This question was submitted on the site in response to the recent GFCI tip. It’s a good question with several possible answers. What do you think?

Hey Bryan,

I have had a few instances where we are firing off a furnace in a new build with a temporary power pole outside with gfci outlets installed that are tripping the gfci on blower motor start up. (In order to get temporary heat for drywall, we run an extension cord to the gfci pigtailed to the furnace). I’ve never had issues doing this with PSC motors, but everytime I’ve tried to use a gfci with a variable speed or true ECM motor, it trips the gfci. I’ve tried multiple motors, different furnaces.

What gives?

Sincerely,
Frustrated in Fort Collins

Howdy Frustrated (This feels like a Dear Abby article but I’m just gonna roll with it),

I haven’t tried this so I haven’t experienced it (No big call for temporary heat in Florida) but it does make some sense. The GFCI is just watching for a difference in current on hot and neutral. When you add in a variable frequency drive (Which is essentially all an ECM or X13 motor is) the circuitry / housing is exposed to a lot of “induced” electromagnetic fields due to the varying frequency / harmonics. Some of this induction (Magnetic Flux) may be routed to ground causing a tiny imbalance. Here is a thread in my buddy Mike Holt’s forum that talks about this very phenomenon HERE

Thanks for participating. Great stuff!

— Bryan

P.S. – What do you think is going on?