NEC 2005, Article 450.4(A) has a confusing clause regarding autotransformer overcurrent protection:

An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B [Output taps] as shown in Figure 450.4.

What, exactly, do they mean by this? It sounds almost as though they're probihiting overcurrent protection on the output, but that makes no sense and this is directly contradicted by the table referenced earlier in the paragraph. Do people actually physically open up these transformers to break the windings so they can put a breaker in the middle, is that what they're prohibiting?

Yup, they are prohibiting an overcurrent device in the middle. If such an overcurrent device were to operate, then the output voltage would rise to the input voltage.

Since the current rating of the shunt winding is less than that of the transformer and the series winding, I can see why someone might want to put an overcurrent device in that location. This would be a case of a safety improvement in a component of a system reducing the over-all safety of the system.

-Jon

Steve, I doubt they're concerned with someone opening the transformer to insert a breaker. Referring to the diagram again, I believe they don't want a breaker between the bottom of the winding and point B; t should be in the line to the left of point B.