transformer for HVAC unit

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SparkyRules

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Hoowdy folks

I am reworking a job where the original electrician bailed on my customer. We are through most of their mischief but have one very large potato still cooking away. The owners thinking they would save some $$$ bought a used HVAC unit (not roof top out back pad mount) however not knowing they bought a 460V (480) unit and this building only has 208/120V wye. A new 10 ton unit would cost them 6K. I can get a transformer used (75KVA) for around 1,000 ish looking on e-bay. I have no clue on how to calculate for the locked rotor current (startup) VS xfmr size a 45KVA would do so per min circ size shown but as hunch being able to get a 75KVA for the same price as a 45 makes me guess a 75 would handle to start up better. HOWEVER I hear there is a difference between step up and step down transformers, all the ones I see are 480 prim to 208/120 sec so I would have to wire this backwards. I can not see using a neutral would be needed on the 208V (now) prim side (wye would it be, bad pun) but the bigger question is this a code violation, a breaking of natural law and if so wye on either? The power company wants way more to put in 480 and counting the switch gear etc a right voltage unit wins over that possibility. My real gut feeling is for them to get the right voltage brand new warrantied unit for 6K but hey will at least try to figure out the option here.

I am deliberately making your mistakes for you, just to help you learn faster, yup I got those big of shoulders...
 
I can get a max 100A breaker without doing major acrobatics, it is a 400A service, no have not done load calc but good point will do so, but the gut feeling is the service can handle this. I can get the max demand from the power company for last 12 months which is really better than a load calc as it is based on reality.
 
SparkyRules said:
I can get a max 100A breaker without doing major acrobatics, it is a 400A service, no have not done load calc but good point will do so, but the gut feeling is the service can handle this. I can get the max demand from the power company for last 12 months which is really better than a load calc as it is based on reality.
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75 kva at 208 V is about 208 amps. at 480V it is 90 amps. do you really need 90 amps for a 10 ton A/C unit? most xfmrs are just labeled with the voltages and not as primary or secondary so you can wire them as you see fit.
 
SparkyRules said:
I can get a max 100A breaker without doing major acrobatics, it is a 400A service, no have not done load calc but good point will do so, but the gut feeling is the service can handle this. I can get the max demand from the power company for last 12 months which is really better than a load calc as it is based on reality.
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You will definitely not be able to start a 75kVA transformer with a 100A breaker.
 
The transformer doesn't actually care, but there are potential issues. Certain winding designs make it so that back feeding it creates even higher inrush current on the low voltage side. The easiest way to tell now is to read the manufactures literature, because starting in the 2014 code if you want to use a transformer to step-up, the transformer must be listed for that. So something from the mfr must state it clearly either on the nameplate or at least in the data sheets.

Your problem, buying on Fleabay, may be that this transformer might be too old to make that clear. It might be OK, but it might be difficult to ascertain and if it's going to be inspected, your AHJ will want to see that documentation.
 
OK so it then IS a code issue (110-3-B) per listing and labeling. Good to know. Although if I think about that it would that have to be excluded as part of the instructions per that vintage of xfmr and only a newer one (built after 2014) would need said listing , correct?
 
ron said:
A 200A breaker on the primary will not startup a 75kVA transformer with a 208FLA. Trip almost everytime.
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Doesnt seem like that would be an issue. What exactly are the conditions? Are you assuming the "used in reverse" scanario and are you talking just energizing the transformer with no additional load? Ive seen reverse connected transformers hooked to breaker rated same as their LV side rated current start no problem.

To the OP: I suspect a 45 kva transformer would be fine. I recently came across a situation where there were 2, 7.5 ton and 1, 5 ton units on a 45 KVA used in reverse and it had been like that for 10 years. If you use a delta -> wye transformer in reverse, make sure to not connect a neutral to the XO and also remove the bonding jumper (if installed) from the XO to frame.
 
Is this HVAC unit an addition or a replacement for similar capacity unit? If a replacement of similar capacity then your service/feeders can probably handle it, otherwise you need to consider the added load though 10 tons is not an extreme number in many cases.

I'd also say the 75 kVA transformer is probably somewhat overkill if only supplying this one load.

Don't know the $$$ situation with the owner, but can see you putting most of the $6K for a new unit into making this unit run, but no warranty on the thing when all is done. Maybe about as well to get the correct unit and try to get what they can out of this one.

If you are backfeeding a 480 delta/208 wye transformer you are going to have to corner ground the output as there is no neutral to ground. Is the HVAC unit able to operate on a corner ground system? Basic across the line motor/compressor starting likely can, but if there are VFD's involved it may not be able to be used on corner grounded system.
 
kwired said:
If you are backfeeding a 480 delta/208 wye transformer you are going to have to corner ground the output as there is no neutral to ground. Is the HVAC unit able to operate on a corner ground system? Basic across the line motor/compressor starting likely can, but if there are VFD's involved it may not be able to be used on corner grounded system.
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Why would you have to corner ground it?
 
Between 250.20 and 250.21 this application is likely required to be grounded, there are three corners of the delta to choose from for the grounding point.
 
Hi So a 45KVA seems like it would work but that new art (450-11 (B) does seems to suggest the xmfr does need to be marked for step up And good luck finding one used that does have that marking. The more I get into this the more a new right voltage HVAC unit with a warranty makes the most sens even if it cost a few $1,000 more, can always sell off the old 460V unit to recoup some of the $$$.
 
I get the corner ground for the delta 480 if sec. and no XO on the LV wye (when uses as prim) in a commercial site having 480 to ground just seems like adding yet another level of hazard and this is a day care... so red flags all over this. I do like to think things thru as how to, but I just can't get a good feeling about doing this. I am gonna tell him to bite the bullet and get a right voltage unit (with warranty) and sell the wrong one. I mean the 45KVA xfmr 1.5K install 1.5K now need 480V Disconnect 3R for HVAC .6K extra work so at best we are saving less than 1/2 the cost of a new unit and if he can sell used 460V unit for 2K its win win... PLus I get to blame this on all you guys smarter than me!!!! heck yeah!!!
 
SparkyRules said:
Hi So a 45KVA seems like it would work but that new art (450-11 (B) does seems to suggest the xmfr does need to be marked for step up And good luck finding one used that does have that marking. The more I get into this the more a new right voltage HVAC unit with a warranty makes the most sens even if it cost a few $1,000 more, can always sell off the old 460V unit to recoup some of the $$$.
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450.11(B) doesn't require the transformer to be marked for step up or for reverse feed, it just mentions "provided that the installation is in accordance with the manufacturer’s instructions".

The main difference between a step up and step down transformer is which winding is placed closer to the core, this will also have an impact on inrush current when energizing the unit.

Most transformers will not have a primary "neutral" tap unless there is a need for one, alternative on site energy sources and back feeding into the utility is making more potential need for that though then there used to be.

Most transformer manufacturers can make you any transformer you want, but they will cost more then what is commonly stocked that sells in higher volume. There is generally more applications that step down from 480 volts to 240, 208 and/or 120 then there is stepping up making those units higher volume sales and easier to find and likely less cost then a step up.
 
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