What diameter non-metalic conduit (schedule 40) is required for 6 romex cables (3 - 20A and 3 -15A)?

Question

What diameter non-metalic conduit (schedule 40) is required for 6 romex cables (3 - 20A and 3 -15A)?

Answer 1

http://books.google.co.uk/books?id=wn94U...

Answer 2

Romex cable does not go through conduit.

tcw

Answer 3

This is a GOOD question; and I'm glad you asked it, as it is important to know the rules for running NM Cable in a raceway.

The first thing you need to do is to MEASURE the actual cable assemblies (each manufacturer's cable will vary a bit, depending on the sheathing thickness).

Next, you must go to Chapter Nine of the NEC, and see Note #9 in the "Notes to Tables" Section. This will tell you that a multi-conductor cable assembly shall be treated as a single conductor; and that the actual dimensions must be used. It also states that for an elliptical cable, the widest dimension shall be used as the radius when figuring the cable's area. (3 or 4-wire assemblies are generally round; two-wire assemblies are generally elliptical.)

Now, go back to high school geometry and figure the cross-sectional area of each of the cable assemblies. (Remember the area of a circle isA=πr^2.)

The NM Cable I just measured has the following dimensions (yours may be different):

14-2 (15 Amps) has a wide dimension of 9/16"

12-2 (20 Amps) has a wide dimension of 5/8"

(Since you didn't mention any 3-conductor cables, I'll stop here.)

To figure the cross-sectional area, you square the radius and multiply by pi (3.1416). 9/16 has a decimal value of 0.5625; 5/8has a decimal value of 0.625. Use half of these values as the radius of the cable. The #14 conductor formula is [0.2813 x 0.2813 x 3.1416 x 3] (conductors) = 0.7458 (I try not to use more than 4 decimal places.) The #12 conductor formula is [0.3125 x 0.3125 x 3.1416 x 3] = 0.9204.

Add the two cable assembly values together and you get a total of 1.6662 sq. inches. Now the hard part is done.

Remember, that when installing more than 2 conductors in a raceway, the raceway fill can only be 40%. Now you have to look in Chapter 9, Table 4; under Articles 352 & 353. There you will find that schedule 40 RNC has the same interior dimensions as HDPE Conduit (new to the 2005 NEC). You must use the 40% fill column and find the next greatest value to 1.6662; which is 2-1/2" RNC (1.878).

The problem is, YOU CANNOT RUN ALL OF THE CABLE ASSEMBLIES IN ONE RACEWAY, if you are using 15 Amp & 20 Amp overcurrent protection devices. You will see in Table 310.15(B)(2)(a) that you must derate the current carrying capacity (Ampacity) of the conductors when run in a bundle or in a raceway. For Type NM Cable (Romex), you must count ALL of the current carrying conductors; so let’s assume that you have all 2-conductor cable assemblies. That gives you 12 conductors that must be derated. According to this Table, you must derate them by 50%. Referring to Table 310.16, you will see that you start derating #14 THHN (Type NM-B is the equivalent of THHN) at 25 Amperes; 50% of 25 is 12.5 Amperes; you cannot install that on a 15 Amp breaker when derated by 50%. #14 THHN uses 30 Amperes as the starting point for derating; 50% of 30 is 15 Amperes, and you cannot install that on a 20 Amp breaker. You also may have to derate the conductors for the ambient temperature for the location they are being installed in (see the “Correction Factors” at the bottom of Table 310.16).

Refigure your pipe-fill for two raceways. You’ll be able to use a smaller raceway, and you won’t have to worry about overheating the conductors, or, under-sizing the overcurrent protection.

Once again, let me say “GOOD QUESTION.”

And, to anyone who says "You can't run "Romex" in conduit", let me point out that this is often required (actually, allowed is a better choice of words) by Code as a means of protection, when protection of Type NM Cable is required.