Ecmweb 6094 Generator Design And Installation Guidelines Pr
Ecmweb 6094 Generator Design And Installation Guidelines Pr
Ecmweb 6094 Generator Design And Installation Guidelines Pr
Ecmweb 6094 Generator Design And Installation Guidelines Pr
Ecmweb 6094 Generator Design And Installation Guidelines Pr

Generator Design and Installation Guidelines

Feb. 18, 2014
There’s more to generator installations than a first glance at Article 445 would indicate.

Article 445 of the National Electrical Code (NEC) provides the electrical installation requirements that apply specifically to generators. A generator is basically a motor that is driven by a motive force to produce electricity instead of being driven by electricity to produce motive force.

The NEC provides the requirements for motors in Art. 430, which is the longest Article in the book. By contrast, Art. 445 is only slightly more than a single page. However, Art. 445 does refer to Art. 430 in several places.

Photo 1. Power Design, Inc. installed this 250kW generator with a 550-gal, integral fuel tank for the emergency systems at its Solaire Wheaton project, a mixed-use facility in Wheaton, Md.

Doesn’t cover it all

Article 445 assumes you understand the complexities of generator installation. It applies to all generators, but there are big differences between a small 5kW residential generator and a big diesel generator providing backup power to a data center. There’s also a big difference between a backup generator (ramps up slowly, runs a long time) and an emergency generator (comes to running speed quickly, can’t run a long time). Article 445 doesn’t address these differences.

Article 445 doesn’t present requirements for sizing a generator, either. This is an engineering decision.

At the simple end, it’s a residential generator, and the owner just doesn’t want all that venison in the freezer to spoil if the power goes out.

For a large critical uptime situation, you may be sizing a bank of backup generators with a redundancy factor (e.g., n+1). You might also accommodate other factors, such as load shedding schemes, or be sizing different types of generators for different types of uses.

Location

Article 445 doesn’t address siting a generator, but it does say that all generators must meet the requirements for motors in 430.14 [445.10], and these are location-related. Obviously, you want the generator to have good protection from the elements and mechanical damage. But you must ensure it has adequate ventilation and is readily accessible for maintenance (Photo 1).

For small installations, correctly establishing “adequate” and “readily” is a matter of common sense. But with installations that are large or mission-critical, any of several industry standards may apply.

In small installations, especially residential, another location factor to consider is the proximity to the service entrance. Locating it close to the service entrance has advantages for power transfer and for bonding to the electric utility ground. If you site the generator outdoors on a pad, be aware of berms, swales, and other water-directing landscape features. The landscape could rule out installing the generator close to the service entrance.

In large installations, locating emergency power generators away from the service entrance (Photo 2) reduces the risk that a single event will take out both your primary power and emergency power. You must also consider fuel delivery, day tank location, fuel maintenance issues, and other factors that complicate the location decision.

Noise can affect location as well. Large industrial generators are often in their own specially designed building with noise abatement features. One limitation is finding enough space for this building and the necessary access driveway(s).

Photo 2. Here, you can see the raceway penetrating the floor in the generator room. The enclosed conductors bring emergency power out to fire pump, elevators, and building life safety systems at Power Design’s Solaire Wheaton project. The flexible metallic conduit is the best choice for protecting the conductors while providing some mechanical isolation from the vibration and movement of the gen-set.

For residential applications, avoid installing the generator on the side of the house facing a neighbor’s bedroom. Where this isn’t practical, house it in a suitable (and properly ventilated) enclosure. The ventilation must be operable (or, if passive, functional) in extreme weather.

Marking

The 2014 NEC introduces new marking requirements for generators. It adds two paragraphs to the single paragraph that was in the 2011 NEC, plus adds requirements to that first paragraph.

While it’s true that these requirements pertain to the manufacturer and not the installer, the purpose is to provide the installer with useful information. Now, for example, the generator will be marked to show whether the generator neutral is bonded to the frame [445.11].

A whole new set of marking requirements applies to generators 15kW or larger:

• Power factor.

• Subtransient and transient impedances.

• Insulation system class.

• Time rating.

Overcurrent protection

Some specifics apply, depending upon which of the listed configurations you have [445.12]:

A) Constant voltage generators. Except for AC generator exciters, protect these by inherent design, circuit breakers, fuses, protective relays, or other identified and suitable means.

