In MN I am using one gang metal boxes in ICF wall. The boxes have one knock-out on top. Can I have two cables come out through one gromet?

If I anchor the cables within 6 inches, do I need to have "holding" grommets?

 

IF you are using steel boxes you are required to use a bushing or connector. I recommend using button connectors like this: NM94 | Product Information They will accept up to 2 - 12/2 or 14/2 cables. I know Menards and Home Depot both carry versions of them.

If you are using handy boxes or a single wall case I would be careful. They do not have a very large cubic inch capacity. Many cases they only have enough room for one cable and one device. IF your using steel boxes, I recommend 4" square boxes with mudrings the depth of your sheetrock. That or use deep wall cases (the ones with ears)

 

I am using the NM 94 grommets with 16.5 cu boxes in the ICF walls. I will be installing a thin synthetic stucco coating to the ICF walls, so no mud ring.

 

You would be better off to use deeper boxes. With two cables you have 5 conductors plus 2 for the device. This gives you 7*2= 14 cu inches minimum on a 15 amp circuit. You would have 15.75 if using #12.

A 3 1/2" deep metal box is 18 cubic inches.

 

Can't use deeper boxes, the eps foam is 2.5" thick. If I was using drywall I could use a mud ring to add .5 inches, but I am not. I am using 12 guage, I have done a bunch, tight, but doable.