Electric furnaces must adhere to testing standards defined by which of the following?

The correct standard governing electric furnaces is UL 499. This standard specifically addresses electric heating appliances and specifies the safety and performance criteria that these appliances must meet. Electric furnaces are classified under heating appliances, thus making UL 499 the relevant standard for ensuring their safety and compliance during testing.

UL 727 pertains to the safety of electric oil heaters, while UL 1996 covers issues related to electric outdoor heat tracing cables, and UL 391 is focused on heating appliances for residential or commercial purposes other than those covered by UL 499. Therefore, it's clear that UL 499 is the appropriate standard for electric furnaces in this context, as it encompasses the essential testing methods and safety measures specific to that category of appliances.