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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.

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    Why do you need conduit, are you planning to run exposed in the garage? Why not us NM in the basement, and switch to condit and individual conductors in the garage? – Tester101 Oct 8 '14 at 4:28
  • @Tester101, this was what I was considering initially. I wasn't sure if running the NM from the main into the subpanel could be ran through conduit. – Lee Harrison Oct 8 '14 at 12:32
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It is acceptable to run Nonmetallic sheathed cable in conduit, in fact code calls for it in some situations.

National Electrical Code 2014

Chapter 3 Wiring Methods and Materials

Article 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS

II. Installation

334.15 Exposed Work.

(B) Protection from Physical Damage. Cable shall be protected from physical damage where necessary by rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, or other approved means...

Conduit fill calculations become a bit more difficult when dealing with cables, but a few notes in chapter 9 provide some direction.

National Electrical Code 2014

Chapter 9 Tables

Notes to Tables

(5) For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.

(9) A multiconductor cable or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

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.²

Conduit 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.²

According 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.

National Electrical Code 2014

Chapter 9 Tables

Notes to Tables

(8) Where bare conductors are permitted by other sections of this Code, the dimensions for bare conductors in Table 8 shall be permitted.

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.²

Since 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".

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The problem with NM-B and conduit is that you have to treat it as a single round wire the diameter of the largest dimension of the oval (or "round-ish" for 3 conductors and ground) for fill calculations - and, let's face it, it is a giant pain to actually pull. Also, it's a dry-location-only type wire and garages are skirting the edge of that being true.

I'd strongly suggest that you should just run conduit the whole way and then use THWN, XHHW, or the like. It will be better protected, and it may also be cheaper (large NM-B often has a significant premium over 3 or 4 conductors of equivalent conduit-type wiring - and the conduit you will use can be much smaller to run 3-4 AWG 6 conductors than one AWG 6 NM-B.)

4 THWN or 3 THWN and a bare ground (AWG 6) will fit in a 3/4" EMT conduit (per code-compatible fill calculations.) I'm having a bit of trouble tracking down a hard number for the diameter to use for NM-B in AWG 6, but I'm fairly sure it comes out to a much bigger conduit (based on checking the same thing in smaller wires.) You might want to run larger conduit for an easier pull, or stick with smaller conduit for cheaper tooling and conduit, or go with PVC conduit if you decide to go larger than 3/4" so you don't need tooling (at least in the form of benders) though you should be able to rent those pretty cheaply for a day if you wanted to do 1" or 1-1/4" EMT for an easier pull.

A $300 (or rent it for a day) drill option:hole-hawg If you already have a cordless drill (so you can skip the batteries and chargers) you can often get the right-angle drill to go with it for under $100 - might take a bit longer but will get your job done eventually if you'd prefer not to rent, without sinking professional electrician tool money into the big guy without having professional electrician income to offset it. Will also be smaller for an easier fit into your tight joist spaces. cordless right angle

  • I thought about doing the whole run with THWN, but the basement ceiling is barely 7' high. I was trying to avoid any conduit hanging down since I plan on finishing the basement at some point. rafters are 12" on center, so getting any conduit up there could be difficult since I'm running perpendicular to them. – Lee Harrison Oct 8 '14 at 1:36
  • You can rent a right-angle drill which will make short work of the holes in the rafters. Google "hole-hawg" and take a trip to the local rental center. (You almost certainly don't need to buy one....) If you feel you must buy something, a much cheaper right-angle drill that's a bit less heavy-duty will do the job for your house once at a far more reasonable price. – Ecnerwal Oct 8 '14 at 1:41
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    I think it would be more useful to have a separate question about drilling/installing in the rafters. I was puzzled about why you included the right-angle drills in your answer until I saw the comments since the original question didn't say anything about drilling into rafters. – Johnny Oct 8 '14 at 3:24

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