How many cables are you allowed to run through a single hole in a wooden floor joist? For example, could I run five 12-2 cables through a 1-1/8" hole? I know they physically fit, but is there a code to regulate this? Is there any limit on this or some table for guidance?
According to the National Electrical Code, you can have 4 12/2 nonmetallic sheathed cables through a single bored hole that is fire- or draft-stopped using thermal insulation, caulk, or sealing foam, or where proper spacing is not maintained for more that 24 in. If you maintain proper cable spacing (which is defined by individual cable manufacturers) where not passing through joists, you can jam as many cables as you like (without damaging the cables) through a bored hole (that is not fire- or draft-stopped using thermal insulation, caulk, or sealing foam).
Article 334.80 tells you that you can use the 90°C column from Table 310.15(B)(16) for derating, but the final value must not exceed the 60°C column. If you look at Table 310.15(B)(16), you'll find that 12 AWG copper wire is rated at 30 amperes at 90°C and 20 amperes at 60°C.
Table 310.15(B)(3)(a) tells you that with 7-9 current carrying conductors, you'll have to derate the ampacity by 70%. So with the information from Table 310.15(B)(16), and Table 310.15(B)(3)(a). The math is quite simple.
4-6 current carrying conductors
30 amperes * 80% = 24 amperes
7-9 current carrying conductors
30 amperes * 70% = 21 amperes
10-20 current carrying conductors
30 amperes * 50% = 15 amperes
Which shows that you can have between 7 and 9 current carrying conductors through the bored hole, before the wires are derated below 20 amperes (from Table 310.15(B)(16) 60°C column). Since each 12/2 cable has 2 current carrying conductors, you can figure that only 4 cables can pass through the bored hole (
2 * 4 = 8).
National Electrical Code 2014
Article 334 Nonmetallic-Sheather Cable: Types NM, NMC, and NMS
334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.
Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a).
Article 310 Conductors for General Wiring
310.15 Ampacities for Conductors Rated 0–2000 Volts.
(3) Adjustment Factors.
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).
To my understanding per the NEC you can put as many wires through the bored hole as you want providing you don't over-stress or damage the wires. I'm not sure the discussion on de-rating is applicable since that applies to wires run through a raceway or a conduit, and the hole would have to be bored through a material that is 2 feet thick before the hole could be considered a raceway. As always however, you need to make sure your local electrical code does not have language regarding this issue which might be more stringent than the NEC.
Article 310 describes the general case of cables that do not maintain spacing for 600 mm / 24". A series of holes through floor joists is not described by that sentence. Moreover, references in the section to holes include firestopping. So the derating would apply to holes between floors that are firestopped but not joists which are not. Consider Gardner Bender's MuliCable Staples that support up to 8 12-2 NM cables in tight proximity. For the balance of the run, just as in between floor joists, the conductors can dissipate heat build up that might occur where the cables are bunched through unfirestopped holes or the multicable staples. So, it seems both situations are tacitly excluded by Article 310. One wonders how being sandwiched within wall insulation gets address and not sure it does though that is where I might also wonder about grouping cables together and consider derating. Seems worse than in raceways.