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| Subject - 3 phase transformers | |
| David Hyatt |
A three-phase delta-wye transformer 480/208/120v has a 45KVA load, the secondary line current would be approximately__________amps? A)54.1 B)31.2 C)73.4 D)125 I know many can answer this question, but show all calculations, how you arrive at this answer. Thanks 
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| tony thrower | 45000/(208 x 1.732)= 124.9 |
| David Hyatt | Cool, I did it a little different but have the same answer. Your way is easier. 45,000/3=15,000/120=125a |
| iwire | 45 x 2.77 = 124.6 Close enough for estimating purposes. 2.77 works for 208 volt 3 phase amps from KVA. 1.2 works for 480 volt 3 phase amps from KVA. 45 KVA @ 480 45 x 1000/480/1.732 = 54 or 45 x 1.2 = 54 |
| Ryan_J | What is the efficiency of transformer? |
| Scott Vickrey | Since this is obviously a test question and the efficiency was not mentioned then we would use 100%, right? However this is a good extension to the subject, so forget the multiple choice answers and let's say the efficiency was .98. Who can walk us through the sizing of the primary and secondary OCPDs? Try to include as many code references as possible. |
| David Hyatt | 45000/(208x1.732x.98)=127.46 =secondary phase current wye connected phase current=line current Transformer has 4 to 1 ratio 127.46/4= 31.865= primary phase current 31.865 x 1.732= 55.190 primary line current primary line current= 55.190 secondary line current= 127.46 Not sure how to size the OCP. Can someone help? Also check my calculation as well, transformers for a residential electrician is not my strong point. |
| Scott Vickrey | David read Ron's response to Dave's similar question at electricalmatters.com.  |
| Ryan_J | Hi Scott. As you may have noticed, I spend quite a lot of time at Electrical Matters (Dave Nix's site). I have always enjoyed reading Ron Shapiro's posts, as I think he is a very qualified designer. In fact, he and I had a nice chat last Friday about the NFPA 72 and the "survivability" concept that has the whole world on edge. Anyway, enough rambling from me. I am glad you enjoyed the efficiency part that I threw in. As it appears you are well aware of the fact that transformers are incredibly effiecient, so I wanted to throw that in in case anyone wasn't aware of it. HAve a great night :) |
| Ryan_J | 208*1.732 is 360...same as a circle. I think this is a number that is well worth memorizing. |
| sled dog | transformers efficiency is expressed as inductance ( the coils resistance to the voltage being passed thru them) three percent is common. by the way 1.732 is the square root of three- three phases, get it? an easy way to size a three phase transformer- volts x 1.732 x amps divided by 1000 equals kva |
| justinelect | Ryan reminded me of my exam prep course and the ways people use to remember things. 208 X 1.732 = the circle or 360. 480 X 1.732 = 831 or the taco house. Quote from the instructor "at work by 8:00 and at the taco house by 8:31." It works I always remember the circle and the taco house. |
| KSsparky | Correct me if I'm wrong, but wouldn't the 98% efficiency of the transformer reduce the available secondary amps from 125 to 122.5, not raise it to 127.5 ? I believe your calculation should look like (45000/(208 x 1.732)) x .98 If you leave the .98 inside the bracket, you will produce 45900 va. |
| JimmyDee | Looks good to me KSsparky. Jim |
| lctrc789 | Grounding new Transformers, I always check article 250-2 (a) and (d). Remember to do as 250-12 states especially older buildings. Clean all surfaces, Grind if you can and I always use NO ALOX. Article 250-24 all of them depending on transformer. Article 250-30 (a) 3 is another one to pay attention to as well as in some areas you have to use 250-28 as well, bonding and your conductor paths. Of course Article 250-36 (b) for the neutral conductor as well. For the secondary side of the transformer Article 250-66 and Article 250-122 for the primary side of the transformer always ahead of the overcurrent. A lot of these articles really depends on what kind of transformer you are setting DELTA, WYE 480 volts etc. When you start on 4160 and higher it is all different. 13,200 to 4160 is much different then 480 to 120/208. I say the entire article of 250 if you have many different voltages WYES AND DELTAS etc. All city/county and local utilities have different rules for grounding bonding etc. |
| Patrick J Baker Jr | I have 3 phase 240Delta and need 3 phase 208Wye. My transformer hook-up produced an arc when tying in B and C pase in. Any suggestion? |
| JimmyDee | You have almost a dead short when you are initially saturating a transformer and it will draw a lot of current. A fairly good spark would come from that, especially when you connect the 3rd phase. How do you have this wired so you can see this? Jim |