Transformer Protection, Disconnecting Means and More

Article 100                Definitions

Article 210                Branch Circuits

Article 240                Overcurrent Protection

Article 250                Grounding and Bonding

Article 310                Conductors for General Wiring

Article 404                Switches

Article 410                Luminaires (Lighting Fixtures), Lampholders, and Lamps

Article 430                Motors, Motor Circuits, and Controllers

The 2005 edition of the General Information for Electrical Equipment Directory
published by Underwriters Laboratories Inc. is also mentioned.

Transformer and secondary conductors overcurrent protection

Q: A 112.5 kVA dry-type transformer is connected to a 480-volt, three-phase circuit to obtain 208Y/120 volt circuits for an office building. Four 500 Kcmil copper conductors with Type THWN insulation are connected to the secondary terminals and extend for 12 feet to a 400-ampere main circuit breaker panelboard. Primary overcurrent protection for the transformer is 150 amperes. Is this installation National Electrical Code (NEC) compliant? If not, why?

A: Based on the information supplied in the question, review the following analysis: Transformer primary full-load current is 112,500 divided by 480, multiplied by the square root of 3 (112,500 divided by 830), which equals 135.5 amperes. Since the overcurrent protection for the primary of the transformer satisfies the requirements of 450.3(B) and Table 450.3(B), let’s check the overcurrent protection for the secondary windings and secondary conductors.

With primary overcurrent protection that is equal to or less than 125 percent of rated primary current, secondary overcurrent protection for the transformer windings is not required. However, the secondary conductors must be protected to comply with 240.21(B)(3). If the supply conductors for the primary are 1/0 or larger Type THWN copper conductors, 240.21(B)(3) applies to the secondary conductors. Item 5 in 240.21(B)(3) requires that a single circuit breaker or set of fuses limit the load current to that permitted by 310.15. This value in Table 310.16 is 380 amperes. Therefore, the circuit breaker size must be reduced to a standard size of 350 amperes, or a non-standard fuse or circuit breaker with an ampere rating of 375 amperes may be used, or the conductor size could be increased to 600 Kcmil copper with THWN insulation. Finally, the 400-ampere overcurrent protection may meet the requirements of the NEC where motor loads are supplied by the feeder, because 430.63 allows an increase in overcurrent protection beyond conductor ampacity where motors are part of the feeder load.

Branch-circuit disconnecting means

Q: Some electrical inspectors are requiring 2-pole, common trip circuit breakers on 15- and 20-ampere, 120 to 240-volt multiwire branch circuits. In another jurisdiction, two single-pole circuit breakers with a handle tie are accepted. One jurisdiction accepts 2 single-pole circuit breakers without a handle tie on multiwire branch circuits. What does the Code require?

A: A multiwire branch circuit is defined in Article 100 as a branch circuit that consists of two or more ungrounded conductors that have a voltage between them and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Under some conditions, single-pole circuit breakers without handle ties are permitted. Where the multiwire branch circuit supplies 125-volt receptacles, 125-volt lighting or both, single-pole circuit breakers without handle ties are acceptable.

Where line-to-line loads such as an electric range are supplied from a multiwire branch circuit, a multipole, common trip circuit breaker or two single-pole circuit breakers with a handle tie are permitted. These rules are in 210.4(A)(B)(C).

Multiwire branch circuits are permitted in Class I, Division I locations where the disconnecting means for the circuit opens all ungrounded conductors simultaneously. This same requirement appears for Class II, Division I locations in 502.40 and Class I, Zone I locations in 505.21.

This discussion indicates that there are various requirements for the disconnecting means for multiwire branch circuits. The three jurisdictions mentioned in the question are probably applying the requirements in the NEC under different conditions.

Series-rated overcurrent devices

Q: Can the let-through current of a fuse (the up-over-and-down method) be used to protect a downstream overcurrent device or must the fuse and downstream overcurrent device be tested and evaluated for a series rating?

A: For an existing installation, the answer is no for testing, if the series- rated combination of overcurrent devices is selected by a licensed professional engineer engaged primarily in the design and maintenance of electrical installations. The series-rated combination must be documented and stamped by the professional engineer. Also, the upstream overcurrent device must be identified and field marked on the end-use equipment.

For other than existing installations, the combination of line-side overcurrent device and load-side circuit breaker must be tested and marked on the end-use equipment, such as switchboards and panelboards.

Series-rated overcurrent devices cannot be used where motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and motor full-load currents exceed one percent of the interrupting rating of the lower rated circuit breaker. For example, a 22,000 ampere interrupting rating circuit breaker is series rated with a 10,000 ampere interrupting rating circuit breaker. Motor loads connected on the load side of the 22,000 AIC circuit breaker and line side of the 10,000 AIC circuit breaker cannot exceed 100 amperes. These requirements are in 240.86(A)(B)(C).

