Q: Should 250.34 or Article 702 apply to a portable generator that is connected to a domestic refrigerator, freezer, television set, and floor and table lamps in a one-family residence?
A: The answer depends on the method used to connect the generator to the appliances. The frame of a portable generator does not have to be connected to a grounding electrode if the installation complies with 250.34(A)(1) and (2).
Part (1) requires that the generator supply only appliances that are cord-and-plug connected and the receptacles are mounted on the generator or the noncurrent-carrying metal parts of appliances and the equipment grounding conductor terminals of the receptacles are bonded to the generator metal frame. Where these conditions do not exist, Article 702—Optional Standby Systems applies.
Revisions to Article 702 in the 2002 and 2005 editions of the National Electrical Code (NEC) require the application of the article to portable generators that are connected to a premises wiring system. Prior editions did not cover portable generators in Article 702 because Code-making Panel 15 did not want to create a conflict with existing requirements for portable and vehicle mounted generators that appear in Article 250.
Part of the scope in 702.2 describes that application of the article: “Optional Standby Systems are intended to supply on-site generator power to selected loads either automatically or manually.”
Although the requirements for Optional Standby Systems occupy only one page in the NEC, it is important to comply with the requirements. The main thing is to comply with 702.6, which requires a transfer switch.
Here are two sentences in 702.6 that are very important: “Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment.”
This sentence also appears in 702.6: “Transfer equipment shall be required for all standby systems subject to the provisions of this article and for which an electric-utility supply is either the normal or standby source.”
Listed manual transfer switches containing two circuit breakers connected to a slide bar that prevents interconnection of the normal and optional sources at the same time are available, as well as multipole double throw safety switches that allow transfer of the load from one source to the other.
A question that is often asked is: “May an extension cord with attachment plugs (caps) on both ends be connected to the generator and the premises wiring?” The answer is no. A transfer switch as described in 702.6 must be used unless the installation complies with the exception, which requires, among other things, that the electrical system be supervised and maintained by qualified individuals.
Also, 406.6(B) prohibits attachment plugs (caps) on both ends of an extension cord by this sentence: “Attachment plugs shall be installed so that their prongs, blades, or pins are not energized unless inserted into an energized receptacle.”
Where the portable generator is a separately derived system (grounded circuit conductor—may be neutral—is switched), a grounding electrode is required to comply with 250.30. Where the generator grounded circuit conductor is not switched, an equipment-grounding conductor must be bonded to the system-grounding electrode only.
The above is a summary of what I consider to be the main requirements in Article 702.
Protection for air conditioners
Q: I installed the branch circuit and overcurrent protection for a remote air conditioning condensing unit. It consists of a sealed hermetic compressor and a condenser fan motor with this data on the nameplate: 240 volts single-phase, 29 amperes branch circuit selection current, 50 amperes branch circuit short-circuit and ground-fault protection.
Branch-circuit conductors are 10 AWG copper with THWN insulation. Branch- circuit overcurrent protection is a 50- ampere, two-pole circuit breaker. The -inspector has questioned the use of a 50-ampere circuit breaker on 10 AWG copper wire. Is it necessary to increase the branch circuit wire size or reduce the ampere rating of the circuit breaker?
A: No. It appears that you have complied with the manufacturer’s instructions and appropriate rules in the NEC. Although the ampere rating of the overcurrent protective device exceeds the ampacity of 10 AWG copper conductors, this is permitted by 240.4(G) and Table 240.4(G), which is titled Specific Conductor Applications. The first item in the table is the air conditioning and refrigeration equipment circuit conductors, and the reference is to Article 440, Parts III and IV. Also, 110.3(B) allows the installation of listed or labeled equipment in accordance with instructions furnished by the manufacturer. Nameplate data on the equipment qualifies as installation instructions.
Q: Our electrical inspector has notified us that concrete-encased reinforcing rods must be used as part of the grounding-electrode system on all new construction where the concrete slab for the building is in contact with the earth. We often do not get a contract for interior wiring until the floor slab is already poured. Is it necessary to break concrete to gain access to the reinforcing rods?
