By Mike Carr & Mike Kirchner, Generac Power Systems

With the popularity and sales of standby power products increasing, electrical contractors are seeing more and more generator installation opportunities. As electricians become comfortable with how automatic transfer switches and generators are configured and connected, installing a variety of systems is becoming a more routine type of job.

Savvy contractors recognize a profitable opportunity when they see it, and that’s why more of them are selling as well as installing such systems and moving up from residential to commercial products. Now that various manufacturers are offering commercial generators through electrical distributors, the potential profits are magnified for these higher-ticket items. One example of this is Generac's new line of QT Series generators that range in output from 20 to 150 kilowatts. The easy availability of these types of products allows contractors to offer their customers a complete installation package at an attractive price while maintaining healthy profit margins.

This article is an overview of the steps required to successfully size, select, and install a generator in a typical commercial application. It is meant to be informative rather than comprehensive. Personnel who handle installations and electrical connections should be properly trained and certified. For a safe and successful installation, please follow manufacturers’ guidelines and comply with all applicable codes and standards.

The First Step: A Load Analysis

A proper installation begins with a load analysis. If the generator has not yet been selected, the nature and extent of the electrical load will have to be determined and the genset chosen based upon that analysis. So what is an appropriate way to select the generator that will meet the requirements of the application?

The first step is to determine the scope of the project in consultation with the end user. Are all the building’s loads going to be backed up by the generator or only some of the key circuits? Two key factors involved in making this first step decision are what the customer can afford to spend on the project and whether it is cost feasible to separate the more critical loads into their own sub-panel. Oftentimes an end-user may want to back up the entire building only to decide otherwise after receiving a quotation. To avoid this step in the process it is often helpful to prepare some initial sizing estimates and budgetary pricing options to verify the end-user’s desire to back up all or just a portion of the loads.

To create an initial estimate of the load requirements for a small to medium-sized commercial customer, two different approaches can be used and correlated. The first approach is to size to 50% of the building’s service size. This may seem too small compared to sizing the generator to match the service capacity, but most commercial applications have services that are typically loaded between 40 to 60%. The second approach is to make an estimate based upon the building’s size. Most fast food outlets, convenience stores, restaurants, and supermarkets can be estimated at 50 kW + 5 watts / square foot. It should be noted that both these methods are “ball park” sizing guidelines for initial budgetary estimates. Actual load sizing will be necessary.

If the initial estimates are within the owner’s budget, additional sizing needs to be considered. There are three common methods of sizing commercial applications —

 

• Analyzing the billing history
• Using a power analyzer
• Measuring current with a clamp-on ampmeter.

Bill history is useful to capture load levels if the customer is on a commercial utility rate structure with a peak demand change. Simply read the peak kW from the utility bill. One full year of peak demands should be examined to account for seasonality.

The power analyzer approach provides significant information though it may be a bit cumbersome to set up. That approach is recommended if the load characteristics are unique or the load profile is difficult to quantify.

The most common method of sizing is to measure peak load with a clamp-on amp-meter. Make sure to fully load the building when taking the reading. When converting amps to kW, do not use the typical .8 power factor ampacity table supplied by most generator manufacturers. Commercial buildings do not operate at .8 power factor but instead at .95 to unity. For three phase power, calculate the power requirements using the following equation —

kW = 1.732 x Volts x Amps x Power Factor / 1000

For example, if voltage = 208 VAC and current = 200 amps, assuming PF = .95, then KW load = 1.732 x 208 x 200 x .95 / 1000 = 68.4 kW

Once the peak load level is established, size the generator 25% larger than the peak load. This allows for a small amount of load growth and additional capacity for load transients that typically occur when starting a motor. To limit voltage dips within the building to 15%, the maximum across-the-line motor start should be limited. The maximum electric motor horsepower should be less than 15% of the generator’s kilowatt rating. Most generator manufacturers offer basic sizing software that can help estimate the effect of starting an electric motor.

Generally, sizing loads for commercial applications is very straightforward. However, care should be taken to limit certain types of loads with respect to the generator when using the general guidelines described above. Limit nonlinear loads (UPS, computer, battery chargers, etc) to 25% of the generator’s rating. If nonlinear loads are a significant part of the load profile, utilize a clamp-on harmonic analyzer to measure the harmonic current level. Provide this data to your generator supplier for a more comprehensive evaluation of generator sizing.

