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Top 10 AHJ Points of Confusion - Part 1 of 2

Top 10 AHJ Points of Confusion - Part 1 of 2


Authorities Having Jurisdiction (AHJs) have an array of different skills, knowledge and experience with the general rules and codes governing generator installations. (We won’t even mention the onerous local regulations that exist.) Over the years, contractors and technicians have shared with us the most common areas where misconceptions or confusion occurs with AHJs.

We developed this Top 10 List so that during an inspection, you can be prepared to address these issues and explain the situation, including code references. This may help alleviate some of the issues you experience on a regular basis.

Since the explanations are quite detailed, we are dividing this article into two segments. In our September newsletter, we will feature Questions 1-5 that cover general misconceptions, sizing and placement issues. In our October Issue, Questions 6-10 will cover electrical issues such as grounding, separately derived systems, raceways and more.

1) Do all generators have to meet the requirements of NFPA 110, the Standard for Emergency and Standby Power Systems?
NFPA 110 is the code Standard for Emergency and Standby Power Systems and classifies these systems as Level 1 systems, where the loss of the system will result in the injury or loss of human life, or as Level 2 systems where the loss of the system is less critical to human and life safety.  Systems that are not classified as Level 1 or Level 2, are not covered by this standard.

NEC Article 700, Emergency Systems are NFPA 110 Level 1 Emergency Systems and NEC Article 701, Legally Required Systems are NFPA Level 2 Standby Power Systems.  NEC Article 702, Optional Standby Systems are not covered by the NFPA 110, and within NFPA 110, Section 1.1.3 (6) states systems that are not classified as a Level 1 or Level 2 system are not covered by the standard.

Optional Standby System generators are utilized when businesses or homeowners are caused discomfort from the lack of electrical power and life safety is not dependant on the operation of the system.  A good example is a restaurant that wants a generator to provide backup power for the walk-in coolers and freezers to prevent the loss of product during a power outage. The generator is not required for life safety and will not be supplying any emergency or legally required loads, as the emergency lighting is supplied by unit equipment. An Optional Standby System generator can be installed to protect the restaurant.

2) What codes cover placement of the generator?
The code requirement for the placement of a generator can be found in NFPA 37, the Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines. 

Section 4.1.4 (view example) states that engines and their weatherproof housings if provided, shall be installed outdoors a minimum of 5 feet from any opening in walls and a minimum of 5 feet from any structure constructed with combustible materials.

There are two exceptions to the placement requirements. A minimum separation from a structure shall not be required if one of the two conditions below is met:

  • the adjacent wall of the structure has a minimum of a one hour fire rating, or
  • the generator enclosure is constructed with noncombustible materials and a Nationally Recognized Testing Laboratory has demonstrated by testing the enclosure that a fire within the enclosure will not spread and ignite any combustible materials outside of the enclosure.

3) Is a Standby System generator feeder the same as the utility service conductors?
The NEC definition of a service is: “the conductors and associated equipment for the delivery of electrical energy from the servicing utility to the premises served wiring system.”  Only the local utility supplies an electrical service, everything else is a feeder. The conductors from the generator to the transfer equipment and premises served are always feeder conductors. The generator should never be considered as a service.

4) For facility applications, does the size of the generator have to match the size of the electrical service?
Article 702.4 states that when an optional standby system generator is installed with automatic transfer equipment the generator has to be sized large enough to either handle the full load that’s transferred by the automatic transfer equipment, or if a Load Management System is utilized, the generator has to be sized large enough to handle the load that will be energized 100% of the time. 

A simple load management scheme is easy to implement. If the generator is close to being overloaded, the generator can temporarily de-energize some of the connected load (load shed) via a load shed contact. This may typically be as simple as a 90% load level output.

5) How are optional standby generators sized?
Article 702.4 states that the generator has to be sized in accordance with Article 220 or by another approved method. The generator can be sized by using the Standard Method calculations in Part III, the Optional Method in Part IV, or by using the demand data or billing history allowed by 220.87 for existing loads. (view example)

If the system utilizes Load Management, the managed load will not be calculated into the sizing calculations. This is due to the fact that the managed load may not be energized by the generator.

The second round of Top 10 issues (6-10) will appear in the October issue of PowerConnect. If you have questions about the Top 10 AHJ Points of Confusions or any code issues, contact your authorized Generac Industrial Power distributor or dealer.