Running Romex and attic rafters?


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

No problem.
Wayne
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Thanks for the help Wayne and the cite. A follow on question:
They use the term "guard strips". Does that mean one strip on each side of the cable or is just one along one side it sufficient? For example, if the cable is running perpendicular to the rafters and I staple the cable say 5 ft off the floor, then nail a furring strip uphill of it on the rafter is that sufficient? The bottom side where the cable is would then be fairly inaccessible.
Also, without the cable being attached to the furring strip, I guess the strip has to be high enough so that the cable can't sag enough between rafters to stick out beyond it. What would normally be used, something about 3/4" thick?

It looks like they can run the romex perpendicular across the tops of those cross members because the tops of the cross members may be 7ft above the floor.

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I would say one is sufficient, but I agree it is not 100% clear.

You can attach the cable to the furring strip, just not the face. E.g., just use a big zip tie around the guard strip and the cable. Just watch out for bundling--don't group together more than 4 of the 2 conductor cables at once.

That is what I would use, a 1x2.
Cheers, Wayne
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