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The number of conductors allowed in the box that is stamped inside the box refers to the total capactity of that box to contain that number of current carrying conductors. However that stamped number in that box does not allow for any reductions for devices, equipment grounding conductors, straps, hickeys, etc. that also must be counted as current carrying conductors.
NEC 370-16-b states approximately the following, more is said in that article but this is the basics.
14 Ga wires must be provided 2 cubic inch of box capacity for each current carrying conductor in that box. 2.25 cubic inch is required for 12 ga wire.
It goes further to say that you must deduct one current carrying conductor for all the bare or green wires in that box as a total of one conductor reduction to you capacity of that box.
A device must deduct 2 current carrying conductors for each yoke [mounting bracket of each recepacle, switch, etc.
A strap or clamp that enters the box 1/2" must also deduct 1 current carrying conductor for a total of all straps in that box counted as one deduction.
A pigtail that originates within that box but does not leave that box is not counted as a current carrying conductor.
Any wire that enters or leaves that box whether wire nutted together or not must be counted as one conductor for each entry or exit with the exception of the rule for green or bare wires.
Any wire that passes through that box but is not cut in that box is only counted as one.
A few other rules apply and are in 370-16 but those are the main rules of this subject.
The above calculation method applies to any conductors 14 ga through 6 ga.
Any box containing #4 or larger conductors must be counted by a different method due to there size.
Hope this helps
[This message has been edited by rpxlpx (edited March 13, 2001).]