The Second Step: Sizing & Selecting the Transfer Switch

The size of the automatic transfer switch (ATS) is determined by the size of the breakers supplying power to it. If the application is to power only a critical sub-circuit, the ATS ampacity is determined by the distribution panel breaker providing utility power. If the application is to power the entire building, the ATS ampacity is set by the building service size. The ATS size rating must also be equal to or greater than the generator breaker size.

The circuit breakers supplying power to the transfer switch protect the switch from over-current and short circuit conditions. The ATS requires these over-current protection devices to be located between the ATS and the source power. For some whole building applications with limited space in the electrical room, it may be desirable to locate the ATS outside and wire the utility transformer directly to the transfer switch. This requires a special ATS that is service entrance rated. These switches are typically twice the cost of non-service rated switches. Another option is to utilize a NEMA 3R service rated disconnect in advance of the outside transfer switch.

The Third Step: Generator Fuel Choices

 Generators for commercial applications come in multiple fuel options. For applications less than 150 kW, spark ignited engines utilizing natural gas or liquid propane (LP) vapor are very popular. They tend to be the market norm in this size range for multiple reasons:

 

 

• Unlike diesel fuel, neither fuel is viewed as an environmental contaminant
• Spark ignited automotive-based engines are less costly
• Exhaust emissions are more environmentally friendly
• Natural gas is an unlimited source providing longer run times
• Unlike diesel fuel, neither gaseous fuel needs to be maintained

For applications greater than 150 kW, the market has traditionally chosen diesel-powered generators due to a significantly lower capital cost. However, recent advances in the market now make spark ignited solutions cost effective when compared to diesel solutions. This is due to the recent availability of modular power systems that allow automotive-based engines to easily connect together into a larger, combined power source.

 The Fourth Step: Generator Location

Generators can be located inside or outside a building. The preferred location for commercial applications is outside the building for multiple installation and reliability concerns. Locating the generator inside requires significant attention to exhaust piping, cooling system airflow, fuel piping, vibration damping, and local and national fire codes. The scope of this article assumes that the generator is located outside in a sound attenuated weather resistant enclosure.

When determining the location of the generator, follow these guidelines —

 

• To minimize installation cost, the generator should be located close to the automatic transfer switch and the generator’s fuel source.
  
• To assure adequate generator ventilation, the radiator’s discharge must be unobstructed by walls or other barriers that could cause the discharge air to re-circulate. Since many commercial customers want to place the generator within a walled area, a strong preference should be given to generators that direct the discharge airflow upward.
  
• To assure adequate space for servicing and national code compliance, the generator should be located at least five feet from walls or building openings.
  
• To avoid exhaust being drawn into the building, the generator should not be located near the building’s air intake system
  
• The generator should be located in an area that is secure from flooding
  

The next step is to consider any local codes and ordinances pertaining to standby generators. Contact your local authorities having jurisdiction (AHJ’s) and equipment suppliers for information that may be pertinent. These local requirements may involve electrical wiring, fuel storage, sound and exhaust emissions.

 The Fifth Step: Placing the Generator

 Support pads are typically rectangular shaped concrete forms that support and distribute the weight of the generator. For generators less than 300 kW, all that is typically required is a four-inch thick pad that extends a minimum of twelve inches beyond the unit’s base on all sides. The power cabling and conduit may be routed underground, then stubbed up through the concrete pad underneath the generator breaker location. Care should be made to coordinate the stub-up location with the genset manufacturer’s installation drawings. Another option is to run the conduit above ground. This option is often used for retrofit projects where the generator is located close to the point of building entry.

Once the site is properly prepared, the installation can proceed. When the unit arrives, inspect it for any freight or shipping damage. Locate any items that were shipped loose and keep them at hand, including the documentation packet and door keys.

The genset is typically delivered with its base frame attached to a wooden pallet, making it easy to move with a forklift. Once it is positioned near the pad, remove the connecting bolts and use heavy-duty straps at each corner of the base frame to lift the unit and put it into place. Affix the genset to the pad using masonry bolts of sufficient length and strength.

The Sixth Step: Piping Fuel to the Generator

 Generators may be powered by natural gas, propane (in liquid or vapor form), or diesel fuel. For gaseous powered generators, a mechanical contractor / licensed plumber may need to be involved in piping the fuel lines. The first step is to coordinate with the end-user’s facility personnel and / or the gas company to assure adequate gas pressure and flow capacity.

