The location in the circuit of the overcurrent protection permitted in the tap rules of Sec. 240-21 raises some questions regarding the effectiveness of the overcurrent protection.
Overcurrent protection of conductors has three purposes: protection against short circuits, protection against ground faults, and protection against overload.
All taps permitted by Section 240-21 (a) through (g) provide short-circuit and ground-fault protection to some degree by virtue of overcurrent protection ahead of the taps. Let’s look at each to determine the effectiveness of this overcurrent protection.
(a) Branch-Circuit Taps. This reference to Sec. 210-19 recognizes 20-ampere taps to 50-ampere range circuits, large enough for the load, and no longer than necessary for servicing.
Also recognized are: 18-inch taps having an ampacity of 15 where tapped from circuits less than 40 amperes, and 20 amperes tapped from 40- or 50-ampere circuits, and feeding individual lampholders or fixtures; high-temperature taps 18 inches to 6 feet in length to a recessed fixture per Sec. 410-67; taps not over 18 inches long to individual outlets other than receptacle outlets; infrared lamp industrial heating appliances; and nonheating leads of deicing and snow-melting cables and mats. These nonheating leads not having a grounded sheath or braid must be enclosed in raceways.
All of these permitted taps obviously have overcurrent protection ahead of them, which will open in the case of a short-circuit or ground-fault, and in each case the smaller conductor is required to be adequate for the load.
Now comes the questionable part. Sec. 210-19(d) Exception No. 2 refers to Sec. 240-4 for protection of cords and fixture wire, where (a) requires that cords and fixture wires be protected at their ampacities. Section 240-4 (b)(1) recognizes the use of cord smaller than the circuit to which they are connected, as part of listed appliances or portable lamps. Again, no problem, for these appliances or lamps will have been investigated by a recognized testing laboratory, and the cords are limited in length.
Sec. 240-4(b)(2) covers fixture wiretapped from branch circuits. Fixture wires are permitted for Class 1 circuits, Art. 725, and Fire Alarm circuits, Art. 760). Up to 50 feet run length of No. 18 or 100 feet run length of No. 16 may be tapped from a 20-ampere branch circuit. Will the overcurrent device open promptly in case of a short circuit or ground fault at the far end of the run? Maybe.
Under Sec. 240-4(b)(3), a listed No. 16 extension cord of any length can be tapped from a 20-ampere branch circuit. These long cords can easily be overloaded; a No. 16 cord with an ampacity of 10 fed from a branch circuit protected at 20 amperes. The likelihood of the branch circuit overcurrent device opening on a ground fault or short- circuit at the far end of a No. 16 cord is certainly questionable.
A 100-foot run results in a length of No. 16 fixture wire, out and back, of 200 feet. At a resistance of about 5 Ohms per 1,000 feet (from Table 8, Chapter 9) the resistance of 200 feet of No. 16 is 1 Ohm, giving a short- circuit current at 120 volts of 120 amperes. The magnetic trip of a 20-ampere circuit breaker starts to trip at about 10 times the breaker rating, or 200 amperes, so it will not trip at 120 amperes.
That leaves the thermal element of the circuit breaker to open this circuit, which could take enough time for the short circuit to start a fire. You might expect a short circuit or ground fault will unleash the floodgate of available fault current, but note that a 1978 Underwriters Laboratories (UL) survey showed that the available fault current at receptacles in dwellings was under 200 amperes in about a third of the cases and under 300 amperes in the next third.
Moving to 240-21(b), Feeder Taps, the 10-foot tap is found under (b)(1). Supplying a 30-ampere ex-op switch from a large feeder in an auxiliary gutter would be impossible if the overcurrent protection had to be located where the conductors receive their supply. The 10-foot tap makes this practical.
The tap conductors must be sized for the load and not less than the rating of the device supplied or the rating of the overcurrent device supplied. This takes care of overloading the tap conductors. The tap conductors can not exceed 10 feet in length, and must be enclosed in raceway, which minimizes the possibility of short circuits and grounds.
Beginning with the 1990 NEC, field-installed tap conductors must be no smaller than 10 percent of the overcurrent protection ahead of the feeder. Thus, a No. 10 copper tap with an ampacity of 30 cannot be tapped from a feeder having overcurrent protection rated over 300 amperes.
These taps usually terminate in a circuit breaker or a fused switch, which will protect the tap against overload, and many of the other loads require their own overcurrent protection: Panelboards [384-16(a) & (b)] and Motor Control Centers [430-94]. Are these tap conductors protected? Reasonably so.
Next time we will look at the rest of the taps permitted by Section 240-241.
SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.