do'es a gfi plug change the fill factor in a box

No. Each yoke or strap counts as two conductors, regardless of its physical size.

It is always a wise idea to use a deep box for GFIs, dimmers, etc.

I wonder why the code doesn't address the extra space that GFIs take up??

Peter

If the box is too small. Theres is always the GFI Breaker. No space problems there

GFCI breaker.

Higher cost, more likely to trip from cumulative leakage currents and you have to go find it if it trips.

I will stick with a GFCI outlets in deep boxes at the point of use.

The majority of our prints require GFCI outlets at each location that needs GFCI protection.

Bob

Yeah, that's the other thing: GFI's in standard boxes.
It's a good idea to use a larger box but not mandatory. Most of us will learn by experience in this regard because you will often have a ground fault after cramming all of that into the box!

We don't need the NEC to dictate our every move do we? That book keeps getting thicker & thicker as it is...

Anyone who has ever installed a GFI receptacle in a standard sized weather-proof box knows that if you have more than one cable entering the box you will have one helluva time getting that receptacle into the box. And so you add a weather-proof extension ring and you're good to go.
Same with an indoor box: Use a 20.5 cu. in. or even a 22 cu. in. if you have more than one romex to contend with. You'll be glad you do at trim-out.

:~)