I'm getting ready to add a 60A subpanel in my attached garage, but I'm conflicted about what type of wire to buy. The wire will run exposed in the rafters in the basement, then through the foundation wall, out into the garage(the garage floor is 4ft lower than top of the foundation). I plan on casing the wire in metal conduit once in the garage. The chosen wire size is 6-3, most likely copper since it easily available locally. The entire run will be dry and indoors.
Can I use NM-B wire for this? I'm confused because I will be running it into conduit once in the garage, which results in a double race-way. Is this acceptable, or should I be using a different type of wire?
edit
I chose NM wire because I wanted to avoid hanging conduit in the basement. The ceiling is low, and the rafters are 12" apart, which could make running conduit difficult. I plan on using conduit in the garage up to around 12' high, to avoid any exposed jackets where they may get caught on something. However, If I were to use THHN wire, I could fully enclose the entire run within conduit.
Best Answer
It is acceptable to run Nonmetallic sheathed cable in conduit, in fact code calls for it in some situations.
Conduit fill calculations become a bit more difficult when dealing with cables, but a few notes in chapter 9 provide some direction.
Note 5 says that you'll have to use the actual dimensions of the cable, to determine if it will fit in the conduit. While note 9 says that you can think of the cable as a single conductor, which means if it's the only thing in the conduit you can use the 1 wire fill percentage of 53%.
Calculate area of NM cable
You should be able to find a spec sheet from the manufacturer of the cable you choose. I found the spec sheet for Southwire’s Romex® SIMpull ® Type NM-B, which lists the sizes of various cables. It lists 6/3 cable as having a diameter of 650 mils, or 0.650". From which you can calculate the cross-sectional area, using the formula πr².
(0.650 / 2)^2 * pi =(0.325)^2 * pi =0.105625 * pi =0.3318307240354219108126167073589 in.²Conduit Fill
Once you know the area of the cable, you can use Table 4 from Chapter 9 of the National Electrical Code to determine the size of conduit needed. Remember because of Note 9, you can use the 1 wire column (53%).
Electrical Metallic Tubing (EMT)
1/2" @ 53% fill = 0.161 in.²
3/4" @ 53% fill = 0.283 in.²
1" @ 53% fill = 0.458 in.²
1/2" @ 40% fill = 0.122 in.²
3/4" @ 40% fill = 0.213 in.²
1" @ 40% fill = 0.346 in.²
Intermediate Metal Conduit (IMC)
1/2" @ 53% fill = 0.181 in.²
3/4" @ 53% fill = 0.311 in.²
1" @ 53% fill = 0.508 in.²
1/2" @ 40% fill = 0.137 in.²
3/4" @ 40% fill = 0.235 in.²
1" @ 40% fill = 0.384 in.²
Rigid Metal Conduit (RMC)
1/2" @ 53% fill = 0.166 in.²
3/4" @ 53% fill = 0.291 in.²
1" @ 53% fill = 0.470 in.²
1/2" @ 40% fill = 0.125 in.²
3/4" @ 40% fill = 0.220 in.²
1" @ 40% fill = 0.355 in.²
Rigid PVC Conduit (PVC), Schedule 80
1/2" @ 53% fill = 0.115 in.²
3/4" @ 53% fill = 0.217 in.²
1" @ 53% fill = 0.365 in.²
1/2" @ 40% fill = 0.087 in.²
3/4" @ 40% fill = 0.164 in.²
1" @ 40% fill = 0.275 in.²
In this example, you'd need 1" conduit to fit the 6/3 nonmetallic sheathed cable.
The other option. Is to run 6/3 NM cable in the basement, then switch to three 6 AWG THHN conductors through conduit in the garage. According to Table 5 in Chapter 9 of the NEC, 6 AWG THHN has an area of 0.0507 in.². So the three current carrying conductors would have a total area of just 0.1521 in.².
0.0507 in.² * 3 = 0.1521 in.²According to Note 8 in Chapter 9, you can use the area listed in Table 8 for bare conductors. So if you install a bare grounding conductor in the conduit, you can use this value instead of the insulated conductor size.
Which means you'll add an additional 0.027 in.², for a grand total of 0.1791 in.².
0.1521 in.² + 0.027 in.² = 0.1791 in.²Since there will be more than 2 wires in the conduit, you'll have to use the 40% fill column. Even with area reduction, you'll still be able to use 3/4" conduit instead of 1".