On Monday, November 4, 2019 at 10:19:00 AM UTC-5, Bob Engelhardt wrote:
I have a Ward-Leonard variac that was originally used, I think, for
theatrical lighting. It is labeled "20A resistive, 14A otherwise". I
suppose "otherwise" means motors, but motors aren't usually run off
variacs. What else could "otherwise" be?

Also, why only 14A?

"Otherwise" would be non-resistive loads - which would be reactive (capacitance) loads, or inductive (Motors and such) loads.

From another website, put simply:

When electricity flows through a circuit, there are points on the circuit, called loads, where energy is drawn away. Loads, in essence, are objects that use electricity--such as light bulbs. There are a variety of classification systems, but one way you can divide loads is into resistive, capacative, inductive or a combination of these types.

So:

A Variac "sees" a resistive load as a constant, whereas reactive and inductive loads will vary, so the amount of current the variac "sees" may be greater than the constant load at onset, shutdown, or when the load varies. Hence the derating.

Peter Wieck
Melrose Park, PA