A: A change in 250.50 in the 2005 NEC requires the use of a concrete-encased electrode on all new construction. Part of 250.50 was revised to require the use of a concrete-encased electrode as part of the grounding-electrode system where present. Previous language used the phrase “where available” to handle situations such as described in the question.
To avoid having to chip out concrete, electrical contractors should ask the general contractor to have about 2 feet of ½ inch or larger reinforcing rod turned up at the expected location of the electric service. Where the service location is unknown, reinforcing rods should be made available at both sides and back of the proposed structure.
Specifications for the concrete-encased electrode should also be provided to the general contractor, such as the minimum size—½ inch and the minimum length—20 feet; the rod material—bare steel or covered with an electrically conductive coating; and a minimum encasement of 2 inches of concrete near the bottom of the slab or footing that is in contact with the earth.
The electrical inspector’s notification that concrete-encased reinforcing rods must be part of the grounding-electrode system is an indication that the 2005 edition of the National Electrical Code is being enforced.
Overcurrent transformer protection
Q: A 50 kVA dry-type transformer with 4 percent impedance is rated 480-volts, three-phase primary with a 120/208-volt wye connected secondary. The total calculated 120/208-volt load is 33 kVA. Is it permissible to use 8 AWG copper wire protected by a three-pole, 50-ampere circuit breaker instead of sizing the primary conductors for the full-load current of the transformer? Is it necessary to use 4 AWG copper conductors for the primary with an 80-ampere three-pole circuit breaker?
A: There is nothing in the NEC that prevents reducing the ampere rating of the primary branch-circuit conductors to accommodate the actual load. However, 240.4 requires conductors to be protected in accordance with their ampacities as specified by 310.15. According to Table 310.16, 8 AWG copper conductors have an ampacity of 40 amperes with 60ºC insulation or 50 amperes with 75ºC insulation. To take advantage of the 75ºC ampacity, wire terminals and equipment must be marked for this temperature rating.
The use of a three-pole, 50-ampere circuit breaker could cause a problem. Since the rated full-load primary current of a 50 kVA, 480 volt, three-phase transformer is about 60-amperes, the 50-ampere circuit breaker may nuisance trip while energizing the transformer because of high inrush (magnetizing) current.
An electrical equipment manufacturer’s representative told me a few years ago: “If you are going to size the primary overcurrent protection at 125 percent or less of full-load current, do not use my circuit breakers.”
To avoid this potential problem, dual-element time-delay fuses can be substituted for the three-pole circuit breaker.
To take advantage of the spare capacity of the transformer, consideration should be given to increasing the wire size to 4 AWG Type THWN copper conductors protected by an 80- or 90-ampere overcurrent device.
Connecting multiple grounding electrodes
Q: Is it permissible to connect ground rods, a concrete-encased electrode and metal water pipe together with a single grounding-electrode conductor? If the answer is yes, what size grounding-electrode conductor is required for a 200-ampere service supplied by 2/0 AWG Type THWN copper conductors?
A: It is assumed that this service supplies a three-wire, 120/240-volt service for a dwelling unit as permitted by 310.15(B)(6) and the accompanying table, thereby, making the overcurrent protection for these conductors Code compliant.
According to Table 250.66, the minimum size grounding-electrode conductor for this service is 4 AWG copper.
The grounded-circuit conductor at the service is connected to the grounding-electrode conductor at one end and to the grounding-electrode system at the other end. This requirement is in 250.24. The grounding-electrode conductor can be connected to any convenient grounding electrode that is part of the grounding system.
However, this grounding-electrode conductor must be installed in one continuous length, or joints or splices must be made by irreversible compression-type connectors or by exothermic welding as required by 250.64(C). Part (F) of this same section allows running the grounding-electrode conductor to any convenient electrode that is part of the grounding-electrode system, provided that the grounding-electrode conductor is not smaller than the largest conductor required for any electrode that is part of the system. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.