Sizing buck boost transformer

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I'll have a 225 amp breaker feeding the transformer and 70 amp fused at the AHU. Just got the unit in today and they had it fused for cooling only at 60 amps, heating requires 70 amp MOP
 
Tulsa Electrician said:
Usally for the application I would factor in 20% of the calculated load for a motor that starts more than once an hour.
If the RTU has a VFD you may be ok.

As far as using a step down in reverse not a choice I would make.

You did say temporary so it is what it is.

When you time read up on reverse wire trans.
Any way, adding the 20% your at 48.8
Feed that from 200 amp buss duct switch to the trans ( how far to transformer) and the location of the transformer. You can fuse it down if need be. Make sure to ground the transformer correctly and run a neutral to the first means even though you don't need it. Then three wire from there to the RTU. This for the wye secondary.

Since your using a reverse fed you will need to do ground the Delta secondary some how. The RTU may not like this. If you use a corner grounded secondary read up the rules for the grounded conductor and if a fused disconnect is used be careful. Running an open delta secondary is a worse idea for a RTU most have a single phase motor or like condinser fan or other components.
I really think doing a reverse fed is a bad idea here. Maybe other will come in an offer an option.
Another to keep my in mind is the primary overcorrect will need to higher on reverse fed. Which leads to increase in wire size etc.
If you pop a fuse when energize the transformer you fuse is small to handle the inrush current. You may also find a voltage dip of 10% on the secondary that you did not factor into your calc. 2% taps will not fix this. Also your taps also run reversed as well so don't go wrong way.
Also do not run a neutral to XO on the primary side and in this case
Do not tie xo to ground or to enclosure, leave it Open. Leave core ground to enclosure in place. I would recommend B phase grounded secondary and ground per NEC.

More to it however this will get you going in the right direction for other to discuss with you.
Hope this helps
Click to expand...
So if I ground B phase on secondary 480 I would only have 480 on A and C phases wouldn't I ? How does that work if 480 3 phase is required. And by grounding B phase does that mean B phase would go to ground only and not to x2? 🤔
 
It will work as long as you understand what happens when you do and follow the necessary step to do it in a code compliant manner. Don't take any short cuts or it can become an hazord.
Just my opinion.
 
Woah. Lots going on here.

Yes, you can use a transformer in reverse, but you need to be really careful.

1) Only some transformers are permitted by the manufacturer to be reversed. They will probably work if not specifically permitted, but....

2) reversed transformers have much worse inrush characteristics. Depending upon what is feeding the transformer this might be a problem when you energize the transformer.

3) When you use the delta as second don't have a neutral available. This means you cannot ground in a balanced fashion. For loads such as motors or resistance heaters this doesn't matter. But VFDs really should be fed with a grounded wye (grounded neutral) source.

4) You can corner ground a delta source. It still works just fine because the L-L voltages are unchanged. The 'grounded b' has 0V to ground, but 480V to the other phases.

Jon
 
No, it is a grounded phase or grounded conductor. Not to be confused with a neutral.
Phase to phase it's 480
Since B phase is grounded for Saftey it will measure zero to ground. You can not fuse it. It can only by opened if opened simultaneously with the other two phases (breaker) the breaker must be straight voltage rated no slash rated since it 480 to ground.
Since it's a grounded conductor it shall be identified as such. Gray in color for 480 as long as this system identification is not being used else where. So corner ground the secondary ( b phase). This is most common. Please lable all enclouser correctly. I prefer White background
with corner grounded symbol.
treat this like any other secondary grounding in way of GEC,SBJ,SSBJ. The only thing is you install SBJ to B phase of the secondary on the Delta in the transformer. If you do not you will need phase monitor. This get expensive.
by grounding a phase intentionally. A second fault will try to clear the OCD. If open delta the first phase fault will ground the system unintentionally and pose a hazord. The the second phase fault will try to clear the system fault. I did say try. This is why it is important to do it per code so it will clear and in time. Don't skimp on the EGC , SBJ and SSBJ. This is what provides the low impedance path to back to the source to clear a fault on the system.

Study up on this and ask for help on installation.
Several code section talk explicitly on corner grounded delta's.
example of one.
250.68 3-c grounded conductor They don't won't you down sizing this grounded (phase) conductor.

3-phase 3- wire delta.
once you know what to look for there is several code references to the corner grounded delta.
 
Read art 200,240,250 etc.
Pay close attention on you grounding and bonding on the 480 volt side. Bonding over 250 volts.
Have fun and be safe. I can still remember my first one. Corner grounded delay that is.

Tulsa Electrcian
 
I'm late on the update but here it is. The 75kva transformer step down transformer worked very well wired in reverse operation. AHU is up and running smooth. Thank you all for the input, very much appreciated. Love this forum!👍😎
 
And just in case you are curious, the larger than normal inrush current that Winnie referred to earlier in this thread is because normally the primary windings that magnetize a transformer are the "outer" windings. The secondary windings are inside those and closer the the iron core. But when reverse fed, like you are doing, the windings that energize the unit are the inside ones. Being closer to the core, they can cause dramatically increased inrush current.

And if you were not curious, and read this far, 1000 pardons ;)
 
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