The genset specifications will define the required flow and acceptable pressure requirements. While some generators operate on gas pressure as low as 5 inches of water column, it is not uncommon for a generator to require 11 inches of pressure. The generator installation manual will provide tables to adequately size the gas piping to minimize piping pressure drops and keep pressure within acceptable levels. The gas flow required will typically be specified in ft 3 / hour or btu / hour (note: 1 ft 3 / hour = 1000 btu / hour). Verify that the existing gas service will support this new generator load in addition to the facility’s existing gas requirements.

Most manufacturers will provide a pre-plumbed coupling in the frame of the generator for the fuel connection. A flexible UL or AGA approved gas line, capable of mitigating any vibration, should directly connect the rigid fixed pipe to the generator. Verify all gas piping for local and state code compliance.

For LP applications, coordinate with the LP tank provider to ensure that the tank has adequate flow capacity for the application. In addition, estimate the generator’s minimum and maximum running time by comparing its hourly fuel consumption rate (at 75% of rated load) with different amounts of fuel in the tank (which could vary greatly at the time of an outage, depending upon if the tank was recently filled or not). Consult with the end user to determine if a larger tank is needed to ensure that the generator can run for a sufficient number of hours if the fuel level happens to be low.

For diesel applications, a sub-base fuel tank is typically recommended due to its ease of installation and highly reliable means of providing fuel to the generator. Coordinate with your local genset supplier to assure compliance with any local code or permitting requirements.

 The Seventh Step: Wiring the Generator

 Generators are available in various voltage and phase configurations. The first step is to ensure that the unit at site matches the building requirements. Wire the generator and transfer switch power cabling per NEC requirements using 75 degree cabling. Verify and maintain correct phase rotation while wiring the system. Care should be taken to support the lugs on the generator breaker and ATS when tightening the power cabling. Most single phase or three phase commercial applications will utilize a 2 pole or 3 pole switch, respectively. In these configurations, the generator neutral is connected to the facility’s neutral at the ATS and should not be bonded to the generator frame. There should be an equipment ground wire between the ATS and the generator connection panel, and the generator frame needs to be connected to an effective ground-fault current path. This requires the generator to have an equipment ground (grounding conductor) typically connected to the generator frame.

As for the wiring of control circuits and auxiliary power, consult the manufacturer’s documentation for specific requirements; however, certain generalizations can be made. Generators typically require 120 / 240 volt auxiliary power to operate the block heater and battery charger. This circuit is typically 20 amps and is powered from a distribution panel that is backed up by the generator. For control wiring, a two-wire start function is typically pulled between the generator and the ATS. This is the wiring that enables the generator to start up after the ATS has sensed that the utility power has failed. Other control wiring may be required based upon application specifics.

 The Eighth Step: Starting Up the Generator & Making Operational Checks

When all system connections have been made, the installation is in effect complete. What remains to be done is to check over the generator and ensure that the system will operate as intended in the event of a utility power outage. Depending upon the size and type of generator, that will be the responsibility of the generator dealer or the electrical contractor.

It is important to check over the generator prior to operation. Use the manufacturer’s start-up checklist that is included in the documentation packet. Key elements to inspect are the engine oil level, the gearbox oil level (if the unit has a gear drive), the coolant level, fan belt tension and the starting battery.

The generator should be operated and tested briefly before being placed in use. It should be run in manual mode with the circuit breaker open to verify correct voltage and frequency, comparing the control panel readings to rated specifications. It should then be run in manual mode with the breaker closed to verify proper phase rotation with the utility at the transfer switch.

To prepare the system for automatic operation, the generator controller is switched from manual mode to automatic and the transfer switch maintenance disconnect switch is also put into automatic, if so equipped. Check the system operation by opening the utility breaker at the main distribution panel. After the appropriate time delays have expired, the engine should start running and come up to its normal operating speed within about ten seconds. The ATS should then transfer automatically to provide generator power to the load. To complete the test, close the utility breaker. The system should automatically retransfer to the utility and go into a pre-programmed cool-down before shutting down.

Conclusion

Automatic standby generators are becoming easier to install as manufacturers strive to streamline the process and make them easier to connect, test and commission. Advanced digital control systems are more reliable than ever and their “smart” capabilities provide a quicker and fuller indication of system status.

As more and more businesses seek backup power protection, commercial standby generators offer a tremendous profit opportunity. By learning more about these products and how to install as well as sell them, electrical contractors have an easy way to win bigger jobs and earn greater profits.