Is it acceptable to use 3/4" steel flex conduit instead of EMT for wiring 240 receptacle plug inside my garage? This would be attached to the garage wall.
The answer is fully dependent on:
- Whether the location is subject to physical damage.
- Conduit fill (for the 3/4" size) and planned amperage of the circuit.
For item 1, flex conduit cannot be used where subject to physical damage.
From NFPA-70:2014 (2014 NEC):
ARTICLE 348 Flexible Metal Conduit: Type FMC
348.12 Uses Not Permitted. FMC shall not be used in the following:
(7) Where subject to physical damage
"Subject to physical damage" is subject to the inspector's opinion since the NEC does not explicitly define these terms, so I would recommend consulting with them.
Generally, wiring at a height less than 8 ft is an unwritten convention that is typically considered subject to physical damage p(rovided there are no other surround protections/enclosures/guard strips and then you're back to the inspector's interpretation).
For item 2, once you have selected the wire size appropriate to supply the required current on the circuit, you will need to calculate conduit fill to determine the allowable size of the conduit/raceway (refer to Informative Annex C of the NEC).