transformer sizing calculation

[tankfarms]

Hello,
Trying to double-check myself on some calculation here. I'm trying to size a transformer for an AC unit. The AC unit has three (3) indoor units and one (1) outdoor unit and are all fed by the same transformer that I'm trying to size here. The specs for the motors for the indoor and outdoor units are as below:
Indoor
208/230V 1PH, FLA 15amps

Outdoor
208/230V, 1PH, FLA 45amps

My question is:
(1)It doesn't specify the corresponding voltage level for the given FLA, so which voltage should I use for KVA calc.?
(2)Since it's single phase, I still need to convert the nominal voltage (in this case 208 or 230) to the line-gnd. voltage (120 or 133) for KVA calculation correct?
(3)Since all the loads in this case are single-phase loads, once I got the KVA, shall I divide the KVA by 3 to get the actual transformer size?

If I follow the steps above, and use 208V (thus 120V line-gnd voltage), and assume I size a 3-phase transformer, so the KVA rating would be: (15*3+45)*120/1000 * (1/3) = 3.6 KVA. So something like a 5KVA 3-phase transformer would be good for this application of feeding the AC?

Appreciate any feedback and comments. Thanks.

 

[charlie b]

You are going to have to deal with an imbalance, no matter how you connect the motors. You just can?t connect those four loads phase-to-phase on a three phase system, and get the same total load on each phase.

But your basic calculation process is not right. The motors are rated to run at either 208 volts or 230 volts, a line-to-line value, so you don?t use the line-to-neutral voltage (i.e., 120) as the multiplier to get KVA. I do not know whether the indoor motor would draw the 15 amps at 208 or at 230, so I would conservatively use the 230.

Indoor motors: (3) times (230 volts) times 15 amps equals 10,350 VA.
Outdoor motor: (1) times (230 volts) times 45 amps equals 10,350 VA.
Total VA: 20,700.

If the loads could be balanced, the phase currents would be 20,700 VA divided by 208 volts, and divided again by the square root of 3. Result: 57.5 amps.

But since the loads are not balanced, I would get a transformer that has a bit more capacity. The next higher standard size (beyond the 20.7 KVA load) is 30 KVA, and that is what I would use.

 

[rcwilson]

KVA is KVA, don't adjust it for voltage or three phase.

Indoor unit: 15A @ 230 V= 3.45 kVA
Outdoor Unit: 45 A @ 230V = 10.35 kVA

Split loads on three-phase transformer:

Phase A-B: Outdoor unit = 10.35 kVA
Phase B-C: Two indoor units = 2 x 3.45 = 6.9 kVA
Phase C-A: One indoor unit = 3.45 kVA.

Minimum size is = 3 x the most heavily loaded phase = 3 x 10.35 = 31 kVA. Go to next larger size and get a 45 kVA transformer. Total load is only 20.7 kVA but the one winding has to push 10.35 kVA, so a three phase transformer smaller than 30 kVA would be overloaded on that one phase.

Or use single phase 120/240V transformer with all loads on L1-L2. 20.7 kVA = 25 kVA, includes 25% safety factor.

I'll leave it to you and others to do the actual circuit calculations with the 25% adder for the largest load, etc.

 

[Smart $]

Given dual voltage spec's of 208/230, I would possibly opt for a single phase 120/240 trannie... but I first have to ask what is the service voltage configuration, i.e. why the trannie to begin with? Seems strange having a 208/120 trannie for just these loads...

The reason for going with a 1? trannie is with the 4 loads given, you cannot balance them on 3?. Best case scenario is 45, 30, and 15... or just 45 and 45... across the lines. Also, your KVA will have to take into consideration both the unbalance and starting inrush current of the motors and/or compressors.

 

[tankfarms]

All the posted comments really help. I think I was just overthinking too much. I've decided that a single-phase 30KVA should do the job. Thanks all for the input.