I want run some additional drops in my garage. The way I want to do this is to drop from the ceiling for each one. For this I would want to daisy-chain from one drop to the next, so I would have 2 - 12-2 romex cables in the conduit of ech drop; AC/incoming and AC/outgoing.

1- Can this be done? If so, what size conduit is required?

2- Is there a better way?

 

Yes. Best practice is to use individual wires not cables in conduit. Cables are harder to pull and take up a lot of room. Non continuous conduit is used to sleeve cable where it may be exposed to damage but in a closed conduit system such as this commonly you would use THHN/THWN wires. ½" or ¾" conduit would be fine for this.

Is this an attached garage?

 

If all you're looking for is physical protection on the walls, 3/4" PVC is all you need.

 

Your plan is not completely clear. Are you installing conduit for each drop from the ceiling to each box only, or are you doing the entire project in conduit?

 

Based on NEC conduit fill, 3/4" conduit is not large enough for 2 cables of 12-2 Romex. NM cable is counted as one conductor and the largest dimension is used to calculate area. Two conductors in conduit is to use a 31% fill. If these are sleeves 24" or less then 60% fill can be used and 3/4" is okay. One 12-2 NM cable has an area of approximately .132 sqin. 31% fill on 3/4" pvc conduit is .157 sqin. and EMT is .165 sqin.

 

If 3/4" is not large enough, then I've been doing it wrong and passing inspection for the past thirty years.
What size conduit would you suggest?

 

I'm just citing what the NEC calls for. Based on that it appears 1" would be needed. Can't say I agree with it because 3/4" seems adequate to me. 1" seems quite large for two 12-2 Romex cables. If my calculations are wrong I'd be happy to be shown what's right. I got the dimensions from Southwire for their 12-2 Romex.

 

Now this is from the handbook. It is not code, but explanatory information.

"The maximum fill requirements do not apply to short sections of conduit or tubing
used for the physical protection of conductors and cables. Cables are commonly
protected from physical damage by conduit or tubing sleeves sized to enable the cable
to be passed through with relative ease without injuring or abrading the protective
jacket of the cable. The requirement of 300.5(D)(1) regarding physical protection of
direct buried cables and conductors as they emerge from below grade is an example of
conduit or tubing being used as a protective sleeve and not as a continuous raceway
system per 300.12. However, a fitting is required on the end(s) of the conduit or tubing
to protect the conductors or cables from abrasion. [See 300.15(C)."

The other thing, if you claim the "protection argument", it calls for sked 80 if you go pvc.

 

I agree, the sizing is for complete conduit systems. A conduit sleeve does not need to meet the same rules.

 

Correct. I misspoke thinking the 24" or less @ 60% fill also applied to sleeves. So it's okay to use a 3/4" conduit sleeve for two 12-2 Romex cables as long as they fit without possible damage/chaffing to the cables.

 

Correct, the sleeve cannot be so tight as to damage the cables.

 

I assume the garage is open construction,if so,conduit with individual conductors into "J" boxes then drop down with SO cord or the like,use the proper connectors.
Geo

 

SO cord cannot be used in place of permanent wiring. Cords are allowed to connect equipment where flexibility is needed.

 

The OP mentioned he wanted to drop down from the ceiling,that's why I suggested the cord with proper fittings, I know cord can't be used for permanent wiring.
Geo