dry type transformer load 1

[Elio581]

I need a transformer 480v/120v to supply a load of 20KVA @120v. I have problems sizing the capacity of the transformer if I only want to use 1phase. Should I use a 1phase or 3 phase transformer? What connection? Thanks in advanced.

 

[waross]

If you use a single phase 480 Volt supply, your transformer must be at least 20 KVA for a 20 KVA load.
If your code allows a maximum of 80% loading a single phase transformer will need to be 25 KVA.
If you use a three phase transformer you lose 1/3 of the capacity and the transformer needs to be 30 KVA for 100% loading or 37.5 KVA for 80% loading.
You may use a double delta or a zig-zag connection to derive single phase power from a three phase transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

For that much load I would use a 30kVA 3 phase 480-208Y120 transformer and distribute my 120V loads evenly across the 3 phases through a panel board. That much load on a Single phase is likely to cause an imbalance in your 480V 3 phase system. But this is assuming your 120V loads are all separate, not one single 208A 120V load. If you have one single load that needs 208A at 120V, that’s going to be tough.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

For that much load I would use a 30kVA 3 phase 480-208Y120 transformer and distribute my 120V loads evenly across the 3 phases through a panel board.

Best by far if this is possible.
Although you need only 20 KVA with this solution, it is doubtful that you will be able to find a three phase, 20 KVA transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Elio581]

Thanks. But All of my load is 120v and the output of the transformer is 120/240. So if I use just one of the secondary windings what’s the maximum load I can use?? 50%?

 

[davidbeach]

One load at 20kVA at 120V? Or a bunch of 120V loads that all add up to 20kVA? If the latter you put some of the load on one of the 120V and the rest on the other and you need something more than 20kVA single phase.

 

[Elio581]

To be specific. I have an industrial 20kva ciberex UPS. The input voltage is 480 3phase and the output is just 120v 166 amps in only one circuit. For the alternate power I need a transformer to step down another supply source of 480v to 120v to support the load in an emergency. And the load is 20kva 120v. I have doubts if is better use a 3phase or a 1 phase transforme.

 

[davidbeach]

Then you clearly need a 480/120V single phase transformer.

 

[waross]

With a single phase transformer, 240/480:120/240 is a standard rating.
You can connect both 120 Volt windings in parallel and use the full output at 120 Volts.
If you use a three phase transformer you can use a parallel double delta connection to provide 120 Volts, but a three phase to single conversion always loses 1/3 of the transformer capacity.
For a 20 KVA single phase load you may use a 20 KVA single phase transformer, but if you use a three phase transformer connected for single phase you will need at least a 30 KVA transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Make sure you have four bushings if you wish to parallel the windings.

 

[waross]

A 20 KVA or 25 KVA 480:124/240 dry type transformer doesn't have bushings.
For a three bushing distribution transformer you change the connections inside the tank. Tools and gloves must be spotlessly clean and dry to avoid contaminating the oil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

For a three bushing distribution transformer you change the connections inside the tank.

I would have expected a 480/120, 20kva or 25 xfm to be a dry type - pretty common ratings.
I don't think I have ever seen a 20kva oil filled - or a 25KVA either

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

"I don't think I have ever seen a 20kva oil filled - or a 25KVA either"
Neither have I.
My comment was meant to differentiate between dry type transformers that typically don't have bushings and higher primary voltage oil filled distribution transformers that do have bushings.
I have reconnected a number of 13 kV:120/240 Volt oil filled transformers with three bushings for 13 kV:120 Volt.
I am sorry if my previous post was not clear.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Leads then? Or are dry transformers not available with tapped windings? Technically this would be a 240/120 rather than 120/240, but I'm not sure everyone knows this little detail (and I always forget which way is which). Two individual secondary windings are necessary to get the full rating at 120.

 

[waross]

Dry type transformers with four windings are quite common. To primary windings and two secondary windings.
Hence 480/240:240/120 Volts.
Three phase dry type transformers with taps on each phase are common.
You may also find 480:240/120 Volt transformers with taps on the primary. This gives you the ability to use full capacity at 120 Volts and still have the advantage of a voltage adjustment taps.

Bill
--------------------
"Why not the best?"
Jimmy Carter