I am installing a new 100A sub-panel. The feed will be from main with a 100A dual-pole breaker. The main and the sub-panel are in the basement. The new sub-pnael
Will be about 25 feet from main.

I already bought the feed cable for this sub-panel. I bought #4/3 With Ground NM-B romex.
The color of this romex cable is black.

Thinking about the cable installation from main to sub to plan the work, at first,
I thought I drill 1.5" holes through 5-6 joists to run this romex wire from main to the
New sub-panel.
Then I thought, the basement ceiling will be open, (no sheet rock), so there is
Some potential for future wire damage. A wire with up to 100A going through it,
Is something I like to protect as much as possible.
Now I wished I bought individual THHN cable and run them through schedule 40 or 80
PVC or metal conduit. But I can't return the cut wire.

Can I install this #4/3 black color romex wire in a conduit under the joist
(perpendicular to joist) from main to the sub? Would this be OK with
NEC 2005 and/or 2008?
My worries are, romex is already bundled the wires together, and being
Inside a conduit, it will not have enough air exposure to be cooled,
Therefor it may overhear overtime.

Romex isn't allowed in conduit. Ever.

Actually Romex behind sheetrock poses a greater hazzard because you don't know what might be lurking there: a PVC drain, electrical, plumbing etc. That's why there are nail plates for that application.

Remember when drilling holes, the center will have the least impact on the strength of the joist. Your local building code may have design creiteria such as a hole no larger that x% of the joist and not within x/y fraction of the edge. That cable won't be easy to fish.

Basement ceilings are usually done with a dropped ceiling, so that just nailing to the edge of the joist should be fine.

One other thing. I don't think #4 will get you the 100amps. #4 is rated @ 85 Amps. You will need to up the wire size to #2 copper for 100 draw.

If I am wrong someone will tell me lol...

Mike

Romex isn't allowed in conduit. Ever.
Sorry keep, but you are wrong. You can install romex in conduit. You just have to do a conduit fill calculation.

What % of the cross-sectional area of the conduit must be free space?

I guess I'll have to look up "conduit fill".

Washington1 is CORRECT on this.

What % of the cross-sectional area of the conduit must be free space?

I guess I'll have to look up "conduit fill".

Depending on the cable, it's the largest diameter or width--then per conduit fill calculation--->>

See 310.15(B)2
See Chp 9 T1 (Notes to table)

Depending on the cable, it's the largest diameter or width--then per conduit fill calculation--->>

See 310.15(B)2
See Chp 9 T1 (Notes to table)

I have a #2/3 W/G romax (color black ) cable that I can use if #4 isn't rated
For the 100A dual-pole breakers. The #2 is like one inch thick.

Question: If I decide to put it in a conduit, would a metal conduit
(is the metal conduit a galvanized steel or aluminum? Does it matter which metal type?)
Be better suited for this romex than a schedule 40 or 80 PVC conduit?

Any hints on how I can calculate the fill, if my cable thickness is one inch?
If romex in a conduit is allowed, I was thinking of using a 2 inch galvanized
Conduit, with compression fittings (to avoid water ever getting in there just in
Case if the washing machine on 1st floor let go). So question is, once
Someone can teach me the fill calculation, do I need to go bigger than 2 inch conduit?

That was code for Go here:

NFPA :: Publications :: necdigest :: View 1993 to 2005 NEC Editions (http://www.nfpa.org/itemDetail.asp?categoryID=860&itemID=21227&URL=Publications/necdigest%E2%84%A2/Access%20the%20NEC%C2%AE%20(members%20only))

Then click the link after Visitors have...

You can read the 2005 NEC code online. No printing, no searching. Section #'s it's much easier.


EDIT: Bold

That was code for Go here:

NFPA :: Publications :: necdigest :: View 1993 to 2005 NEC Editions (http://www.nfpa.org/itemDetail.asp?categoryID=860&itemID=21227&URL=Publications/necdigest%E2%84%A2/Access%20the%20NEC%C2%AE%20(members%20only))

Then click the link after Visitors have...

You can read the 2005 NEC code online. No printing, no searching. Section #'s it's much easier.Those links are for NFPA members only. At least the 1993 is.

See chp 9 T1 note 9

Which means: The percent of cross section of conduit in this case would be 53%.

Been,

I will post the calculation later on today, it's 6:34AM I have to get to an estimate.

I don't think you can't link to the free link directly. Added BOLD type to my post.

1.5" PVC
2" Ridgid metal conduit
Although it's allowed, I would never run NM in conduit for this application. (Warning:NM cable should be installed in normally dry locations)

1.5" PVC
2" Ridgid metal conduit
Although it's allowed, I would never run NM in conduit for this application. (Warning:NM cable should be installed in normally dry locations)

Thank you Washington1 for doing the math, and everyone for providing
The valuable feedback for my project, I appreciate it.

I assume Washington1, you are a licensed electrician, correct?
That's where experience worths a lot, as well as it creates a built-in intuition to guide us.
Since you mentioned "never", that's good enough reason for me to just install
My NM-B cable through joist, and forget about conduit. Fortunately, most of the run
Is in the ceiling, away from several harms way.

Thanks
-been