A generator

Electrical Inspectors and Generators for Existing Dwellings

What is a generator and how do we inspect it on a residential application? Simply put, a generator is composed of two main parts, a prime mover, and an alternating-current or direct-current motor. The prime mover spins the motor causing an electromagnetic field to be induced onto the magnetic pole(s) of the motor. The number of poles the motor has, determines what the generator produces such as, single-, two-, or three-phase power. This is a very simple explanation. The generators can be permanently connected or portable. As inspectors we normally will not be looking at the internals of a generator since most are a permanently connected listed piece of equipment. Some gen sets use a combustion engine for prime mover or possibly a wind turbine, which could be fueled by natural gas or liquified petroleum gas (LPG). For this blog, we will discuss a permanently installed combustion engine type generator set being added to an existing residential dwelling. These installations are referred to in the NEC as optional standby systems and covered under article 702. The rating of most residential generators is not typically over 22 kilowatts (KW) and may be considered a gas fired appliance by the mechanical code. Frequently these installations require more than one inspector.

When I was inspecting in the field, one of the initial items I requested was the installation instructions for the unit. This would provide additional information like clearances from buildings, windows, or doors, as well as specific wiring requirements. After proper clearances were verified, I would look for the disconnecting means required by the 2020 National Electrical Code® (NEC®), section 445.18 and they are:

  • Emergency shutdown of prime mover – this disconnecting means is designed to disable the prime mover from inadvertently starting again and requires a mechanical reset to reengage the prime mover
  • Remote emergency shutdown – this disconnecting means is applicable to generators over 15 KW rating and is located outside of the generator enclosure or equipment room, so this may affect larger one- or two-family dwellings.
  • Emergency shutdown in one- and two-family dwelling units – this disconnecting means is for any generator at a one- or two-family dwelling and must be outside the dwelling in a readily accessible location.

These disconnecting means should not be confused with the transfer switch, or the overcurrent protective device located within the generator housing for the feeder conductors running to the transfer switch.

Being somewhat familiar with other codes I would frequently ask the installer or owner if they had contacted the utility company to determine if the current gas meter or regulator was sufficiently sized to handle the increased gas consumption created by the generator. Often, they had not. This may not seem important but asking this question may have saved them from being without the generator or heat when it was needed. Gas meters and regulators are typically sized for a specific cubic feet per minute flow at the time of installation, based on the amount of British Thermal Units (BTUs) required by the appliances in the dwelling. By adding additional BTUs to an existing gas meter without being upgraded could mean the gas fired appliances will not have sufficient gas flow to function properly. Inspectors asking questions, even when it is not within the area of your expertise, can almost always help avoid future problems for the customer.

Moving to the transfer switch, I would determine if there was as an integral main overcurrent protective device (OCPD) and then inspect clearances around the switch. When a transfer switch contains an OCPD it frequently means the switch is an automatic transfer switch and is the service disconnecting means. For our scenario this is the case. I then would request a load calculation or provisions to automatically manage the load, sometimes called “load shedding.” Once that information was gathered, I would verify that the switch had a “suitable for use as service equipment” (SUSE) label, and that it had proper capacity, ratings, listing, and labeling. Since the transfer switch is the service disconnect it may also be serving as the emergency disconnect required in the 2020 NEC, section 230.85. Because the transfer switch is being used for both the service disconnect and the emergency disconnect, it must be marked as: EMERGENCY DISCONNECT, SERVICE DISCONNECT. Those markings should be on the exterior of the enclosure and comply with section 110.21(B). Adding emergency disconnects to the exterior of a dwelling is a way that the NEC allows first responders to safely disconnect all power within the structure, which will save time and lives in the event of a fire.

Within the automatic transfer switch, I would also be verifying conductor sizes from meter, generator, grounding electrodes and new feeder to the old service panel as well as properly torqued terminations. The old service panel, typically inside the dwelling, new or existing, now has feeder conductors providing it with power instead of service conductors. Therefore, all neutral conductors must terminate on separate terminal bars from the equipment grounding conductors and the main bonding jumper between the neutral conductor and the panel enclosure must be removed if the panel is existing. If everything checks out, then I would have the installer initiate a power outage scenario to make sure all systems were functioning properly.

This was not an overly deep dive into residential generator and optional standby system inspections, but a good overview of what to look for when conducting them. When on the job do not be afraid to ask questions; they can often lead to the discovery of a potential problem. Catching such issues early in the process allows us to readily address and fix the problem before it has the potential to harm us and others.

To learn more about this and other related topics, go to the NFPA electrical inspection webpage and join us on NFPA Xchange where you can collaborate with other industry professionals, ask questions, and network with like-minded individuals We look forward to hearing from you!

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Dean Austin
Dean Austin
Senior Electrical Specialist

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