Sizing buck boost transformer

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Just going by MCA, kva, etc that gives some assurance a 45kva will suffice. In fact, with motors & compressors you have to take into account LRA and other factors pertain to the transformer itself to be assured. Motors and compressors require oversizing but, as far as I know, there is no magic number.
 
Crossboss said:
Update: Ok I do have a 200amp source off the bus duct. Now should i have a 45kva or 75 kva transformer and need to find size of wire for secondary side to the AHU. MOP IS 60 MCA is 49
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45 kVA is enough for the running current. But you also need to consider the starting current for any motors.

How much of the load is for _motors_, and how much is for things like resistance heat strips?

Jon
 
It's a shame standard 3 phase transformer sizes jump from 45 to 75.
IF I was doing the install, I would install the 45 but, as noted, you do need to be aware there is a possibility of inrush problems.
(In the event of an in-rush problem, a HVAC soft-start might eliminate the problem)
 
augie47 said:
It's a shame standard 3 phase transformer sizes jump from 45 to 75.
IF I was doing the install, I would install the 45 but, as noted, you do need to be aware there is a possibility of inrush problems.
(In the event of an in-rush problem, a HVAC soft-start might eliminate the problem)
Click to expand...
there is not enough difference in cost between a 45 and a 75 kVA transformer to matter much. That extra cost is dwarfed by the labor costs.
 
Update so I got a s step down 75 kva transformer and gonna wire in reverse. After alot of studying the consensus was to disconnect the bonding jumper but not sure why. Has anyone wired a transformer in reverse and can you tell me why we would not want it bonded?
 
augie47 said:
It's a shame standard 3 phase transformer sizes jump from 45 to 75.
IF I was doing the install, I would install the 45 but, as noted, you do need to be aware there is a possibility of inrush problems.
(In the event of an in-rush problem, a HVAC soft-start might eliminate the problem)
Click to expand...
10’s, 15’s then 25’s...
never saw a 20
 
Crossboss said:
Update so I got a s step down 75 kva transformer and gonna wire in reverse. After alot of studying the consensus was to disconnect the bonding jumper but not sure why. Has anyone wired a transformer in reverse and can you tell me why we would not want it bonded?
Click to expand...
A step down used in reverse is really not a good idea. Try and get the correct transformer.

IMO 45KVA is fine. Typically, actual running current is 66% of MCA. A 75 KVA is $100 more in no load losses per year....
 
electrofelon said:
A step down used in reverse is really not a good idea. Try and get the correct transformer.

IMO 45KVA is fine. Typically, actual running current is 66% of MCA. A 75 KVA is $100 more in no load losses per year....
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Cannot get a step up. Do you know of any specific reason why its not a good idea?
 
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
 
Fuse selection critical as well as the ocd for buss duct. You may take down the buss duct ocd. Evaluate all the equipment on the buss ducts connected load. May have a mad boss on your hands.
 
Did a quick comparison on inrush.
No engineer here. Just give you an idea what your dealing with on inrush.
If fed normal 541 amps
back fed 5488 amps
Numbers based on 10% primary current and 37% secondary current 45kva transformer. No other items added.
 
electrofelon said:
Why not? That is a super common transformer.

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I asked for a 208delta/480wye step up and senior project manager got me a used step down and says to wire it in reverse. Did as much research as I can up to this point and seems most electricians are divided on it's a bad idea and that yeah it works the same in reverse just disconnect the bonding jumper at xo and I will have no neutral.
 
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