Tech Question – Why Do ECM Motors Trip a GFCI?

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 startup. (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 every time 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?

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.

Also, on a furnace with a flame rectifier, a very small amount of current (microamps) is purposely routed through the flame to prove the flame. That has nothing to do with it being ECM, though.

Thanks for participating. Great stuff!


P.S. – What do you think is going on?


Joe Reinhard
Joe Reinhard
1/7/17 at 10:51 PM

Understanding of GFIs (the 1st acronym before “C” was added) have been (and specs read for 120v AC GFCI receptacles and a 240v AC GFCI breaker for a hot tub) say GFCI supposed to trip w/ 4-8 milli-amps detected btwn neutral and equipment or safety ground not “L” or hot leg. This is great since well below the m-amp amount that causes human heart fibrilation.
MHolt link shows “kundework Jr member” calling the ground “PE” but that acronym still excapes me.
The thread was helpful reminding me any VFD or ECM motor are non-linear loads and produce harmonics which is basically current and voltage riding and amplifying the fundamental AC (alternating current) 60 Hz sign wave. So the harmonics answer on why a GFI trips makes sense. Curious as to what harmonics (3rd, 5th, 7th,….11th?) cause the most m-amp current imbalance btwn the neutral and ground. Anyone else have this perception or understanding, I’d be interested in some confirmation. Thanks. As always, very interesting topics posed by HVACR School.cim.

    Gary L Reecher
    Gary L Reecher
    1/8/17 at 03:40 PM

    Is the extension cord being left on the ground in a coil or in one of those wind up reels? Even though there is insulation surrounding the cord there is still a magnetic field being generated around the cord and that magnetic field is being amplified which may be a problem.

    Would also be interested in what gauge extension cord is being used. If it is only 14 gauge it might be an option to use a larger gauge such as 12 or even 10 gauge.

Matthew Patchell
Matthew Patchell
1/8/17 at 06:45 PM

I am chiming in not having a chance to read Holt’s thread but I’m a senior service tech in the greater Boston area an ECM motors fluctuate amps/ohms which does at least to my understanding effect voltage? ECM motors shouldn’t effect voltage from main power since all the major is being controlled through the board an motor so I’m curious only because I work on ECM blower motors all the time along with variable speed draft inducers an understand what’s being said here but I may be way off but the board should be regulating all Hz / amp / ohms to my understanding? Please correct me if I’m off on this as I’m speaking from my understanding an experience which is roughly 13 yrs but not specifically with ECM motors an the correlation with GFCI circuits I’m interested because it’s the first I’ve heard of it being a problem I understand the concept and could see how that is a possibility but I feel like how would be causing that?
Thanks Guys an

mark wolf
mark wolf
1/8/17 at 07:46 PM

If you are using the furnace for the home I am glad you are having trouble making them work.This is not ok. You are dragging drywall dust & construction debris into the ductwork,the motors etc.essentially you are screwing the homeowner over.drywall dust is a indoor air quality issue,it is an abrasive that takes years off the life of the motors.It is corrosive.voids most manufacturer warranty.A brand new unit should NEVER be used as a temporary construction heater.

William Ball
William Ball
1/13/17 at 12:56 AM

Why is a furnace connected to a GFI? New construction? Furnaces are not designed for this power connection. And, ground fault receptacles are not designed for an application (furnace) that should be hardwired and switched to an electrical panel with a HACR breaker. An extension cord is not a substitute!

Chris m
Chris m
1/13/17 at 07:32 AM

My understand of gfci is they pretty much they count in then out electrons. One in one out. Alls good. But if 10 go in but only 9 come out. They shut down assuming it went to ground. Or if 11 come out.
Inverters cause nightmares to electric power. Groups of electrons are held back or pushed out depending on what the inverter is up to. Capacitors loading and unloading to change the pulse or hertz while driving the voltage higher and lower to keep motor rolling. Gfci sees this as fault. It likes even flow.
Like starting a big saw. The electrons race into the windings. Trying to get motor to spin. The get backed up in the wire. A Crap load go in few come out. As motor starts to turn. Then they all race out. Again causing an imbalance. Shutdown.

Alls a gfi knows is it does not see the same on each leg. Where it went? it does not care. It’s missing,shutdown.
Inverters rip electrical systems apart. Filters help. But they can cause back currents eddy current. Think noise in wires that can be measured all the way back to the powerhouse.
At low speed. Say, 5 hertz 25hp motors can bounce, due to the inverter pulse impacts. Higher they smooth out. Think using a shot gun to spin a pin wheel.
Pump inverters mess with digital programmers on the same electrical service. Filter filter filter.

Just my opinion.

Alex Murphy
Alex Murphy
1/30/18 at 08:19 AM

I came to the same conclusion you did Bryan, but I don’t have a fancy oscilloscope like the guy in Mike Holt’s thread. Anytime I wired it up to a non-GFCI protected outlet, it ran fine. We eventually found an outlet that worked with the furnace that was not as sensitive, but it was a doozy. Another thought popped up as I was writing this response. We install a bunch of Navien tankless water heaters… which are modulating pieces of equipment and per the NEC we have to run new GFCI’s in mechanical rooms or make sure we are plugged into one. We have to be careful when installing these with existing plugs due to the sensitivity of old GFCI’s. Thanks for posting (and responding to) this question as well as playing along with my lame Dear Abby reference.

Thanks for the replies and concern with running furnaces during drywall. I am very aware of this issue and agree with you, yet ultimately have no control over it. If I told our builders to use some other form of heat in their mcmansion, they would just use another HVAC company. The extension cord was a 12/2 100’ cord laid out flat along the ground. I tried 3 different gauge cords including a 10/2 with no luck.

11/17/19 at 07:29 PM

I was curious about this and came across this, which sheds some light on the why.
Thought I’d share.

4/13/21 at 05:09 AM

I recently bought a VOM that had a hz feature built in. When I test my generator the meter display is all over the place. (It has GFCI outlets). Furthering my investigation, I tested my household kitchen GFCI outlets and found them to react in a similar way. Thinking it was a faulty meter I almost gave up but tested a NON GFCI outlet to find it reading a constant 59.9hz. What makes a GFCI outlet do THAT??? 🙂

4/23/21 at 08:53 PM

Mike Holts link appears to be broken or the article has been removed ?
I am getting a permissions error:

“Oops! We ran into some problems.
You do not have permission to view this page or perform this action.”



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