re: the NEC doesn't require GFCI protection for the fan unless it's
located over the tub, and the manufacturer requires it
I'm not pushing back...just trying to understand the wording above.
The "and" in that sentence seems to indicate that the manufacturer has
the final say. In other words, the NEC requirement is based on 2
criteria: location and the manufacturer's requirement.
Are there fans that can be installed above a tub but not require a
GFCI protection because the manufacturer doesn't require one?
The NEC requirement is just the general one that equipment be
installed according to manufacturer's requirements. Manufacturers
typically say that their fans may be installed over showers if they
are installed on a GFCI. I don't know if there are any that allow
installation over a shower without GFCI protection.
A good description of manufacturer's requirements being mandatory, not
There are also requirements from the agency approving the device. That
is usually UL. Those requirements are also mandatory.
Bath fans and GFCIs came up about a month ago:
From the UL "White Book"
"Fans intended to be mounted over tubs or showers have been evaluated
for such purposes and are marked 'Acceptable for use over a bathtub or
shower when installed in a GFCI protected branch circuit."
The manufacturer will always require GFCI protection for a fan over a
tub. If the manufacturer misses it, it is required by UL. [Old fans may
have been made when the UL standard was different.]
(The UL "White book" is available at:
5.5M and not real easy to use)
The "final say" may be NEC, manufacturer, or approval agency.
My in-laws just had a new roof installed, so I didn't want to cut any holes
and risk causing a leak. So we routed the duct out through a sidewall.
Except for a short bump up over a beam, the 8' duct slopes mostly towards
the outside wall vent.
So, there's no chance of a leak, other than maybe condensation working it's
way back. However, the tripping doesn't seem to be related to shower usage,
which makes me doubtful of a moisture related cause.
I am skeptical that a properly working fan should trip a properly
working GFCI when the fan is turned off. If there is no ground fault
in the fan or switch, there can be no imbalance of currents (conservation
of electrons). A double pole switch should not make a difference.
I might suggest an intermittent ground fault in the
fan or switch when the fan is turned off.
Please let us know what the problem is/was when/if you figure it out.
Give MQ's theory a try. I would agree with his suggestion that you
might have an intermittent ground fault in the switch itself.
Cheap & easy to try it out. Just swap the switch with another SPST
from somewhere else in the house.
This is incorrect; see an earlier post regarding the collapse of the magnetic
field in the motor windings when power is switched off. The consequent
backcurrent generated does indeed produce a brief, slight current imbalance.
This is evidently of sufficient magnitude and duration to exceed the GFCI's
Of course it should. By opening both the neutral and the hot, a DPST switch
prevents the backcurrent from reaching the GFCI, thus preventing the GFCI from
ever seeing the imbalance.
Perhaps; but the explanation referenced above is IMO much more likely to be
Very doubtful. Given that it's a single-pole switch, this would require an
intermittent, high-resistance short to ground that doesn't draw enough current
to make an audible arc. Hard to imagine how that could happen.
Easy enough to test -- but the results are predictable: it won't make any
It's possible that this particular GFCI is a little more sensitive than it
should be, and swapping it with a different one may cure the problem.
Switching both the hot and the neutral through a double-pole switch is almost
certain to fix it.
Doug Miller (alphageek at milmac dot com)
Only if there is leakage to ground. The leakage could be capacitive
currents from winding to motor poles. Else, as M Q wrote, how could the
current be different in the 2 wires? Field collapse can create a high
voltage spike but it creates a current in the circuit through both wires.
Could be high voltage produces current from high side (hot) to grounded
motor poles by capacitive current?. Spike is short duration (high
frequency) increasing the capacitive current. Size of voltage spike
depends on where in the current sine wave the switch opens.
Assuming timing of both poles opening is close enough together that the
GFCI doesn't trip.
NOT correct. A GFCI trips any time there is an imbalance between the hot and
neutral currents, that exceeds the thresholds for duration and current. It is
NOT necessary for there to be ANY leakage to ground.
Yes, it does -- and with current reaching the motor through an SPST switch,
*one* of those two wires is disconnected from the GFCI when the switch is
opened, but the other is not. And that's why the GFCI sees an imbalance -- and
why I think a DPST switch will eliminate the problem.
Possible, I suppose... but I'm not convinced.
Pretty good assumption, I'd say.
Doug Miller (alphageek at milmac dot com)
Of course. But how do you get a different current in the wires of a bath
fan without ground leakage.
Still not explained - how is the current not the same in both wires
unless there is some form of leakage to ground. The current is the same
in both wires for a light bulb. Why not the fan.
Wall switches generally operate slowly - apparently intentionally. (You
can typically move the handle to a position that causes the contacts to
arc - not that arcing is relevant.) I have read the intent is that the
contact break be slow enough to reduce inductive kick from magnetic
field collapse. GFCIs may operate in 1/2 cycle. It is not obvious to me
that slow operation and random variation would not result in timing
difference between poles of 1/2 cycle in some switches.
Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
As always, it trips when the fan switch is turned "OFF", not while it is
I've tried two different GFCI's, and three different switches, and they all
have the same results.
I'm baffled. The wiring is new and in good condition. The only two things I
can think are the fan itself is bad, or condensation is draining back into
the fan and causing problems.
Still, it seems like the DPST switch would completely isolate the fan from
the GFCI. There shouldn't be any current flow even if there was a problem
with the fan. And there have been absolutely no problems while it is
running. Only when it is turned off.
Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?
As you would expect. The incoming hot wire goes to one pole of the switch,
the incoming neutral wire goes to the other pole. Then the switched hot and
neutral go directly to the fan.
The ground wire is directly tied to all other grounds.
The switch clearly indicates which two terminals are the "line" and which
two terminals are the "load". Outgoing hot wire is on the same pole as the
incoming hot wire, and the outgoing neutral is on the same pole as the
I don't see any other way you could wire the switch?
Seems unlikely. I've never had a GFCI fail before, so I can't imagine two
being bad, especially since they are two different brands.
That seems to be the only thing remaining, though I don't understand how it
could trip the GFCI if the DPST switch cuts it out of the circuit
I wired the two bathrooms in our house the exact same way and we've never
had a problem with the fans tripping the GFCI's.
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