Reducer washers on RMC

Q: Do reducer washers installed at panelboards provide grounding continuity for rigid metal conduit (RMC) containing 12 AWG copper branch-circuit conductors?

A: Yes. The 2005 edition of the General Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc. has this paragraph under Conduit Fittings (DWTT): “All metal fittings for metal cable, conduit and tubing are considered suitable for grounding for use on circuits over and under 250V and were installed in accordance with the NEC, except as noted for flexible metal conduit fittings and liquidtight flexible metal conduit fittings.”

Switching 240-volt luminaires

Q: Is a single-pole switch acceptable for controlling a 240V lighting fixture?

A: No, a 2-pole switch must be used. There are two different rules in Article 410—Luminaires (Lighting Fixtures), Lampholders, and Lamps. The first applies to switched lampholders and reads, “410.48 Double-Pole Switched Lampholders. Where supplied by the ungrounded conductors of a circuit, the switching device of lampholders of the switched type shall simultaneously disconnect both conductors of the circuit.” Auxiliary equipment for electric-discharge luminaires (lighting fixtures) must use double-pole switching where supplied from circuits with two ungrounded conductors. This requirement is in 410.54(B).

Switches used to control electric-discharge lighting systems operating at more than 1,000 volts are required to open all ungrounded primary conductors to comply with 410.81(A). Switches with a marked off position must disconnect all ungrounded conductors to the loads they serve. This requirement is in 404.15(B).

Dwelling-unit grounding electrodes

Q: The grounding-electrode system for a one-family dwelling unit consists of a concrete-encased electrode and a single ground rod with a resistance to earth of more than 25 ohms. The water pipe is plastic and the home is wood frame construction. Therefore, there are no other grounding electrodes. Is it necessary to drive another ground rod because the single driven rod has a resistance of more than 25 ohms?

A: There is no requirement for a ground rod where at least 20 feet of ½ inch or larger reinforcing steel is the grounding electrode. Specifications that the concrete-encased electrode must meet are given in 250.52(A)(3). They are that at least 20 feet of ½ inch or larger steel reinforcing rod located near the bottom of the concrete foundation or footing that is in contact with the earth and connected to a grounding-electrode conductor sized according to 250.66(B).

The 6 AWG copper grounding-electrode conductor from the ground rod, although not required, may be connected to the 4 AWG copper grounding-electrode conductor from the concrete-encased electrode or to the system-grounded conductor at the service.

Parallel conductors

Q: With a calculated load of 490 amperes, I installed two 250 Kcmil copper conductors with Type THWN insulation per phase with two 2/0 AWG copper conductors with Type THWN insulation for the neutral in a single 3-inch rigid metal conduit. These are the service conductors for a retail store. The service disconnect is a 600-ampere switch with 500-ampere fuses. This job was rejected with the comment that the conductors were too small. According to Table 310.16 each phase conductor has an ampacity of (255 x 2) 510 amperes. The inspector said that I have to derate the conductors because there are more than three in the conduit. I contend that I have 4-circuit conductors, but the neutral of this 208Y/120-volt system carries only the unbalanced current; therefore, there are three current-carrying conductors. Since the conductors are joined electrically at both ends, there are four conductors in the raceway—with the neutral carrying only unbalanced current; no derating is required. Is this a correct interpretation of the rules?

A: Although 310.4 points out that parallel conductors are electrically joined together at both ends to form a single conductor, 310.15 points out that there are six current-carrying conductors in the conduit. This sentence appears in 310.15(B)(2): “Each current-carrying conductor of a paralleled set of conductors shall be counted as a current carrying conductor.” Also, 310.15(B)(4) allows the neutral conductor to not be counted as a current-carrying conductor where it carries only the unbalanced current from other conductors of the same circuit. Since there are six current-carrying conductors in the raceway, 310.15(B)(2)(a) and Table 310.15(B)(2)(a) require the six current-carrying conductors to be derated to 80 percent of the ampacity shown in Table 310.16. This results in a corrected ampacity of 408.

It will be necessary to replace the 3-inch conduit with two 2-inch conduits or add another 3-inch conduit and pull in three 250 Kcmil and one 2/0 AWG conductors in each raceway or increase the conduit size and install six 350 Kcmil and two 2/0 AWG copper conductors, all with Type THWN insulation.            EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.


About the Author

George W. Flach

Code Q&A Columnist
George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical in...

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