Re: 9 current carrying conductors
You can have more than 9 current carrying conductors in a conduit, it just isn't overly practical.
Lets look at 20 Amp circuits as an example. For the sake of simplicity I will assume all conductors are current carrying (which is not always the case.)
So, we start with 9, #12 THHN conductors which will fit in a 1/2" EMT. per table c.1
Per table 310.15(B)(2)(a) we must adjust the ampacity of our conductors to 70% if we have 7-9 current carrying conductors. If we look at table 310.16 we find #12 THHN has an ampacity of 30 amps in the 90C coulumn (which we are allowed to use for this calculation as THHN is rated for 90 degrees.)
so, 30 x .7 = 21 Amps. Our circuits are protected at 20 Amps, which is less than 21 amps, so all is well.
No we add one more current carrying conductor, for a total of ten.
Looking back at table 310.15(B)(2)(a), we find our adjustment factor for 10-20 conductors is 50%
so. 30 x .5 = 15 amps. No good on a 20 amp breaker.
However, if we bump up to #10 THHN we come up with a starting ampacity of 40 Amps (310.16 again)
and. 40 X .5 = 20 Amps, which is fine with our 20 amp OCPD.
Per table C.1 we are only allowed 5 #10's in a half inch EMT. Therefore we have to upsize to a 3/4"
Of course, if you add a non current carrying conductor (lets say it's an EGC) to the equation, you would have 11 #10 thhn's in that pipe and would have to upsize the conduit to 1"
It is quite possible to go beyond more than 9 current carrying conductors. Nothing in code prohibits it. But, to do so you have to be well versed in what code defines as current carrying / non current carrying, and well versed in allowable pipe fill. Perhaps more importantly, you would have to plan ahead to do such a thing, or you will likely not have a large enough conduit.
FWIW, I try to set my HR's up for two networks tops, and run them all in 3/4" conduit (unless I am pulling larger wire / cable for equipment feeds, subpanels, etc. That is more of a design preferance than anything, but it seems to be a very common design preference.
edited to overcome confusion caused by a sticky "0" key on my computer :roll:
[ August 18, 2005, 10:06 PM: Message edited by: cselectric ]
