conduit size with Romex

I have a small stockpile of old metal electrical conduit that measures @ 5/8" inside diameter. I think they call it 1/2" conduit. I am finishing new construction on my house and have wired everything fine and now want to add some outlets in the basement in an unfinished area. I want to protect these wires as they come down the concrete walls and was wondering if I can put two sets on 12/2 with ground Romex inside this conduit. It fits - but is a little tight. I only ask because I already have the conduit and all the fittings that would connect it to metal boxes etc. I need two sets of wires so I can run back out and to the next drop. I don't have access to any code tables to look this up. Any help would be appreciated.

 

No.

I don't have my code book with me now, but I'm sure you can't put two 12/2 cables in 1/2" conduit. I'll look it up later to see if it's legal to even put one in.

If you go to the library, look at the tables in chapter 9 of the NEC.

 

I checked the 2002 NEC. You are allowed to fill 53% of the area of that conduit. For 1/2" conduit, this is only 0.161 square inches, Even a single 12/2 NM cable is counted at 0.2 square inches. So code doesn't even allow you to put one cable in this conduit.

If you go up to 3/4" conduit, then you can put one 12/2 in it.

If you want to put two 12/2 cables in conduit, you can only fill 31% of the conduit and thus you'd need 1.25" conduit to be legal.

All of the above is from Chapter 9.

Article 310 will still allow you to use 20-amp breakers with four conductors per raceway. You only need to derate to 80% and that is still sufficient to avoid reducing the breaker size in most ambient temperature conditions.

 

Thanks John....I traded a friend some 1/2" for 3/4" and 1- 1/4"even up. This may work out after all. I was suprised at how little wire you can put in the conduit compared to the space (31% fill!), and am equally suprised at how much you must derate just because of the extra wire running parallel to each other. I can't say that this makes complete sense to me but I am very thankful for the information and I am equally eager to meet or exceed all codes. The closest library that has a NEC is over 30 miles away as I live in the out in the woods away from any towns. Thanks for saving the trip. Would you like to share your opinion of why the conduit needs to be so empty and why you must derate. I can see that if wiring would heat up because of near capacity loads, that this could be the reason, and they would want air space between wiring for this...is this why?

 

You could install a handy box at the top where the ceiling meets the wall. Then install your Romex in and out of that handy box. Then install your 1/2 conduit down to a handy box where your receptacle is installed using 12 THHN single conductors in the EMT instead of Romex. This would solve your conduit fill problem. As John was referring to Romex is treated as per the NEC as a round conductor when it concerns conduit fill. 12/2wGrnd is about 1/2" wide then you must consider this Romex to be a single round conductor equal to 1/2" diameter. This is why you are limited in conduit fill while using Romex.

The deration is becuase each current carrying conductor will heat each other. This is the reason for the deration in ampacity required in Table 310.15.B.2.A for more than 3 current carrying conductors in a raceway.

As for your comment about near capacity loads needing air space, you are actually closer than you think. What you are installing are general lighting style receptacles that are of unknown use. There may be a load applied to this receptacle ranging anywhere from a minor load of a lamp to a major load of a vacuum sweeper motor or even more. The actual load of this convenience receptacle is unknown the entire life of that receptacle. At any time it may be loaded to its maximum. In wiring design to ensure safety we must design for the worst expecting the possibility of that maximum load being applied to any one of those convenience receptacles.

Hope this explains more of what John was saying and the why's

Good Luck

Wg