# I Could Use Some Help With Transformer Selection



## CoolWill (Jan 5, 2019)

Assuming you want the bare minimum of capacity, 10 kVA should get by. 25 kVA would get you almost 70 amps.


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## 460 Delta (May 9, 2018)

If it were me, I would probably install a 15 KVA 208/120 3 phase xformer with a 3 phase panel. The single phase motor should have a 208 rating. Later on if and when it goes bad you’ll be all set with 208 3 phase. 
That’s what I would do but others may disagree.


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## joebanana (Dec 21, 2010)

What is your total load?


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## splatz (May 23, 2015)

How about this one 

https://www.fptonline.com/Images/TechDataSheets/P27LF21-15.PDF


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## micromind (Aug 11, 2007)

460 Delta said:


> If it were me, I would probably install a 15 KVA 208/120 3 phase xformer with a 3 phase panel. The single phase motor should have a 208 rating. Later on if and when it goes bad you’ll be all set with 208 3 phase.
> That’s what I would do but others may disagree.


I would do the same thing........15KVA @ 120/208 will give 42 amps per phase, this will allow for future expansion. Then 480 current is 18 amps and you'll need 3 phases and a ground, no neutral. 

It'd be easiest if you got the 120/208 panel with a 50 amp main; it'll likely be a basic 50 amp breaker that's backfed. 

A less expensive route would be a 10KVA single phase unit, it's the same 42 amps per phase but it's single phase, not 3Ø.


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## 460 Delta (May 9, 2018)

micromind said:


> I would do the same thing........15KVA @ 120/208 will give 42 amps per phase, this will allow for future expansion. Then 480 current is 18 amps and you'll need 3 phases and a ground, no neutral.
> 
> It'd be easiest if you got the 120/208 panel with a 50 amp main; it'll likely be a basic 50 amp breaker that's backfed.
> 
> A less expensive route would be a 10KVA single phase unit, it's the same 42 amps per phase but it's single phase, not 3Ø.


Doesn’t Square D make a combo xformer and breaker panel? If they or a competitor makes such a unit, that would be the happening system.


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## Cow (Jan 16, 2008)

I usually do these little buildings with a 15kva single phase trans.

If your air compressor is actually a 2HP 230v single phase it will be much less than 24 amps. A 15kva would be more than plenty. My personal preference is to stick with stepped down single phase rather than 3 phase since you already have 3 phase 480v present.


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## micromind (Aug 11, 2007)

460 Delta said:


> Doesn’t Square D make a combo xformer and breaker panel? If they or a competitor makes such a unit, that would be the happening system.


They do, I've installed several of them. 

Not sure but I think 30 or maybe 45KVA is the largest one, they come with both primary and secondary breakers and they take QO breakers. I've never seen one with bolt-on breakers, they might make them or not. 

They are HEAVY though, quite a bit more than a basic transformer and the transformer part is encapsulated........no vents. 

I've seen both single and 3 phase models.


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## Cow (Jan 16, 2008)

460 Delta said:


> Doesn’t Square D make a combo xformer and breaker panel? If they or a competitor makes such a unit, that would be the happening system.





micromind said:


> They do, I've installed several of them.
> 
> Not sure but I think 30 or maybe 45KVA is the largest one, they come with both primary and secondary breakers and they take QO breakers. I've never seen one with bolt-on breakers, they might make them or not.
> 
> ...


Mini power zones.

We've used them a couple times, they are convenient, but the price and the usual long lead times makes them hard to justify more often than not.


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## JonCOLH (Feb 8, 2019)

joebanana said:


> What is your total load?


The sub panel will use a 50A main breaker. Total load will be closer to 30A with the compressor being the only continuously drawing load. I believe the 24A listed is if it's wired @120V, so most likely it'll be closer to 12-14V at 230V. 



Cow said:


> I usually do these little buildings with a 15kva single phase trans.
> 
> If your air compressor is actually a 2HP 230v single phase it will be much less than 24 amps. A 15kva would be more than plenty. My personal preference is to stick with stepped down single phase rather than 3 phase since you already have 3 phase 480v present.


Looking at it I believe you're correct. The 24A is most likely the 120V amperage. Not sure if they're adding the inrush to that. I would bet @ 230V it's closer to 12-14A. I'm also leaning towards a single ph transformer since the 3ph is already there. I deal with small control transformers regularly, but not often, so not terribly familiar with step downs for source power.


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## The_Modifier (Oct 24, 2009)

JonCOLH said:


> Any point in the right direction is greatly appreciated.


I would go with the mauve one- everyone around here uses the grey ones. Stand out and use the mauve one.:biggrin:


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## 460 Delta (May 9, 2018)

The_Modifier said:


> I would go with the mauve one- everyone around here uses the grey ones. Stand out and use the mauve one.:biggrin:


My preference is CH-CH series, they are an attractive tan color.


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