The most common use of this rule that I have seen is for the secondary side of the POCO transformer when one transformer feeds more than one building. In some locations it is the EC's responsibility to install the secondary wiring thru the meter socket to the main disconnecting means of that service. Other locations require us to bury the conduit(s) for the underground service, the thePOCO installs the wire and terminates both ends. Since the POCO in not under the NEC wire sizes are often different than what an EC would have to run. My best guess is that the code panel did not want to get into an discussion with the POCOs and the NESC people.
Re: Tap Rule 2002 NEC Section 240.21(B)(5)#83668 02/13/0305:27 AM02/13/0305:27 AM
In my opinion, 240.21(B)(1) and 240.21(C)(4) were placed in the code to let electrical contractors be more competitive with the utilities. The utility installation was never covered by the NEC and they have not normally provided overcurrent protection for these conductors. Now the electrical contractor comes along and wants to make the same type of installation in a campus or other type distribution system and he has to provide an OCPD for his conductors at the line end of the conductor. With these two tap rules, the electrical installation can be installed in the same manner no matter who installs it. If it is a safe installation for the utility, it is also a safe installation for a contractor. Don
Re: Tap Rule 2002 NEC Section 240.21(B)(5)#83670 02/21/0309:51 PM02/21/0309:51 PM
Lets take this a step farther. You have a 480V feeder to a step down transformer. On the load side of the trans is say a flex. metal conduit feeding a trough with 3 disconnects. Can someone install another metal flex out of the transformer to another disconnect? Will this fall under the 10 foot tap rule? Will it fall under the 25' tap rule? For the sake of convinece make the load calcs all work out for the exisiting feeder. (i.e. the feeder to the trough is the correct size, the wires are the correct size and the disconnects on the existing were all the correct size.) The question basically is, Can you add another feeder cable out of the transformer to pick up one more disconnect in a different pipe?