Originally Posted by thekctermite
It sounds like that code is trying to avoid what we commonly term "bundling." In my opinion, in a pipe or tightly concealed location, bundling is an issue (heat buildup, etc). In an unfinished basement where there's good airflow...Not so much. But, code is code and you should always make an effort not to group wires too close together. If it isn't too late in your project, you should probably try to remedy the issue. Whether or not the inspector will catch it is hard to say...It can be subjective sometimes, but if you follow the code you're 100% safe.
Thanks for this. The term "bundle" got me a lot further in the NEC Handbook. By the way, my situation is obviously SFR/residential.
I also found a couple good links:
Most of the following is drawn from those two references, with me double-checking 2008 NEC.
The reason for all this, as I imagined, is heat -- current running through magnetic fields ends up eddying (useful knowledge from my 4 years of grad physics), effectively increasing the resistivity/impedance, and therefore increasing the heat generation.
0. The main NEC references are
a. Heat and amperage de-rating:
NEC Tables 310.16, & 310.15 (B)(2)(a)]
NEC Sections 303, and 310.15
(Section 310.15(b)(2) contains the 24-inch limitation -- apparently unlimited wires can be bundled for 23 inches.)
NEC Chapter 9, Tables 1-4.
1. The ABS thing is a BIG no-no -- ABS and PVC (white/black plastic) are no good -- only listed raceways such as Schedule 40 Rigid Nonmetallic Conduit (the grey stuff) are OK. [See NEC 110.3(B)].
2. NM-B is rated at 194 degrees F. (See notes on cladding of individual conductors, under external sheathing.) It must not be operated at a temperature higher than the listed temperature of the raceway [NEC 352.12 (E)].
3. We are allowed a pipe-fill of 40% of that area, or 0.794 Sq. In. [See NEC Chapter 9, Tables 1 & 4].
4. To find the cross sectional area of the Type NM cable (“Romex”), [see NEC Chapter 9, Table 1, Note 9]: You must use the major dimension of the elliptical cable as the diameter for each cable assembly.
(From link A, On 12-2 the major dimension is 15/32”. That means that you must use 15/64 as your radius when calculating the area of each cable assembly. 15 divided by 64 equals 0.2344. That is your radius; 3.1416 (PI) X 0.055 (.2344 squared) = 0.173. ==> If you have 7 conductors in a 1.5-inch pipe, then the total pipe fill area of the wires is 0.173 X 7 = 1.211 Sq. In. verses the allowable fill 0.794 Sq. In.)
5. Also see the Ambient Temperature adjustment factors [at the bottom of NEC Table 310.16]. E.g. if you are running this conduit into an attic, further derating is probably required; if the attic temperature ever exceeds 122 degrees F - not unusual in a lot of the US - [NEC section 352.12 (D)] does not allow RNC, unless it is listed for higher temperatures.
6. Don't use plumbers glue on the fittings.
7. Re expansion fittings for conduit -- Take NEC Chapter 9 Table 2 into consideration when bending RNC with an approved method (not heating with a propane torch, for example); when installed in long, straight runs, you should also check NEC 352.44 and NEC Table 352.44(A) to determine the need for expansion fittings.
8. Removing the cladding from the wires running through the raceway is a BAD idea and will make things worse if any wire heats up.
9. If more than two NM cables pass through a hole that is to be fire or draft stopped, then you have to derate [according to NEC Table 310.15(B)(2)(a)].
10. "If you exceed approximate values listed then you should consider calculating for voltage drop. If you stay within these distances then voltage drop should not be a concern on a general rule. 150 feet for 125 volts or 250 feet for 240 volt or 600 feet for 480 volt." [no cit., from unchecked source].
A couple more links with discussions of bundling scenarios and de-ratings, etc.:
temperature rating for Romex
Remaining questions --
1. What is the required separation (if *any*) for nm-b romex stapled side-by-side?
2. Likewise, can holes through 2x4s, etc, e.g. carrying 1-2 romex 12-2 wires each, be arbitrarily close together (that seems silly to me, since magnetic fields go through wood about the same as they go through air)?