Continuing with our review of taps from branch circuits, Sec. 210-19 and from feeders, Sec. 240-21(b) and (c), we turn now to 240-21(d), which is a reference to Sec. 230-91, Service Conductors. Service conductors are necessarily taps from the utility conductors, for it is impossible in most cases to provide overcurrent protection at the point where the taps receive their supply.
Thus, 230-91 permits the overcurrent protection to be integral with the service disconnecting means or located adjacent thereto, and Sec. 230-70(a) requires that the disconnecting means be at a readily accessible location either outside the building or inside nearest to the point of entrance of the service conductors. Because these service entrance conductors are assumed to have no overcurrent protection, the wiring methods [230-43] and their length within the building are limited [230-70(a)], and protection from physical damage is specified. [230-49, 230-50]
The overcurrent protection at the load end does protect the conductors against overload, and they are protected against short circuits or ground faults only by the utility protection, which may allow them to burn down before it opens.
Sec. 240-21(e) Busway Taps.
This reference shows that Sec. 364-11 permits not more than 50 feet of a busway to be reduced in ampacity in industrial establishments only, with the ampacity to be not less than one-third the ampacity of the overcurrent protection ahead of the tap, and the busway free from contact with combustible material. There is the possibility of plug-in units being attached to this busway tap which will overload it, but the limitation to industrial establishments probably makes the assumption that there will be competent supervision of the installation.
240-21(f) Motor Circuit Taps.
Sec. 430-28, “Feeder Taps” is referenced, which allows taps to individual motors from a feeder, following the same rules for a 10-foot tap as in Sec. 240-21(b0)(1), as for a 25-foot tap as in Sec. 240-21(b)(2), and for the 100-foot tap as in Sec. 240-21(b)(4). Also referenced is Sec. 450-53, “Several Motors or Loads on One Branch Circuit.” The requirements of (a)(b) and (c) are quite restrictive, and (d) requires that taps to an individual motor be not more than 25 feet long, be protected from physical damage, and be not less than one third the ampacity of the branch circuit conductors from which they are tapped.
240-21(g) Conductors from Generator Terminals.
This is a reference to Sections 445-4 and 445-5, which state that the conductors between the generator and the first overcurrent device must have an ampacity of at least 115 percent of the generator nameplate rating.
Article 240 Part H, Supervised Industrial Installations. New in the 1999 NEC, these locations are defined in Sec. 240-91 as the industrial portions of an installation, with the condition that only qualified persons monitor and service the installation, and with minimum size, power, and voltage requirements.
Feeder and branch circuit taps are permitted to be protected as in Sec. 240-21, plus [240-92(b) Transformer Secondary Conductors of Separately Derived Systems.] short circuit and ground fault protection can be provided: (a) by the transformer primary where the secondary conductors do not exceed 50 feet or (b) by a differential relay where the secondary conductors do not exceed 75 feet or (c) by engineered calculations determining that the system overcurrent devices will protect the conductors where the secondary conductors do not exceed 75 feet.
Overload protection can be provided by (a) terminating the secondary conductors at a single overcurrent device that will limit the load to the conductor ampacity, (b) terminating the secondary conductors at not more than six grouped circuit breakers or sets of fuses the sum of which does not exceed the conductor ampacity, (c) overcurrent relays, which will limit the load on the conductors to their ampacity, or (d) engineered calculations determining that the system overcurrent devices will protect the conductors.
All of these conductors shall be suitably protected against physical damage.
240-92(c) Outside Feeder Taps.
This is the same as Sec. 240-21(c)(4) except that, instead of a single overcurrent device, the secondary conductors are permitted to terminate at not over six circuit breakers or six sets of fuses, the total rating of which does not exceed the conductor ampacity.
This review of Code-recognized taps illustrates the varying degrees of protection against ground faults, short circuits, and overload of the tap conductors. To offset reduced protection, there are tradeoffs, including limiting the length of the tap, requiring protection against physical damage, limiting the size relative to the overcurrent protection next ahead on the line side, requiring that only qualified persons service maintenance and supervision, and limiting the wiring methods used.
The 18-inch fixture tap and the 10-foot tap are indispensable. As for the other permitted taps, following the rules should result in a safe installation. But in some cases, the short-circuit and ground-fault protection is questionable, as is the overload protection, where up to six sets of overcurrent are permitted at the load end of the tap, so consider in each case whether using a permitted tap rule is worth the risk.
SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.