Transformer rating.

[Rewired]

Hello!
I haven't been on here in a LONG time but I have a quick question regarding a transformer or rather a group of transformers at my friend's metal fabrication shop.
What he has is 3 separate 15 KVA 600V - 120V dry type transformers wired in " Delta-Wye " to give him your typical 120/208V 3 phase 4 wire service. No problems there I understand that, BUT what I can't figure out is will that setup be considered to be equivalent to a " 15 KVA 3 phase" transformer, or a "45 KVA 3 phase" transformer.. I am just curious because if its going to behave like a " 15 KVA 3 phase " transformer, I believe its dangerously overloaded. If its going to be behaving like a "45 KVA 3 phase" transformer, all will be well as far as loading, the only thing that may be wrong would be a code violation regarding the secondary conductor size. He hasn't had any problems as of yet BUT last time I was there he just parked a small rooftop A/C up top and I want to know what I have to work with as far as what actual "size" his transformer bank really is before I hook anything up! ("small rooftop" being 20 amp @ 208V SINGLE phase)..

Thank you for any help anyone can provide!

A.D

[HayZee518]

All three transformers will be sharing the same load so the rating overall is a 15KVa even with three connected in that configuration. I'd have to consult a transformer book to determine the proper loading and what to expect from the installation. I'll look into it and repost to this thread.

[Rewired]

Hello HayZee!
Thank you for the information you have provided! Any more you can will be GREATLY appreciated!

I have a chart for Full load currents of different size transformers, It says a 15 KVA 3 phase is only good for 41.6 A @ 208V... Not really much for driving a machine / welding shop loaded with metalarc lighting in the shop, some tools, baseboard heating in the office , Oh ya, almost forgot the electric stove and dryer !!!![:0]

[HayZee518]

done some searching and he's what I can up with
kVA Full Load Current - Amperes
Rating 120 Volt 240 Volt 480 Volt 600 Volt
KVa 120 240 480 600
3 8.33 7.22 3.61 2.89
6 16.7 14.4 7.22 5.77
9 25 21.7 10.8 8.66
15 41.6 36.1 18 14.4
30 83.3 72.2 36.1 28.9
37.5 104 90.2 45.1 36.1
45 125 108 54.1 43.3
50 139 120 60.1 48.1
75 208 180 90.2 72.2

[HayZee518]

Calculations for transformers
1 KVa = 1000 va

You stated he had three 15KVa Xfmrs
15 X 1000 = 15000va

Take your secondary voltage and multiply by 1.732 which is a three phase constant to get voltage

208 X 1/732 = 360.2 volts

15000va divided by 360.2 = 41.363Amps (P divided by E = Amps (I)