What is an autotransformer

How does the NEC relate to autotransformer?

From Merriam-Webster dictionary:

Definition of AUTOTRANSFORMER
: a transformer in which the primary and secondary coils have part or all of their turns in common

These have specific purposes, why do you ask?

An auto transformer is a transformer that only has one coil. It still has a normal input and output and works just like a normal transformer. It is cheaper and lighter but has the drawback that under fault conditio0ns the supply voltage can appeare at the output. EG a 110v:12v autotransformer could give an output of 110v if a short occurs! They are normally used where a slight change in voltage might be needed say to boost voltage at the end of a long line when higher load is present! This url might help!

http://www.allaboutcircuits.com/vol_2/chpt_9/5.html