conduit fill - romex?

We're working on a kitchen renovation in a house built about
1900. The kitchen space has been gutted down to the studs
and overhead joists and is nice and easy to wire with romex.
There are sufficient kitchen circuits to warrant a new
sub-panel which will be located in the basement just
below the kitchen. The basement has a plaster ceiling.
The question is how to handle the penetrations through the
plaster ceiling and into the wall cavity above while
maintaining a proper fire-stop.
At the main panel on the opposite side of the basement,
the existing branch circuits run in romex going right into
the ceiling and plastered right up to the romex. There
is also older BX armored cable. I am comfortable with
plastering around BX, but romex seems to be a different
animal.
One way I can handle the penetration(s) up from the new
sub-panel is with a short length of 2-inch conduit passing
from the sub-panel through the ceiling and into the wall
cavity above. A bushing would be used on the end of the
conduit and the romex would be run in the conduit from
the sub-panel and then fan out to the various outlets,
switches and fixtures.
There are about 15 branch circuits for the kitchen and
the question is how many 12/2 and 14/2 romex will fit into
a 2-inch conduit and still comply with the NEC conduit
fill limitations and not be so tightly packed as to require
de-rating. I can do the calculation for individual #12
and #14 conductors but I don't know how to figure in the
romex's outer jacket. And I seem to recall that the jacket
on NM-B romex is rated at 60-degrees C even though the
indivual conductors are rated at 90-degress C.
(Could the romex jacket be removed where it enters the
conduit and just the inner conductors and ground run through
the conduit and into the sub-panel if necessary?)
Suggestions on how to handle the floor penetration(s) for
so many circuits while maintaining a proper fire-stop would
be appreciated. Thanks.
Bear

See NEC Chapt 9 Tables: Here are some notes: "(4) Where conduit or tubing nipples having a maximum length not to exceed 600 mm (24 in.) are installed between boxes, cabinets, and similar enclosures, the nipples shall be permitted to be filled to 60 percent of their total cross-sectional area, and 310.15(B)(2)(a) adjustment factors need not apply to this condition. (5) For conductors not included in Chapter 9, such as multiconductor cables, the actual dimensions shall be used. (6) For combinations of conductors of different sizes, use Table 5 and Table 5A for dimensions of conductors and Table 4 for the applicable conduit or tubing dimensions. (7) When calculating the maximum number of conductors permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal of 0.8 or larger. (8) Where bare conductors are permitted by other sections of this Code, the dimensions for bare conductors in Table 8 shall be permitted. (9) A multiconductor cable 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." And don't forget the derating.

Do not use romex in conduit! Use single conductor stranded wire. The only way I would even use solid wire in a conduit is if it came with the conduit like armored cable (but even them sometimes I buy the plain armor and run stranded wire through it.

From what I can see, he's using a short piece as a chase, not a complete conduit system....

That is exactly correct. The conduit is just to handle the distance from the top of the sub-panel to the bottom of the partition above.
The objective is to manage the floor penetration with easier fire-stopping than drilling a dozen 5/8" holes in close proximity to one another and filling around the romex with caulk or rock wool.
The reason that I cannot use single conductor wire is that I would need to splice it to the romex running around the kitchen -- which would require a box and there is no way to make such a box accessible because it would be behind the base cabinets.
[Thanks to all who responded. The code citation for using the actual dimension of the romex was very helpful.]

Yes, that's what I thought....
I've seen a website somewhere that gives the cross sectional area of Romex, but it should be close enough to use width X thickness if you're concerned about the fill percentage...

OH FUCK! Use fire caulk and forget the nipple. You are trying to engineer a disaster. If nothing else ask your local AHJ about what is acceptable.
We're working on a kitchen renovation in a house built about 1900. The kitchen space has been gutted down to the studs and overhead joists and is nice and easy to wire with romex.
There are sufficient kitchen circuits to warrant a new sub-panel which will be located in the basement just below the kitchen. The basement has a plaster ceiling.
The question is how to handle the penetrations through the plaster ceiling and into the wall cavity above while maintaining a proper fire-stop.
At the main panel on the opposite side of the basement, the existing branch circuits run in romex going right into the ceiling and plastered right up to the romex. There is also older BX armored cable. I am comfortable with plastering around BX, but romex seems to be a different animal.
One way I can handle the penetration(s) up from the new sub-panel is with a short length of 2-inch conduit passing from the sub-panel through the ceiling and into the wall cavity above. A bushing would be used on the end of the conduit and the romex would be run in the conduit from the sub-panel and then fan out to the various outlets, switches and fixtures.
There are about 15 branch circuits for the kitchen and the question is how many 12/2 and 14/2 romex will fit into a 2-inch conduit and still comply with the NEC conduit fill limitations and not be so tightly packed as to require de-rating. I can do the calculation for individual #12 and #14 conductors but I don't know how to figure in the romex's outer jacket. And I seem to recall that the jacket on NM-B romex is rated at 60-degrees C even though the indivual conductors are rated at 90-degress C.
(Could the romex jacket be removed where it enters the conduit and just the inner conductors and ground run through the conduit and into the sub-panel if necessary?)
Suggestions on how to handle the floor penetration(s) for so many circuits while maintaining a proper fire-stop would be appreciated. Thanks.
Bear

Some Jurisdictions don't allow "stacks" coming out of panels as you describe.
In that case you may want to consider running 4 - 3/4" conduits from the panel to 4 boxes where outlet or devices will be. That will handle 16 circuits easily while drilling only 4 - 1 1/8" holes. Use 4 11/16" x 2 1/8" deep boxes with P rings and run 4 circuits in each conduit, then take 4 circuits out of each box with Romex. Box fill will be within allowed limits and conductor derating will not be a problem.

This is the rule 312.5(C)Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) or more than 3.0 m (10 ft) in length, provided all the following conditions are met: (a) Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway. (b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling. (c) A fitting is provided on each end of the raceway to protect the cable(s) from abrasion and the fittings remain accessible after installation. (d) The raceway is sealed or plugged at the outer end using approved means so as to prevent access to the enclosure through the raceway. (e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm (1/4 in.). (f) The raceway is fastened at its outer end and at other points in accordance with the applicable article. (g) Where installed as conduit or tubing, the allowable cable fill does not exceed that permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto.