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Old 07-13-2010, 12:37 PM   #1
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Question How do I size for transformer 480Vx240V

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.

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Old 07-13-2010, 12:46 PM   #2
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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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Old 07-13-2010, 01:01 PM   #3
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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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Old 07-13-2010, 02:32 PM   #4
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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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Old 07-13-2010, 04:28 PM   #5
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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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Old 07-13-2010, 05:53 PM   #6
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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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Old 07-13-2010, 05:55 PM   #7
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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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Old 07-13-2010, 06:11 PM   #8
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..............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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Old 07-13-2010, 07:01 PM   #9
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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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Old 07-13-2010, 07:11 PM   #10
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So have we gotten to the size of the transformer yet?
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Old 07-13-2010, 10:36 PM   #11
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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.

Last edited by micromind; 07-13-2010 at 10:39 PM. Reason: Added P.S.
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Old 07-13-2010, 11:41 PM   #12
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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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Old 07-13-2010, 11:46 PM   #13
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with 3 phase you divide by (voltage * 1.73)
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Old 07-14-2010, 07:08 AM   #14
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duh...wow i need to get some sleep..... thanks though
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Old 07-17-2010, 11:21 AM   #15
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Originally Posted by Workingman View Post
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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