I am planning to add an outlet circuit to my garage. Strangely, when they built the house, they only put two outlets in the entire garage, both of which are on shared circuits with the rooms on the other side of the wall (living room and kitchen). I have more than 100 feet of 12-2 Romex left over from other wiring projects, and wanted to know what my options were for using it in a surface mount application. I really don't want to open the walls up in the garage. Quick research indicates that it's NOT against code to run Romex in conduit, but it's not best practice for heat dissipation purposes and can be a pain to pull. I know THHN is relatively cheap, but not if I need to buy 3 separately colored spools of it rather than buying one larger spool and remarking the wires (which is against code, correct?)


  • What path do you plan to take the run the new outlet circuit? Commented Dec 23, 2019 at 17:23
  • 1
    I plan to run the new circuit along the perimeter of the sides and back wall of the garage. There is a stairway at the back of the garage that goes into the basement, and above the door is wooden construction rather than the block bottom walls in the rest of the garage. I would most likely run the romex thru the wood and land it in a junction box immediately on the other side, then branch off to do the back and side walls in exposed conduit (I suppose I could put the junction block on the basement side, then run conduit thru the wood as well). I hope this is what you were asking...
    – clwhoops44
    Commented Dec 23, 2019 at 17:33
  • Are the garage walls block all the way up, or do they transition to frame at some point? Commented Dec 23, 2019 at 17:53
  • They are only block for a couple of feet, then drywall. I'm thinking about 3 cinderblocks high, then framing for the rest of the ~12 foot ceilings.
    – clwhoops44
    Commented Dec 23, 2019 at 18:08
  • What is the source of the objection to having to patch drywall in the garage? Commented Dec 23, 2019 at 18:22

1 Answer 1


You need big conduit

A flat cable like Romex is treated the same as a circular wire of the large dimension.

  • With 1 cable in round conduit, the conduit inside diameter must be at least 138% of the widest cable width.
  • With two same cables in round conduit, the conduit ID must be 259% of the widest cable width.
  • with three same cables in round conduit, the conduit ID must br 274% of the widest cable width.

Note that at minimum sizes, it will be a bugbear to pull. You may be wise to upsize further for ease of pulling.


If you are using non-round conduit, then you need to look up the cross sectional area of the conduit and compute the cross section of the (rounded out) cable. Compute

cross section of cable = (cable widest width ^ 2) * 3.14159 / 4

Cable cross section can't exceed 53% of cable cross section for 1 wire in conduit; 30% for 2 cables in conduit, and 40% for 3 or more.

  • Is it permissable to strip the jacket off the romex exposing the internal THHN(?) wires and run them inside the conduit?
    – Jasen
    Commented Dec 24, 2019 at 2:13
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    @Jasen No, because the interior wires of Romex are not marked anything at all (and in fact that insulation is totally unfit for the physical strain of being pulled in a conduit). Wires without markings are illegal because they're uninspectable, they could be anything,the insulation could be foreign rubbish with no insulating power, durability or wet resistance. Wire without markings is weedwacker string. Commented Dec 24, 2019 at 17:38

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