NEC Code Quandry

Questions and Answers - January 2000

Q1. Is it acceptable to terminate two wires on a single screw or lug?

Q2. Is it permissible to terminate two circuits on a single circuit breaker?

A. Sometimes, Figure 1.

According to 110-3(b) "listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling" and 110-14(a) states "terminals for more than one conductor shall be so identified."

The only time two wires can be installed under a single screw or lug is when the terminal is identified for this purpose. Circuit breakers rated not more than 30 amperes are often identified for the termination of two conductors. This can be verified by reviewing the circuit breaker manufacturer’s catalog.

Neutral and equipment grounding lugs for panelboards are often suitable for two and sometimes three wires. This information is contained on the label affixed within the panelboard or on the packing container of the equipment ground lug. Often the instructions will identify the number of conductors, the size of the conductors, the conductor material as well as the torque requirements.

Note: Split-bolt lugs are generally only rated for two conductors, but some manufactures list these devices for three wires. (i.e. Ilsco catalog page 105 Type “SEL”). 

Q. Can a panelboard be installed with the main breaker upside down so the "up" position of the handle is the "off" position?

A. No. Sections 240-81 and 380-3 require "circuit breakers that are operated vertically be installed with the "up" position of the handle to be the "on" position.

Q. Does the NEC require GFCI protection for a receptacle located within 6 feet of a sink in school laboratories?

A. No. GFCI protection is only required at the following locations:

Item Reference
Agricultural Buildings 547-9
Carnivals, Circuses and Fairs 525-18
Commercial Garages 511-10
Dwelling Units and Bathrooms  210-8
Elevator Pits 620-85
Health Care Facilities  517-20(a), 517-21
Portable or Mobile Signs 600-10
Roof Top Receptacles 210-8(b)
Swimming Pools  680-6(a)
Temporary Wiring  305-6

Q. A person makes contact with the hot wire and the return wire at the same time. Will the GFCI protection device open the circuit and protect the person making contact with the two wires?

A. No, Figure 2. A ground-fault circuit-interrupter protects against electric shock from an energized conductor or from energized metal parts that are not effectively grounded. It operates on the principle of monitoring the imbalanced current between the ungrounded (hot) and grounded (neutral) conductors. In a typical 2-wire circuit, the current in amperes returning to the power supply will be the same as the current leaving the power supply (except for small leakage). If the difference between the current leaving and returning through the current transformer of the GFCI protection device is 5 milliamperes (+ or – 1 milliampere), the solid-state circuitry activates the shunt trip feature to open the switching contacts of the GFCI, thereby de-energizing the circuit.

Severe electric shock or death can occur if a person touches the energized (line or hot) and neutral conductor at the same time, even if the circuit is GFCI protected. This is because the current transformer within the GFCI protection device does not sense an imbalance between the departing and returning current and the switching contacts remain closed.

Q. Does the NEC require phase conductors to be color-coded?

A. No. There is no NEC rule requiring color coding of phase conductors, however, 210-4(d) states that “where more than one nominal voltage system exists in a building, each ungrounded conductor of a multiwire branch circuit, where accessible, shall be identified by phase and system. This means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means and shall be permanently posted at each branch-circuit panelboard.”

This rule applies primarily for commercial and industrial buildings that have 208Y/120 volts for receptacle circuits and 480Y/277 volts for lighting and other equipment loads, Figure 3. If two system voltages are not present, then identification of multiwire branch circuits is not required.

Note: The high-leg conductor must be identified with the color “orange,” see 215-8 for feeders, 230-56 for service conductors, and 384-3(e) for switchboards or panelboards. When two voltage systems are installed in the same raceway (or enclosure), one system's neutral can be white or natural gray and the other system's neutral must be white with a readily distinguishable different color stripe (other than green), or identified with other effective means; see 200-6(d) and 210-5(a).

Q. Can a single equipment grounding conductor be used for a nonmetallic raceway that contains multiple circuits?

A. Yes. This issue is covered in 250-122(c). When multiple circuits are installed in the same raceway, a single equipment grounding conductor can be used for all of the circuits. The single equipment grounding conductor is sized to the largest overcurrent device protecting the conductors in the raceway in accordance with Table 250-122, Figure 4.

Q. Is it a code violation to wrap a stranded No. 12 wire around the post of a wiring device screw?

A. No. This is not directly covered by the NEC, but 110-3(b) requires all equipment to be installed in accordance with the manufacturer’s instructions. The screws on wiring devices are listed for the termination of solid or stranded No. 14 or No. 12 wire.

Q. I am installing a permanent generator at my home to be used for optional standby power. I’ll be using a manual double pole/double throw switch with a solid neutral as my transfer switch. The feeder to the subpanel from the main and the generator is 4-wire (two hots, neutral and ground) and there is no neutral-to-ground connection in the subpanel, Figure 5.

Q1. Is my generator considered a separately derived system?
A1. No.
The National Electrical Code requires a neutral-to-ground connection for a separately derived system, such as a generator, at the separately derived system or at the first disconnect after the separately derived system [250-30(a)]. Fine Print Note No. 1 to Section 250-20(d) warns the NEC user that an alternate ac power source is not considered a separately derived system if the neutral from the generator is solidly interconnected to a service system neutral.

Q2. Should I make a neutral-to-ground connection at the generator?

A2. No. Since the neutral is not opened in the transfer switch, the neutral from the generator will be solidly interconnected to a service system neutral. Under this condition, the generator is not considered a separately derived system and a neutral-to-ground connection must not be made at the generator or at its disconnect.

If a neutral-to-ground connection is made at the generator, this will create a parallel path for neutral current to flow on the neutral conductor as well as objectionable (neutral) current to flow through the grounding path in violation of Section 250-6(a), Figure 5.

Q3. Is the generator required to be grounded to the earth?

A3. No. Since the generator is not a separately system, the generator does not require a grounding electrode (earth).

The above questions and answers (including graphics) by Mike Holt are in EC&M magazine. Click here for more magazine articles.

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