Sourcing transformers for 120V => 100V Buck Boost Transformers

[Mikek400]

I have an item that uses 100VAC 140W 50/60Hz. 

I would like to keep the transformer as small as possible so that it can fit inside the unit. And having a mounting bracket would be a plus too.

I have built buck boost transformers but I don't know how you know how much power you can run through one.

For example a 120V => 20V 1A transformer.  Can you draw 1A through it at 100V in a buck transformer configuration?

Was looking at this one
http://www.ebay.com/itm/HC-Controls-Step-Down-Transformers-120V-to-24V-HCT-01P0AA00LP117-30VA-50-60-HZ-/252210900690?hash=item3ab8f0eed2:g:UGYAAOSwp5JWbODW

[IanB]

If your item draws 140 W at 100 V then it will pass a current of 1.4 A. If your buck transformer is bucking 20 V, then it will need to have a VA rating of about 20 x 1.4 = 28 VA to safely handle the load. Another way of saying this is that your transformer secondary should be rated for 1.4 A or more.

[Mikek400]

Thanks for confirming. That's how I thought it worked.

The other question is about a 120V to =>24V Transformer.

I have made buck boost transformers using 100V =>24V transformers and it bucks roughly 20-24V.

If I use a 120V => 24V transformer will the ratio be the same? I'd guess maybe not.

TIA