Home > Resources and Tools > Engineer Resources > PowerConnect Newsletter > March 2015 > Understanding NEC Code for Standby Generators: Cabling, Sizing, Startup and Disconnect
Understanding NEC Code for Standby Generators: Cabling, Sizing, Startup and Disconnect

Understanding NEC Code for Standby Generators: Cabling, Sizing, Startup and Disconnect


The National Electric Code (NEC) was not written with generators as a major point of focus. As a result, it is necessary to reference multiple sections of the NEC code when designing and installing power generation equipment. This article will reference some key areas while offering insight into typical generator code-related questions.

Is the cabling from the generator a feeder or a service?
This is a basic question that has a significant impact with respect to interpreting generator installation requirements. NEC 100 defines a service as being supplied by an operating utility. Therefore, the conductors from the generator are classified as a feeder.

How should the cabling be sized?
Most generators have a single, fully rated output breaker or multiple smaller ones. When the generator has a single output, the breaker is normally sized 100% to 125% of the rated output amps. The cabling is selected to match the generator’s output breaker(s). Many contractors and engineers will typically size to meet the full breaker output capacity. Others will size using the NEC 240.4(B) roundup rule which allows the generator cabling to roundup to the next nominal breaker size (aka a 500 mcm cable rated at 380 amps could be used on a generator’s 400 amp breaker).

The other option for cable sizing is to follow the guidelines in NEC 445.13 which sizes the cabling at 115% of the generator’s rated amps. This method is typically utilized when the generator does not include on output breaker.  NEC 445.13 also allows the cabling to be sized for 100% of the generator’s rated amps provided the generator is designed to prevent overloading. With today’s generator digital controls, it is fairly easy to apply overcurrent protective trip functions. We would recommend coordinating with your local authority having jurisdiction (AHJ) prior to sizing with this method to assure consensus. 

The final cable sizing method is chosen when utilizing main bus bar (no breaker configurations) to better facilitate selective coordination. In these applications, it is possible to cable multiple disconnects to the generator bus following the NEC feeder tap guidelines.

What does the NEC require for generator sizing?
The answer to this question varies based on the type of load being powered. Emergency systems require the generator to be sized for the entire connected load (NEC 700.4). But if the emergency system is a healthcare application, NEC 517.30(D) requires the generator to meet the peak demand of the load and waives the requirements of NEC 700.4. This was a direct result of healthcare applications being too lightly loaded. 

Legally required standby and optional standby have traditionally required the generator to be sized to meet the peak demand of the load intended to be operated. This requirement has been strengthened in the 2008 code for optional standby applications. These applications now require compliance with Article 220, “Branch-Circuit, Feeder, and Service Calculations”, or another AHJ method. This change was driven by a desire to protect the consumer in the residential market from installing a generator smaller than the potential load. For commercial/industrial optional standby applications, we typically find that a simple load calculation to Part III of Article 220, billing / demand history calculation, or a Professional Engineer (P.E.) stamped drawing  are considered acceptable sizing practices by most AHJs.

How quickly must a generator startup and transfer? 
The common answer to this question is 10 seconds. Emergency systems (NEC 700.12) and fire pumps (NFPA 20 require a 10 second start and load transfer. Legally required loads require a less demanding 60 second response (NEC 701.12) and optional standby does not require an automatic startup and transfer time. Meeting a 10 second startup time is generally not an issue for single generator applications; however, applications with multiple generators often take a few extra seconds to parallel generators. To meet the 10 second requirement, these applications need to transfer the emergency system loads onto the first generator and then add lower priority loads later.

Is a disconnect required on the generator?
NEC 445.18 shows a preference for the generator to be equipped with a disconnecting means but does allow that disconnect (typically the generator breaker) to be removed provided the generator can be readily shutdown, can be locked out from restarting, and does not operate in parallel with other sources. Though the NEC does allow the generator to be provided without an output breaker, standard industry practice is to typically include this disconnect. 

In addition to the generator breaker, is another disconnect required at the point of building entrance?
This is an interesting question because the market implementation of this requirement seems to be split – half the market requires an additional disconnect and half does not. Since the cabling from the generator is a feeder, NEC 225.31 requires disconnect at the point of building entry unless exempted in another part of the code. Enter the local interpretation of the requirements for an outdoor housed generator set (NEC 700.12(B)(6), 701.12(B)(5), 702.12) which states: “Where an outdoor housed generator set is equipped with a readily accessible disconnecting means in accordance with 445.18, and the disconnecting means is located within sight of the building or structure supplied, an additional disconnecting means shall not be required”.  The interpretation issue is based on what needs to be visible from the building. Many markets accept generator visibility as acceptable. In these markets an additional disconnect is not required provided the generator is within 50 feet of the building. If the market norm is to only accept disconnect (versus generator) visibility, an additional disconnect would normally be required as most generator breakers are not visible from the building. 

Another way to consider the local variation on this issue is based on intent. The intent of 225.31 was to ensure power being fed from one building into another could be quickly and easily disconnected in emergency situations. Since the generator is normally placed near the point of building entry, many markets may be comfortable with disconnect capability built into the generator. Again, check with AHJ to be certain.

If you have questions about the NEC code, contact your authorized Generac Industrial Power distributor or dealer