B) Two-wire DC generators. These can have overcurrent protection in one conductor if the overcurrent protective device (OCPD) is actuated by the entire current generated (other than the shunt field current). The OCPD cannot open the shunt field.

C) 65V or less. If driven by an individual motor, it’s considered protected by the motor OCPD, but only if the OCPD operates when the generator is delivering no more than 150% of its full-load rated current.

D) Two-wire DC generators used with balancer sets and 3-wire systems. The OCPD must disconnect the 3-wire system in case of unbalanced voltage or currents.

E) Three-wire DC generators. Install one OCPD in each armature lead, connected so it’s actuated by the entire current from the armature.

Conductor ampacity

Subsection 445.13 has four requirements:

1) The conductors running from the generator terminals to the first distribution device (containing OCPD) must have an ampacity at least 115% that of the nameplate current rating of the generator.

2) You can size the neutral per 220.61.

3) Conductors that must carry ground faults cannot be smaller than required by 250.30(A).

4) If neutral conductors of DC generators must carry ground faults, then they must be at least the minimum required size of the largest conductor.

When passing those conductors through enclosure openings, make sure to use bushings that protect the conductors from abrasion. With engine-generator sets, there’s an additional caution. If oil or grease may be present, use bushings that can tolerate exposure to oil [445.16]. Oil degrades rubber, so bushings containing rubber are not suitable.

Guards and protection

Protection against accidental contact with live parts is an NEC requirement [445.14], as are guards for attendants [445.15]. Typically, small generators have built-in protection against accidental contact, and there usually are not attendants. Take care when removing and replacing covers that you leave this protection intact. In larger installations, such as those that provide power for a hospital or a data center, the generator and controls are usually specified as a package. Take care that on-site modifications during installation don’t leave live parts exposed and do leave guards intact. Don’t leave this to chance; include inspection for this in your pre-startup checklist.

Terminal housings

For generators operating at 600V or less, the terminal housings must comply with 430.12. Subsections (B) through (D) of this section address minimum spacing requirements and refer to Tables 430.12(B), 430.12(C)(1), and 430.12(C)(2). Subsection (A) requires the housings to be made of metal and of substantial construction. Subsection (E) requires you to provide a means of attaching an equipment grounding conductor termination.

Disconnect

You must provide a disconnect that can be locked open. It has to disconnect the generator, all protective devices, and control apparatus supplied by the generator [445.18]. But you don’t have to do this if both of the following conditions apply:

• The generator is arranged to operate in parallel, and

• The driving means for the generator can be readily shut down.

New with the 2014 NEC is elaboration on that second requirement. The driving means must be lockable in the OFF position per 110.25 and rendered incapable of restarting. Also new with the 2014 NEC, you don’t have to provide this disconnect if the generator is a cord- and plug-connected portable generator.

Generators supplying multiple loads

The 2011 NEC passed along a previous NEC requirement regarding multiple loads [445.19]. Basically, it gave permission to supply a switchboard with separate sections or individual enclosures with overcurrent protection tapped from a single feeder. The 2014 NEC dropped Sec. 445.19 entirely.

GFCI

In the 2011 NEC, subsection 445.19 was the last subsection and last requirement in Art. 445. The 2014 NEC adds 445.20 in its place, but this applies only to receptacles on portable generators that are 15kW or smaller. Typically, these are cart-mounted, and there’s no wiring for the electrician to do. This requirement is aimed at manufacturers.

Read Chapter 7 first

To ensure your generator installation is successful, don’t start with Art. 445. For example, suppose your generator supplies power to the premises wiring in the event of electric utility power failure. In that case, the generator system must be set up so that it cannot backfeed power to the grid.

So where do you find this transfer switch requirement? That would be in Art. 705 [Interconnected Electric Power Production Sources]. You must review several Chapter 7 Articles before even looking at Art.445.

You need to know if your generator is for an emergency system [Art. 700], legally required standby system [Art. 701], or optional standby system [Art. 702].

Which generator model you select will depend upon whether you need a rapid-starting but short-running emergency generator (to carry the load from the UPS after a short ride-through) or a slow-starting but long-running backup generator. A critical facility may have both.                

Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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