# How do I size for transformer 480Vx240V



## wildleg (Apr 12, 2009)

you need to know the existing load so you know if you have enough power.

you need to know the load that will be put on the 200A panel (you can size to 200A but that's not necessary if the load is nowhere near that)

are you sure you want 240 ? 208/Y120 would be what we would normally put in around here.


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## Workingman (Jul 13, 2010)

Thanks Wildleg for the reply.
As of now the building is empty. He's looking at bring in 2 paint coating machines that will run off of the 400amp/480 panel. Each one of them is 60amp/3ph.
He asked for the 120/240V panel because he is looking at air compressor that run off of 240V/3ph.
Right now thats all I have to work with.


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## Bob Badger (Apr 19, 2009)

I would go with 208Y/120 and push him to order the compressor in 480 or deal with providing the 240 for it separately from the office.

Not really a great plan to feed an office from the same panel / supply as a compressor. It could cause a number of voltage drop issue each time it starts.


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## Murphy (Dec 10, 2009)

breaker for panel isnt hard to figure out.. you know that the primary side of the transformer will only have half as much current as the the secondary. table 450.3 (B) explains how to protect transformers


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## NolaTigaBait (Oct 19, 2008)

Murphy said:


> breaker for panel isnt hard to figure out.. you know that the primary side of the transformer will only have half as much current as the the secondary. table 450.3 (B) explains how to protect transformers


Or you could divide the kva of the transformer by 830..if its 480y


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## Bob Badger (Apr 19, 2009)

NolaTigaBait said:


> Or you could divide the kva of the transformer by 830..if its 480y


That will only provide the capacity of the transformer not provide the info needed to select the rating of the primary breaker.


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## 480sparky (Sep 20, 2007)

Workingman said:


> ..............He asked for the 120/240V panel because he is looking at air compressor that run off of 240V/3ph...........


It's a common misconception that the 'low voltage' of three-phase systems are 120/240. My guess is the compressor is actually either 240 single phase, or 208 three phase. Lay a pair of eyeballs on the nameplate before proceeding any further.


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## NolaTigaBait (Oct 19, 2008)

Bob Badger said:


> That will only provide the capacity of the transformer not provide the info needed to select the rating of the primary breaker.


Oh, I thought if you had a 45kva tranny and you fed it with 480...you would get 54 amps...Use #4 and protect @ 70 amps?


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## Wingnut (Jan 31, 2010)

So have we gotten to the size of the transformer yet?


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## micromind (Aug 11, 2007)

Wingnut said:


> So have we gotten to the size of the transformer yet?


We cannot size the transformer until we know the load it is intended to supply. Simply saying 200 amps is just a guess. 

I've seen a large number of installations like the OP described that have a 75 KVA transformer, 3 phase, 480 to 120/208 wye. The 480 breaker is usually a 100, and the secondary feeds a panel with a 225 amp main. 

The vast majority of these have about 30 amps on the secondary. Way overkill. 

If the expected load allows for it, a 45 KVA can be used. It takes a 70 on the 480 side, and a 150 on the secondary. 

There's a very good possibility that a 30 KVA will do. This takes a 45 on the primary, and a 100 on the secondary. You'll find that a panel with a 100 amp main is a lot less money than one with a 150 or 225. 

The compressor, if it's 3 phase, almost certainly has a dual voltage motor. Usually, a 460-480 volt kit can be ordered. It has a 480 coil for the starter, and new heaters for the now-lower amperage. Easy to install, and it'll run much better on 480. Plus, the transformer can be smaller, and no problems with office equipment.

Rob

P.S. Transformers come with 120/208 as well as 120/240 secondaries. The 120/208 is by far the most common. If you get one that's 120/240, it'll have a high leg. Two of the phases will be 120 to neutral, the other will be 208 to neutral. Not suitable for an office.


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## Murphy (Dec 10, 2009)

NolaTigaBait said:


> Oh, I thought if you had a 45kva tranny and you fed it with 480...you would get 54 amps...Use #4 and protect @ 70 amps?


 
im just learning so im not being smart.. but where is this coming from? wouldn't the load on the primary be divided by 480, which would give you 94 amps.. and twice that on the secondary.


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## wildleg (Apr 12, 2009)

with 3 phase you divide by (voltage * 1.73)


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## Murphy (Dec 10, 2009)

duh...wow i need to get some sleep..... thanks though


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## HAND (Jul 15, 2010)

Workingman said:


> I'm looking at a 400amp/480V/3ph panel in a warehouse. The owner wants to get a 200amp/240V/3ph panel added to he's building so he can have can build a office inside he's warehouse.
> 
> How do I figure out the size of the breaker to install in the 400amp/480V panel to feed the primary side of the transformer and size for the secondary side feeding the 200amp240V panel?
> 
> Thanks for the help.


450.3b, t450.3b


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