I would like to know what size pvc conduit I need for 12/2 wire with ground .Thanks for the help.

 

Any size it will fit in. It's a tight fit in 1/2", so you should probably use 3/4". Better yet, except in very limited situations, don't put 12/2 in conduit at all. It's almost always better to use THHN instead of NM-B in conduit. Can you give us more details of your project?

 

I am putting in a whirlpool tub and runing a line along the ceiling in the garage. Thanks

 

I agree with John Nelson - I would always pull individual conductors if using conduit, and it will be less money than NM. Also, many whirlpools I am familiar with require a 30 amp circuit. #12 is 20 amp max. Look in your instruction manual, or on the nameplate on the unit, to find out what size "Max OC", OCD or OCPD (All are acronyms for overcurrent protective device - a technical term for fuse or breaker.) is recommended. #10 wire is required on a 30 amp breaker. And with your reference to 12/2, it sounds like a 120v unit, which I also have rarely seen. It might require a 200v circuit, which is two hots, a neutral and a ground. Either individual conductors or a 12/3 w-ground NM cable.

Juice

 

I thought code didn't allow NM cable to run through conduit due to potential overheating issues, and that THHN was required if conduit was being used. But I guess I'm wrong since both John and Juice are saying that it's ok. Am I remembering something a little out of context?

Joe

 

I've heard that when running any type of wiring through conduit you should use the next larger size gauge because of heat dissapation. I think it also depends on the duty cycle, if you're pushing the limits of the wiring your using, and how long the wire will run in the conduit. If the hottub requires a 30 AMP circuit, then 10 gauge is the way to go. However, if you'll be running the wire in conduit for a long run, I might go with a bigger wire. Take a look at the manual that came with the hot tub. See what it mentions in the electrical section. With electrical it's much better to be safe than sorry.

 

The exact parameters of when NM is allowed in conduit are hotly debated among electricians, and I surely hope I don't start a debate of that issue here. But it is very clear that NM is allowed to be sleeved in conduit where required for physical protection. Exactly how far that sleeve can go is one of the subjects of debate. But I think most people agree that it's a bad idea to use NM in conduit between two junction boxes, where THHN is so much more feasible. And because of the difficulty and risk of damage of pulling NM very far through conduit, distance should be limited. How limited is not clear. And of course you should never assemble the conduit around the cable one piece at a time, especially with PVC, because PVC glue is harmful to NM cable.

 

It is not required automatically to increase wiring size if running conductors in conduit. It depends on ambient temperature expected in the area where the conduit passes through, and on how many current-carrying conductors are in the conduit. In most common applications, no derating is required.

NM in conduit is frowned upon, though not specifically prohibited, because the outer sheath traps heat, and on top of that conduit traps heat. #10 is required as a minimum for a 30 amp circuit. If the conductors are to be derated due to ambient temperature or conduit fill, then you would use #8 for that 30 amp circuit. #12 is for a 20 amp circuit. If it is to be derated, then you would go up to #10.

John Nelson recently advised that when running NM cable in conduit, the conduit should simply be what fits. However, the notes to the Chapter 9 tables, which specify allowable conduit fill, state that (note 9) "A muliconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have eliptical cross sections (flat oval shape, as type NM is), the cross-sectional area calculation shall be based on the major diameter of the elipse as a circle diameter." In other words, if your NM cable is 1/2" wide at its widest point, you must calculate the % cross-sectional area as if it were a single conductor 1/2 inch in diameter, dramatically increasing the conduit size required. If running over 2 conductors (as would be the case with, say, 12/2 with ground, the % fill is 40% max. 12/2 w/ground is .19 inches by .41 inches. You would calculate based on a .41 full round diameter. Area = pi*r^2 = 0.132 square inches. 1/2" conduit = 0.217 square inches. Max fill at 40% would be 0.0868 inches. Too small. 3/4" conduit is 0.409 square inches. 40% of this is 0.1636. So 12/2 with ground requires a 3/4" conduit, minimum. These conduit figures are based on schedule 80 pvc. Other than PVC, most homeowners generally use EMT conduit. The internal area for 1/2" is 0.304 square inches. At 40%, you can fill it up to 0.1216 square inches. Too small. 3/4" is .533 square inches. At 40%, you can fill it up to 0.219 square inches. 12/2 w/ground would fit.

Without going into the individual calculations, #14/2 w/ground is .1075 square inches, and would fit in 1/2" EMT. But not sch. 80 PVC. You'd still go with 3/4" PVC.

10/2 w/ground is 0.1886 square inches. You would go with 3/4" EMT, but it is too small for 3/4" sch. 80 PVC. 1" PVC is 0.785. At 40% you can go up to .314 square inches. Go with 1" PVC for 10/2-g.

It is also important to note that the only types of conduit permitted for physical protection of type NM cable is rigid and intermediate metal conduit, electrical metallic tubing (EMT), and schedule 80 PVC. Schedule 40, by exclusion from this article, 334.15(B), is apparently not permitted with NM.

Hope that helps.

Juice

 

My "whatever fits" advice was based on a situation that did not involve a "complete conduit system". Such situations are exempt from the conduit fill rules. A "complete conduit system" is not defined by the code, but is generally understood by most people to be conduit that goes from one junction box to another junction box. If the conduit is open at either or both ends, it is not ususally considered such a system.

 

John, I agree. In cases where your NM cable will bass through a roof, foundation, etc., physical protection is required and can be in the form of a 12" piece of condit. Since it is open at both ends, yes, you would use whatever size fits. Heat entrapment in such situations is not an issue. I did not define my terms, as in this case was the requrements for a complete conduit system.

Juice