Here are the latest short answers to questions posted on our Web site. In our discussion, we will cover topics from Secs. 110-3(b), 110-14(a), 210-8, 210-8(b), 305-6, 511-10, 517-20(a), 517-21, 525-18, 547-9(c), 600-10(c)(2), 620-85, and 680-6(a) of the National Electrical Code (NEC).
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. According to Sec. 110-3(b), "listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling" Sec. 110-14(a) states "terminals for more than one conductor shall be so identified"
The only time you can install two wires under a single screw or lug is when you identify the terminal for this purpose. We often identify circuit breakers rated not more than 30A for the termination of two conductors. You can verify this 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 appears on the label affixed within the panelboard or on the packing container of the equipment ground lug. Often, the instructions identify the number of conductors, size of the conductors, conductor material, as well as torque requirements. Note: Split-bolt lugs are generally only rated for two conductors, but some manufacturers list these devices for three wires (i.e. Ilsco catalog, page 105, Type "SEL").
Q. Does the NEC require GFCI protection for a receptacle located within 6 ft of a sink in school laboratories?
A. No. The NEC only requires GFCI protection at the following locations: Agricultural Building 547-9(c); Carnivals, Circuses, and Fairs 525-18; Commercial Garages 511-10; Dwelling Units 210-8; Elevator Pits 620-85; Health Care Facilities 517-20(a), 517-21; Portable or Mobile Signs 600-10(c)(2); Rooftop Receptacles 210-8(b); Swimming Pools 680-6(a); Temporary Wiring 305-6.
Q. If a person touches the hot wire and the return wire at the same time, will this trip the GFCI?
A. No. 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 current imbalance 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 5mA (+ or - 1mA), 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 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 doesn't sense an imbalance between the departing and returning current and the switching contacts remain closed.