Max Voltaire ) said...

The only ones that I
found that were short enough were 4" boxes. So last time I had to use
a 4" metal box, put an adapter/fitter on it to house a duplex
receptacle and plaster over what was left around the outlet. That is
probably not up to code.

That is perfectly acceptable under code (CEC, though I suspect the NEC
is similar in this respect).

I was using this for phone/LAN/TV outlets as I had a supply of 4"
square boxes and "outlet rings" (the piece that fits on a 4" box and
provides a single-outlet opening 1/2" out from the surface of the box).

When my electrical rough-in was inspected, the inspector said that
they wanted more room for GFCI outlets than was provided by the 3"
deep boxes we were using. He suggested either using a 3.5" deep box
OR using the 4" square boxes with the outlet ring on them.

--
Calvin Henry-Cotnam
"Never ascribe to malice what can equally be explained by incompetence."
- Napoleon
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