Chris Lewis wrote:
Strictly speaking, running things like furnaces off "cords" is
frequently a code violation, but most inspectors will consider
"emergency preparedness" to be a good reason. Just make sure
that the installation is otherwise in excess of code. Eg: _adequate_
power cords - SOW cable for "appliance cord", not lamp cord,
with appropriate measures to prevent damage to it.

Double pole, double throw, center off switches; that are suitable for
use as transfer switches for individual loads; are readily available and
when wired to a flanged inlet they make a fully code compliant way to
run a furnace from a portable generator.
--
Tom H