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Concerned about Y2K? Utility deregulation? Live out in the country where it seems every thunderstorm or ice storm puts you in the dark? Customers are turning to standby power sources to keep their essential loads functional during a crisis.

An electrician friend remarked that his most common request for help these days comes from homeowners. They call to ask about the cost of installing an outside receptacle on their house. He has learned to translate this request into an informational presentation on optional standby systems. The Code indicates that "No receptacle shall be installed so as to require an energized attachment plug as its source of supply." See NEC Section 410-56(g). Homeowners typically are not familiar with the requirements of the NEC. They are looking for a low cost way of providing power during an outage. Our job is to ensure that the homeowner can accomplish this in a safe, code-compliant manner. Electrical installations of this type typically require a building or construction permit and inspections of acceptance. Consult your local building department for information regarding permits and inspections.

Optional standby systems provide power to public or private facilities or property where life safety does not depend on the performance of the system. Such systems supply on-site generated power to selected loads. The source is normally a standby generator that may be permanent or portable. The transfer of power between the normal supply and the standby source may be automatic or manual. See NEC Section 702-2 and FPN.

Equipment used with optional standby systems shall be approved for the intended use. It is critical that transfer equipment be designed and installed to prevent the inadvertent interconnection of normal and alternate sources of supply. Utility workers can be subject to shock or electrocution if this requirement is not met. Authorities having jurisdiction typically use evidence of listing and labeling by an approved third party testing laboratory as the preferred method of approval. Section 110-3(b) requires that listed or labeled equipment be both installed and used in accordance with the instructions.

Photo 1. Typical portable generator 5,000 watts

Photo 2. Fixed generator more common for commercial installation with transfer switch.

Photo 3. Transfer equipment for use with portable generator

Transfer equipment used on the line side of the service equipment shall be suitable for use as service equipment and be marked accordingly. If located on the load side of the branch circuit overcurrent protection, transfer equipment is permitted to contain supplementary overcurrent protection having an interrupting rating sufficient for the maximum available fault current that the generator can deliver. See NEC Section 702-6.

A sign shall be placed at the service-entrance equipment that indicates the type and location of on-site optional power sources. Individual unit equipment installed for standby illumination is permitted without such a sign.

Smaller generators are not intended to be connected as a separately derived system. The generator used to provide power typically has both grounded (neutral) and equipment grounding conductors. The grounded conductor from the generator is normally solidly connected through the transfer equipment. The non-separately derived system typically shares the grounding electrode system with the normal source.

Generators shall be grounded as specified in Section 250-34. See also Section 250-20(d) for grounding portable generators that supply a fixed wiring system. Section 250-20(d), FPN No. 1 indicates that if the neutral is solidly interconnected to the service-supplied system neutral, the generator is not a separately derived system. Where the grounded conductor from the stand-by source is connected to an electrode that is remote from the source, there shall be a sign at the grounding location. The ground reference of the separately derived system must be maintained. The sign shall identify all normal and stand-by sources connected at that location. See Section 702-8.

Using portable generators to supply a permanently installed system presents a problem. Generally, the generator manufacturer bonds the neutral to the frame. Most of transfer equipment sold for residential type applications breaks only the phase conductors. The result is a neutral conductor referenced to ground in two locations, when the generator is plugged in. Yet if the bonding jumper is removed, the fault protection may be compromised for any cord- and plug-connected loads that are supplied from receptacles on the generator! The AHJ must carefully weigh both hazards when evaluating the suitability of the installation.

Generator installations shall comply with NEC Article 445. The generator shall have a nameplate giving critical data such as:

1. Manufacturer’s name

2. Rated frequency

3. Rating in kilowatts or kilovolt amperes

4. The normal voltage and amperes that correspond to the rating.

Constant voltage, AC generators shall be provided protection from overloads. Such protection may be through inherent design, circuit breakers, fuses, or other suitable overcurrent protective means or devices that are suitable for the conditions of use. Since generators are required only to have overload protection, the Code contains special provisions for sizing generator conductors. The ampacity of the conductors from the generator terminals to the first overcurrent device shall be at least 115% of the nameplate current rating of the generator. Neutral conductors are permitted to be sized in accordance with Section 220-22.

Generators are required to have a disconnecting means. The disconnecting means shall disconnect the generator, all protective devices, and any control apparatus entirely from the circuits supplied by the generator. No disconnecting means is required if the driving means (engine) can be readily shut down and the generator is not arranged to operate in parallel. See Section 445-10.

The generator may be located inside or outside of the building. The applicable building code requirements shall be met in either case. Generators are considered a hazard. There may be building code requirements that require isolation and ventilation to address safety issues. Manufacturers of portable generators recommend against operating the generator inside of a building. The instructions may also recommend spacing the unit away from operable windows or air intake openings. In both cases, carbon monoxide and the increased potential for fire are the hazards.

Some authorities having jurisdiction apply the requirements of Article 225, Part B to any generator that is located outside of the building. The AHJ may determine that an additional disconnecting means is required at the building served. If so, Section 225-36 requires that the disconnecting means be suitable for use as service equipment. In the case of a portable generator, a suitable cord body and attachment plug may be judged suitable due to the limited amount of available fault current.

Some electrical authorities judge that a generator is not a separate building or structure if the generator serves only one building and is close to that building. In this case, the generator conductors are connected directly to the transfer equipment. The transfer equipment is located on the load side of the service disconnect and generally near the service panelboard. The transfer equipment must be suitable for the intended purpose. In either case remember, Section 445-10 requires all generators be either equipped with a disconnect or have means readily available to shut down the engine.

Contact your local electrical inspector for additional information.

References
1 National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented only by the standard in its entirety.

Tom Garvey, a registered professional engineer and an electrical inspector for the state of Wisconsin, represented the IAEI on NEC Code-Making Panel No. 11 during the 1999 cycle. He also represents the association on the NFPA 79 committee and is participating in the harmonization effort. He is certified by the state of Wisconsin and the ICBO as an electrical inspector.

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