MCA for convenience outlet on HVAC equipment 1

[ypwtb]

Hello,

Why is Trane using 150% of FLA for the electrical requirement of a convenience outlet on a rooftop unit in page 36 of this document?

http://www.trane.com/Commercial/Uploads/Pdf/1025/iom.pdf

Isn't this supposed to be the contribution of the outlet to the total MCA (minimum circuit amps) for the unit?

They list the imposed load of a 15-amp, 115 volt convenience outlet on a 208/60/3 unit as 7.21 amps.

Shouldn't it be 1.25 * (115/208) * (15 amps / sqrt(3)) = 5.985 ... say 6 amps?

It is 6 amps on page 13 of Carrier's document here:

http://hvactroubleshooters.com/CARRIER/48,50HJ004-014 WITH 62AQ060-300 PACKAGES.pdf

Comparing the MCA values in the 3rd and 4th lines of Table 1A for a model 48HJ004 with 208/3/60 power:
[ul][li]The 3rd line says 27.2 MCA with _no_ convenience outlet.[/li]
[li]The 4th line says 33.7 MCA _with_ a convenience outlet.[/li][/ul]
The difference is 6 amps = 125% of FLA.

Which seems to contrast with:

7.21 / ((115/208) * (15 / sqrt(3))) = 1.506

So why is Trane saying 150% of FLA for the electrical requirement of the convenience outlet?

I am a mechanical engineer who is trying to figure out how to get the right numbers to the electrical engineers on my projects. I thought that I understood the MCA calculation until I found Trane's number at what seems to be 150% of FLA.

Please help me to understand why Trane's number is higher than I expected it to be for the convenience outlet.

Thanks,
Yittri

 

[alehman]

I think you've got an extra sqrt(3). The only way I can match their number is to assume 12 amps of load (80% of a 15 amps circuit as permitted by NEC) and 200 volts at the unit. This would be an unusual transformer ratio.

15 * 0.8 * 120/200 = 7.2 amps

 

[gepman]

I don't believe that there is any specific load requirement for a convenience outlet for HVAC equipment. Therefore the air conditioning companies can list the load requirement of whatever transformer or fuse rating which supplies the convenience outlet. The only NEC imposed load requirement for receptacles that I know is NEC

220.14 (I) Receptacle Outlets.

Except as covered in 220.14(J) and (K), receptacle outlets shall be calculated at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke. A single piece of equipment consisting of a multiple receptacle comprised of four or more receptacles shall be calculated at not less than 90 volt-amperes per receptacle. This provision shall not be applicable to the receptacle outlets specified in 210.11(C)(1) and (C)(2).


For a 208V, 3 phase unit they probably are just using a single phase to neutral assuming that a neutral is required to connect this unit. If a neutral is not required then they are using a single phase 208 to 120V transformer with a neutral on the secondary.

They aren't required to use the 125% factor for the convenience outlet, this is a requirement for sizing motor feeders.

430.24 Several Motors or a Motor(s) and Other Load(s).

Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.


Remember that the convenience receptacle is there to meet the following section of the NEC.

210.63 Heating, Air-Conditioning, and Refrigeration Equipment Outlet.

A 125-volt, single-phase, 15- or 20-ampere-rated receptacle outlet shall be installed at an accessible location for the servicing of heating, air-conditioning, and refrigeration equipment. The receptacle shall be located on the same level and within 7.5 m (25 ft) of the heating, air-conditioning, and refrigeration equipment. The receptacle outlet shall not be connected to the load side of the equipment disconnecting means.

Exception: A receptacle outlet shall not be required at one- and two-family dwellings for the service of evaporative coolers.


NOTE THAT IT STATAES THAT THE RECEPTACLE IS NOT TO BE ON THE LOAD SIDE OF THE EQUIPMENT DISCONNECT, WHICH WOULD DISCONNECT POWER TO THE CONVENIENCE OUTLET WHILE THE SERVICEMAN IS TRYING TO SERVICE THE UNIT. I HAVE HAD THIS PROBLEM WITH A/C UNITS PREVIOUSLY ALTHOUGH I THINK THAT ALL MAJOR MANUFACTURER'S HAVE CORRECTED THAT PROBLEM.

 

[WoodrowJWeen]

If you spec a factory installed disconnect switch, Trane and others will wire the transformer for the convenience receptacle upstream of the disconnect switch. Of course, if you get a field mounted disconnect switch, the transformer and receptacle would be switched off as you said above. I usually ask our mechanical engineers to specify RTUs with a receptacle that is field wired and then I show it wired to another 120V source entirely.

 

[NCDesign]

ypwtb I have a couple of things I will throw out.
1st off are you sure it is a 15A outlet and not a 20A outlet and its 115 and not 120? I did some quick math and the Trane catalog is consistant between the three voltages. They are carrying 2597.5 Watts for the receptacle. If you assume a 92% efficient transformer that gets you 20 Amps at 120 Volts and 7.21 Amps at 208/3 phase.

Further more a factory powered outlet would not carry a 125% mulitplier in a MCA calculation of a rooftop unit. Only the largest load gets the 125% multiplier all other loads are added in at 100% values. (Check page 37 of the Trane iom for a great primer on MCA and MOCP calculation.)

Carrier's catalog data doesn't change between 208 and 230 volt units. This tells me that they aren't really rating the unit at either voltage or they are rating the unit at one and just carrying it through both voltages. The 208 voltage rating should have any amperage approximately 10.5% higher [(220-208)/208]. In reading Carrier's IOM in a little more detail their units are factory wired for 230V and if you connect to 208V power you need to rewire the transformer to the 200V tap. If you use 6A at 200/3 phase and the same 92% efficient transformer you get a 20A outlet at 115V.

Both the Carrier & Trane data leaves out some crucial data for this specific calculation. So a lot of what I have done is see if I can make some numbers fit. Neither manufacturer tells you if they are figuring the outlet at 115 or 120 volts or the transformer efficiency. In Carrier's case they aren't really clear at what voltage the unit is actually rated.

With all that being said, I usually give the catalog data for the unit I am specifying to the EE's and then state in my drawings and specifications that although other equipement may be equal it is the contractor's responsiblity to coordinate any electrical changes that may be required due to use of equipement other than what is scheduled.

Also, Trane seems to leave the MCA and MOCP calculation to the engineer on a lot of their equipement. If you aren't comfortable calculating it get your rep to do it. If you want a CYA have them make the factory do it and send it to you. That is probably especially good advice for some of their chillers and larger equipment. If you calculate it yourself a couple of times and get them to do it as well you will have some double checks. You do this enough and you probably won't have to have them do it for you anymore (at least not regularly).

One final note, be aware that the Trane unit says that the unit supply voltage should be within 10% of the rated voltage and they only supply a 230/3 phase unit. If you are running 208V you are on the ragged edge of the voltage limit and if you are under 207V you are technically out of range.

I hope this helps.

 

[burnt2x]

NCDesign nailed it! Star to you!