A client requested that I inspect a generator installation that was determined to be the cause of a fire. The client’s concern was whether the installation identified as an optional standby system complied with the National Electrical Code (NEC).
Let’s look at the violations of the generator installation. The installer cord-and-plug connected a portable generator into a three-phase outlet and attempted to supply certain loads fed from a panelboard. The generator was sized with enough capacity to operate the load in the panelboard, but the flexible cord, attachment cap, and outlet and outlet wiring were undersized. Because of this undersizing of wiring methods and components, the insulation of the circuit conductors in the wall began to overheat, and a short circuit developed between conductors that ignited the framing members and finished wall materials.
Another problem was the manner in which the generator was connected by the existing wiring through a circuit breaker in the panelboard. Wiring the panelboard in this manner allowed a back feed from the generator into the utility lines, creating a hazard for linemen. A transfer switch also was not installed per 702.5(B)(1) or (2) and 702.6, which governs a permanently installed optional standby system.
Manual transfer equipment is available to connect an optional standby system with enough capacity to supply all operated loads. The advantage of such a switch is the user can select the loads the generator serves. If installed, automatic transfer equipment must be capable of supplying the full load that is transferred if the normal power supply is lost. A standby source employed to manage the load must have a capacity that sufficiently powers the maximum load connected by the load management system.
A portable generator that is not grounded to a grounding electrode per 250.34(A) can supply only hand tools and similar equipment electricians use for construction and maintenance activities. Note that only equipment mounted on the generator’s frame can be cord-and-plug connected through receptacles mounted on the generator. Noncurrent-carrying metal parts of equipment and tools plus the equipment-grounding conductor terminals of the receptacles must be connected to the frame of the generator.
A temporary connection of a portable generator is permitted without the use of transfer equipment where conditions of maintenance and supervision ensure only qualified persons service the installation. The normal supply must be physically isolated by a lockable disconnecting means or by disconnection of the normal supply conductors in a manner that prevents a back feed of generator power into the utility power lines per the exception to Section 702.6.
A permanently installed generator must be classified as either a separately derived system or a nonseparately derived system as required by 250.35(A) or (B) and 702.10(A) or (B). A separately derived system (NEC 250.20(D)) is considered an alternate source, such as an on-site generator provided with transfer equipment that includes a grounded conductor. Note that it is not solidly interconnected through the transfer equipment (usually a four-pole switching mechanism) to the service-supplied grounded conductor. The grounded conductor is switched by the transfer equipment and connected to the earth-grounding electrode of the generator and not connected to the earth ground of the service equipment.
A nonseparately derived system (NEC 250.20(D), FPN 1) is a situation where the alternate source transfer equipment (usually a three-pole switching mechanism) does not include a switching action in the grounded conductor, which allows it to remain solidly connected to the service--supplied grounded conductor when the generator is in operation and supplying all connected loads served.
Supply-side and load-side grounding
A conductor that provides an effective ground-fault current path per 250.4(A)(5) must be installed with the supply conductors from a permanently installed generator to the first disconnecting means based on the generator being identified as a separately derived system or classified as a nonseparately derived system. If there is not an overcurrent protection device at the grounded generator protecting the conductors routed to the first disconnecting means, the equipment-bonding jumper, grounding-electrode conductor and grounded conductor must be sized from Table 250.66 based on the size of the conductors from the generator. Based on the size of the overcurrent device, Table 250.122 is used to size such conductors if the device is located at the generator and protects the conductors from the generator. Review 250.142(A)(3), 250.35(B) and 250.30.
Final verdict: The optional standby system was installed without a transfer switch and wired incorrectly as well as grounded inappropriately, so it failed to comply with the NEC for a permanently installed generator.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.