I'm helping a friend on a project and he needs to run Romex from an
existing outlet to a new location about 20 ft away in an attic. The
attic is accessed via stairs, contains a furnace/AC, and has plywood
flooring in the small area by the furnace.

The easiest, direct run would be to use the rafters and the run would
be perpendicular to them. Is it permissible to just staple to the
underside of the rafters or do you need to nail a furring strip and
then staple to that?

I know you can't directly staple in that way to the bottom of joists
in a basement. But, on the other hand, the house in question is 4
years old and the attic wiring for the attic lights was run
perpendicular over the top of the 2X8 cross members that are about 7
ft high and no furring strip was used. The cross members are spaced
many feet apart, probably 4X the distance between rafters, and the
romex is unsupported in between. That apparently passed
inspection.

If a furring strip is required across the rafters, then just directly
stapling to the tops of the cross members like the existing wiring
would be an alternate option. But I'm wondering if it's code
compliant, as I don't see the difference between wire stapled to the
underside of rafters spaced 18" or so and stapling it to the top side
of cross members that are 4 feet apart?

Also, are there any special rules about clearance between Romex and
the furnace? The easiest path would probably put it about 3 feet
above the furnace.

On 2010-08-11, wrote:

The easiest, direct run would be to use the rafters and the run would
be perpendicular to them. Is it permissible to just staple to the
underside of the rafters or do you need to nail a furring strip and
then staple to that?

Actually, you nail the furring strip alongside the cable.

2008 NEC 320.23(A) in part: "Where run across the top of floor joists,
or within 2.1 m (7 ft) of floor or floor joists across the face of
rafters or studding, in attics and roof spaces that are accessible,
the cable shall be protected by substantial guard strips that are at
least as high as the cable."

The cross members are spaced many feet apart, probably 4X the
distance between rafters, and the romex is unsupported in between.

The romex should be supported every 4.5' (NEC 334.30).

Also, are there any special rules about clearance between Romex and
the furnace? The easiest path would probably put it about 3 feet
above the furnace.

No problem.

Wayne