #12 copper conductor and are protected by the maximum OCPD shown on the nameplate, so we would have #12s on a 35 amp fuse and be in compliance with the NEC rules.
Don
 
Okay I understand to a point the calculation.

I did go and ask my engineer on how he calculates the VA.

Here is what he told me to do:

Take the Amps given { 23.9 } and times it by the voltage { 208 }. Then he took that number by 1.732 (3 phase) and got his VA.

He did not (once again) take into consideration the continuous load - 125%.

So, what is the correct calculation again?

What I listed for information is what the Mechanical Department lists on a schedule.

Edit: for some reason a graphic was in place of my 8 in 208.
 
You have to know what amps was used on the nameplate to know if the 125% factor was already incldued or not. For 3 phase, your equation is correct:

VA = amps * 208 * 1.732

Now the question is what are you using the VA for and do you want it to include increases for continuous loads. If you're trying to make a panel schedule to determine your feeder amps, then I believe it is required to take into account continuous loads at 125%.

So if that nameplate amps was MCA, the 125% factor was included in the amp value and you're done. This is probably what your engineer was assuming. If it was FLA, then it gets more complicated and you need to know what portion of those amps belongs to the largest motor (or compressor which is a motor). That portion gets 125% added and then plug the total result into the VA equation above.

Most modern roof top units probably list MCA unless they are real huge. You should get the mechanical department to always use MCA for HVAC amps for both compressors and units with resistance heat. That way, people will know the 125% factors have been included, and it will put the onus on them to figure it out because the only way to do it is to look at the nameplate or the nameplates on each component to figure it all out. That is difficult for you if you can't look at the equipment.
 
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Mark

I think I'm okay with not having to do the 125% because the Mechanical Department lists on their schedule exactly what shows for information on the model on the Trane website. I can't tell you off hand however if the amps they list are Max. But I don't they don't list the minimum circuit amps, which I almost started to use for calculations.

If I understand you right, the way this calculation is done is correct.
 
Are you providing one feeder to the roof to supply all units. If so take 125% of the largest plus the sum of the others. However the 23. whatever that is listed on the unit already has the 125% added so you could recal all units or just use the given and be slightly oversized.
 
adrian33773 said:
Sorry, NEC does not allow 12 AWG on 35A OCPD. Minimum would be 8 AWG.
Click to expand...

In this instance the OCPD could be 35 amps with # 12 conductors. The OCPD is there to provide ground fault and short circuit protection only. The sizing of the conductors has to do with the MCA on the nameplate of the unit and not the size of the OCPD.
 
Table 210.24. The whole idea is for the breaker to trip or the fuse to burn before the insulation on the wire. Once the insulation burns we are talking fire. I'm sure 12 wire can handle the ampacity, no problem. But the insulation can not, especially on the roof where the ambient temperature is much higher than normal. You can actually fry eggs on a RTU. The engineers at my company agree.
 
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The sizing of the conductors has to do with the OCPD, not the equipment amp rating. That would be like saying I can use 22 AWG door bell wire on a 15A breaker because my load is only a clock radio. I've never seen less than 10 AWG on any RTU and those were 2 Ton Units. I read the entire articles 210, 240 and 440 and didn't see anything saying it was okay to use 12 AWG on a 35A fuse or breaker.
 
The difference is that most of these exceptions to 240.4(G) have a built in thermal trip device. In an HVAC unit, there should be a thermal cutout in the compressor winding. Motors have controllers with thermal elements, or a thermal cutout in the winding.

Should these get tight bearings, or the load somehow gets too high, the thermal unit will trip and thus protect the wire. If you read articles 430 and 440, you'll see that wires are sized per FLA + 25% or the HVAC MCA value. You'll then see that overcurrent protection is permitted to be above this amp value, sometimes substantially larger. If wires had to be sized by the breaker rating, then the code would say to calculate the breaker size first and then select a suitable wire. But it doesn't say that.

The 240.4(D) and (G) references let you ignore the normal #10=30A, #12=20A, and #14=15A breaker rules. But you can't ignore the 60C or 75C ampacity of those wire sizes.
 
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adrian33773 said:
The sizing of the conductors has to do with the OCPD, not the equipment amp rating. That would be like saying I can use 22 AWG door bell wire on a 15A breaker because my load is only a clock radio. I've never seen less than 10 AWG on any RTU and those were 2 Ton Units. I read the entire articles 210, 240 and 440 and didn't see anything saying it was okay to use 12 AWG on a 35A fuse or breaker.
Click to expand...


Suemark hit the nail right on the head. For an example take a look at table 430.52 and check out the values listed for an OCPD for a motor. You will see that, for example, an instantaneous trip breaker on a three phase motor can have an OCPD rated at up to 800% of the full load current. Certainly you would not need to size the conductors at 800% also.
 
Adrian,
Table 210.24. The whole idea is for the breaker to trip or the fuse to burn before the insulation on the wire.
Click to expand...
How would 210.24 even apply to this installation? It only applies to branch circuits that serve 2 or more outlets.
The engineers at my company agree.
Click to expand...
If you are saying that they require the use of a larger sized conductor, that is fine and is a very common practice, but if they are saying that using a 35 amp breaker with #12 for this application is a code violation, then they are incorrect.
I read the entire articles 210, 240 and 440 and didn't see anything saying it was okay to use 12 AWG on a 35A fuse or breaker.
Click to expand...
First start with 210.4, the first sentence, 210.4(D) and 210.4(G), then move on to 440.6(A) and Exception #1 for the size of the conductor, next to 440.22(C) for this installation, and lastly to Table 310.16 for the conductor ampacity.
Don
 
Okay Don, whatever 210.4 (D) & (G) were, they no longer exist. They were scratched from the 2005 NEC. Moving on to 440.6(A) Exception #1. It agrees with exactly what I've been saying. It says the branch circuit selection current, which would be 30 amps because they don't make a 23.9 amp breaker, SHALL, which means you have no choice, be used instead of the of the rated-load current (23.9 amps) to determine the rating or ampacity of the branch circuit conductors. Which means 10 AWG CU minimum. Of course the tables 310.16-310.19 also do not include voltage drop. So probably up that to 8 AWG CU. Refer to 310.15(A) FPN No. 1 and 310.15(B) FPN also. Coordination with system OCPD. By the way, the conductors running to the AC equipment is a BRANCH CIRCUIT.
 
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