Step Down Transformer With Nail Core Trips Breaker When Plugged In—Why?

I built a step down transformer with a nail core and enameled copper wire but when I plug it in to test it my breaker trips why?

A nail? sounds like a solenoid, not a transformer ;-)

Here are some resources for properly designing a transformer:

http://www.diystompboxes.com/smfforum/index.php?topic=34112.0;wap2

http://geek.scorpiorising.ca/lcms/index.php?page=windingtransformers

Hi Matthew,
Congratulations on starting to build transformers !

Please would you upload a picture and/or diagram of your transformer, and give us the following specifications:
The size (diameter) of the wire you used;
The length of the wire;
The number of turns of wire on each of the primary and secondary windings;
The input and output voltages.

Then we can do the necessary calculations and we will probably be able to diagnose your problem.