Article 220 Branch Circuit, Feeder, and Service Calculations; Article 240 Overcurrent Protection; Article 250 Grounding and Bonding; Article 404 Switches; Article 700 Emergency Systems; Guide Information for Electrical Equipment Directory (White Book) published by Underwriters Laboratories Inc. also is mentioned.
Use of multipole snap switches
Am I allowed to use a 2-pole snap switch to supply a fluorescent luminaire and an exhaust fan? This equipment is in a bathroom in a fast-food restaurant. The fluorescent luminaire is 277 volts, and the exhaust fan is 120 volts. The switch is rated for 277 volts.
This cannot be done unless the switch is rated for 277 volts and is marked “2-circuit.” Section 404.8 states: “(B) Voltage Between Adjacent Devices. A snap switch shall not be grouped or ganged in enclosures with other snap switches, receptacles, or similar devices unless they are arranged so that the voltage between devices does not exceed 300 volts, or unless they are installed in enclosures equipped with identified, securely installed barriers between adjacent devices.” Part (C) of this same section is new and is directly related to this question. It is new in the 2008 edition of the National Electrical Code (NEC) and reads, “(C) Multipole Snap Switches. A multipole general use snap switch shall not be permitted to be fed from more than a single circuit unless it is listed and marked as a two-circuit or three-circuit switch or unless its voltage rating is not less than the nominal line-to-line voltage of the system supplying the circuits.”
Either two single-pole switches with proper voltage ratings may be used to control these loads where a permanently installed barrier is in the box that separates the switches, or a single switch that is marked “2-circuit” with a voltage rating of at least 277 volts, or the two-pole switch has a voltage rating of 480 volts.
The Guide Information for Electrical Equipment Directory, published by Underwriters Laboratories Inc., states under Snap Switches (WJQR), "Multi-pole, general-use snap switches have not been investigated for more than single-circuit operation unless marked ‘2-circuit’ or ‘3-circuit.’”
Bonding gas piping
Is the gas piping in a residence considered to be grounded if the electric gas heater connected to the gas piping is grounded by the equipment-grounding conductor in the branch circuit that supplies the central heater?
Yes, the gas piping is bonded. This is covered by part (B) of 250.104, which reads, “(B) Other Metal Piping. Where installed in or attached to a building or structure, a metal piping system(s), including gas piping, that is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122, using the rating of the circuit that is likely to energize the piping systems(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.”
Exposed metal parts of the electric heater are required to be grounded and the size of this grounding conductor, which also serves as the bonding conductor for the gas pipe, is listed in Table 250.122 and based on the size of the overcurrent device protecting the central heater branch circuit.
Feeder overcurrent protection
A dwelling unit with a 200-ampere service has a feeder to a detached garage that is protected by a 100-ampere circuit breaker. The calculated non-motor load is 37 amperes. What is the minimum conductor size required for this feeder?
The minimum conductor size is based on the ampere rating of the overcurrent protective device protecting the feeder. A 100-ampere overcurrent device would require 3 AWG copper conductors with 75°C insulation where the terminals on the connected equipment are suitable for 75°C terminations to conform to 110.14(C).
If the tap conductors are not more than 10 feet long, they do not have to be larger than 8 AWG copper or if the tap conductors are not over 25 feet long, 8 AWG copper conductors also can be used.
Where the feeder is located outdoors, except at the point of termination of the conductors, the length of the feeder conductors is not limited and the size of the conductors is not specified, but a single circuit breaker or a single set of fuses must be provided to limit the load to the ampacity of the conductors.
If none of these conditions exist, the 100-ampere overcurrent device may be replaced with a 40-ampere overcurrent device, and 8 AWG copper conductors may be used for the feeder. Or, the 100-ampere overcurrent device can remain, and the feeder conductors can be 2 AWG copper conductors.
All of these possibilities for sizing feeder conductor taps are in 240.21(B)(1), (2) and (5).
Disconnecting means location for generators
Does the disconnecting means for an emergency generator have to be located on the outside of the generator housing to be considered within sight from the building it supplies if it is visible through a window in the generator building? The building for the generator is 35 feet away.
The requirements for outdoor housed emergency generator sets are in 700.12(B)(6) and read, “Outdoor Generator Sets. Where an outdoor housed generator set is equipped with a readily accessible disconnecting means located within sight of the building or structure supplied, an additional disconnecting means shall not be required where ungrounded conductors serve or pass through the building or structure. The disconnecting means shall meet the requirements of 225.36.” The disconnecting means must be marked “suitable for use as service equipment” to comply with 225.36.
Readily accessible is defined in Article 100 as “Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.”
Within sight also is defined in Article 100 as “Where this Code specifies that one equipment shall be ‘in sight from,’ ‘within sight from,’ or ‘within sight of ’ and so forth, another equipment the specified equipment is to be visible and not more than 15 m (50 ft) distant from the other.”
If the door to the structure has a simple latch (no lock and key) and the disconnect can be seen through the window of the generator housing, it may serve as the disconnecting means required by 700.12(B)(6). The number of disconnects in the generator feeder should be kept to a minimum to increase reliability.
Loads on an emergency generator
Can a legally required emergency generator with a single transfer switch supply loads, such as food freezers, computer receptacles, and a gas furnace, in addition to exit and emergency lighting in an office building?
An emergency generator must be connected to an automatic-transfer switch that supplies only emergency loads to comply with 700.6(D). To supply the non-emergency loads mentioned in the question, an additional manual or automatic transfer switch is required to comply with 702.5 and 702.6.
Where nonessential loads are connected to the emergency generator, it must have adequate capacity to carry all loads that are operated simultaneously. Selective load pickup and load shedding are permitted for the optional standby loads where the generator capacity is not adequate for the total connected load.
Load calculations—kVA or kW
Why are some loads in residences added together when some loads are in kilovolt-amperes (kVA) and some are in kilowatts (kW)?
A change in the 2008 edition of the NEC allows kilovolt-amperes to be expressed as kilowatts for load calculations for electric clothes dryers and electric ranges in 220.54 and 220.55 because these loads are primarily resistive and kW and kVA are pretty close to the same value. When calculating loads for these appliances, either the kVA rating (volts × amperes) or the kW rating may be used because demand factors are used to calculate the load on the branch circuit, feeder and service.
EGCs for parallel feeders
What size equipment-grounding conductors are required for two sets of 500 kcmil copper conductors in parallel installed in nonmetallic conduits? This is an 800 ampere feeder supplied from a 208Y/120 volt service.
The minimum size equipment--grounding conductors are based on the ampere rating of the overcurrent device protecting the circuit.
Assuming the 500 kcmil conductors are copper with Type THWN insulation and the ampere rating of the overcurrent device protecting the 500 kcmil conductors is 800 amperes, according to 250.122(F) the equipment-grounding conductors must have a minimum size based on the ampere rating of the overcurrent device protecting the circuit. One of the sentences in part (F) of 250.122 reads, “Each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable in accordance with Table 250.122.”
The table requires a 1/0 AWG copper or 3/0 AWG aluminum conductor in each raceway for the equipment-grounding conductor.
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.