Key Takeaways
A commercial standby generator needs to be engineered around the building, not simply selected from the amperage printed on the main disconnect. We evaluate the service voltage, phase, calculated load, equipment being backed up, motor starting requirements, transfer equipment, fuel availability, generator location, conductor sizes, grounding and bonding, and applicable NEC and NFPA requirements.
A 125 kW three-phase generator is a size we commonly consider for larger commercial applications, including certain buildings with loads in the neighborhood of 300 amps, but generator sizing is determined by the actual electrical load and operating characteristics. Briggs & Stratton’s 125 kW platform is currently available in several configurations, including 120/208V three-phase and 277/480V three-phase.
We also install larger generator systems when the building requires more capacity to withstand power outages.
When the utility power goes out at a commercial property, the question isn’t simply whether the lights go out. Depending on the business, an outage can shut down refrigeration, HVAC equipment, pumps, security systems, computers, access control, communications, commercial kitchens, manufacturing equipment, elevators, lighting, and other systems the building depends on every day.
At Pinellas County Electric, The Panel Pros™, we design and install commercial standby generator systems throughout Clearwater and Pinellas County. We work with three-phase electrical systems, automatic transfer switches, commercial distribution equipment, natural gas, diesel, and LP generator installations, and the electrical infrastructure necessary to keep important building loads operating when utility power is lost.
Briggs & Stratton is one of the generator manufacturers we trust for these applications. Their commercial lineup includes three-phase equipment at multiple voltages and capacities, allowing us to design around the actual electrical requirements of the property instead of trying to force every business into the same generator package.
A 300 Amp Building Doesn't Automatically Need a 300 Amp Generator
This is one of the first things I explain when we’re designing commercial generator systems. Someone may tell me: “It’s a 300 amp building, so I need a 300 amp generator.”
That’s not necessarily how we size it. A building’s service rating tells me something about its electrical system, but it doesn’t automatically tell me the actual load the generator needs to carry.
I want to know what operates during normal business conditions and, more importantly, what needs to operate during an outage.
Are we backing up the entire building? Are we only backing up selected loads? Are there large air conditioning compressors? Pumps? Elevators? Refrigeration compressors? Three-phase motors? Commercial kitchen equipment? Medical equipment? Large lighting loads?
That’s where the load calculation begins.
Why 125 kW Three Phase Is an Important Commercial Generator Size
A 125 kW three-phase generator can provide substantial commercial standby capacity.
For example, Briggs & Stratton’s current 125 kW specifications show approximately 434 amps at 120/208V three phase, 376 amps at 120/240V three phase, and 188 amps at 277/480V three phase. The three-phase units are rated at a 0.8 power factor.
That illustrates why I don’t like sizing a generator by amperage alone.
Voltage matters. Phase matters. Power factor matters. The actual load matters.
The same 125 kW generator produces very different amperage depending upon its operating voltage.
For many 120/208V commercial properties around a 300 amp load, a 125 kW unit can provide useful capacity and potentially some operating headroom, but we still perform the load calculation and evaluate motor starting before selecting the generator.
The building tells us what it needs. The calculation tells us what generator belongs there.
NEC Article 220, Start With the Load
The NEC gives us requirements for calculating electrical loads under Article 220.
Before we decide what generator should serve a commercial building, we need to understand the site preparation, required permits, and loads that generator will actually be expected to carry.
This becomes particularly important with commercial properties because a building may have several large loads that don’t necessarily operate simultaneously.
If we’re designing a whole-building generator, that’s one calculation. If we’re designing a generator to serve selected standby loads, that’s another design decision. Either way, I don’t want to sell somebody a generator based on a guess.
NEC Article 445, Generators
NEC Article 445 is one of the primary NEC articles governing generators.
It addresses generator installation requirements including matters such as equipment location, conductors, overcurrent protection, disconnecting means, and generator marking.
For example, NEC 445.18 addresses generator disconnecting means and shutdown requirements. The exact requirements depend on the generator and installation.
Article 445 is one of the first places we’re going when we’re designing the electrical side of a commercial standby generator installation.
NEC Article 702, Optional Standby Systems
Many commercial generators fall under NEC Article 702, Optional Standby Systems, when the generator is being installed for convenience, business continuity, property protection, refrigeration, HVAC, or other loads that aren’t legally required emergency or legally required standby loads.
Article 702 addresses the installation and capacity of optional standby systems. That distinction matters.
A generator installed because a restaurant owner doesn’t want to lose refrigeration during an outage isn’t automatically classified the same way as a generator serving legally required life-safety equipment.
We determine what type of standby system we’re actually dealing with before designing it.
NEC Articles 700 & 701, When the Generator Serves Required Loads
Some commercial properties have loads that fall into different categories.
NEC Article 700 covers Emergency Systems, while NEC Article 701 covers Legally Required Standby Systems.
Those systems can have additional requirements for equipment, transfer, wiring, reliability, operation, and how quickly power must become available.
This is why I don’t like somebody simply ordering a large generator online and then calling an electrician afterward.
Before we order the equipment, we need to know: What is this generator actually being required to do?
Interestingly, Briggs & Stratton states that its current 125 kW platform may be used for certain NFPA 110 Level 2 applications with the appropriate accessories, but specifically states that this generator set is not authorized by Briggs & Stratton for NFPA 110 Level 1 applications.
That is exactly the kind of manufacturer’s limitation we need to know before equipment is purchased.
NEC Article 702.4, Generator Capacity Matters
For optional standby systems, generator capacity needs to be appropriate for the loads that are transferred onto it. This becomes especially important when automatic transfer equipment is involved.
If the utility fails and the automatic transfer switch transfers the entire building onto the generator, that generator needs to be capable of handling the energy demand we’re asking it to carry, or the system needs an approved load-management strategy.
That’s why load management can become extremely useful on larger commercial projects. We may not necessarily need every large electrical load operating simultaneously during an outage.
Automatic Transfer Switches
The automatic transfer switch, or ATS, is one of the most important components of the standby power system.
The transfer switch monitors the normal utility source.
When utility power fails, leading to power outages, the system signals the diesel generator to start. Once the generator reaches the appropriate operating conditions, the transfer equipment transfers the designated electrical loads from utility power to generator power.
When utility power returns and stabilizes, the transfer equipment returns the building to the normal source and the generator goes through its appropriate cooldown and shutdown sequence.
From the customer’s perspective, this can all happen automatically.
Behind that automatic operation, however, is a substantial amount of electrical engineering.
The transfer switch needs to match the voltage, phase, amperage, poles, system configuration, available fault current, and generator design.
Three Pole Versus Four Pole Transfer Switches
This is another area where grounding and bonding become extremely important. Depending on whether the generator is configured as a separately derived system, we may need to determine whether the neutral is switched at the transfer equipment.
A three-pole and four-pole transfer switch aren’t interchangeable simply because they’re both rated for the same amperage. Whether the neutral is switched affects how the generator neutral is treated and where the system is grounded and bonded. This brings NEC Article 250 into the design.
That’s why I want to understand the entire grounding and bonding arrangement before we order the transfer switch.
NEC Article 250, Generator Grounding & Bonding
Generators aren’t exempt from grounding and bonding requirements.
NEC Article 250 governs grounding and bonding, and generator installations can require careful consideration of whether the generator is a separately derived system.
The transfer switch configuration becomes an important part of that determination.
If the neutral is switched, the grounding and bonding arrangement can be different from a system where the neutral remains solidly connected through the transfer equipment.
This is not an area where we simply add another ground rod and call the generator grounded.
Grounding and bonding need to be designed as a complete system.
Motor Starting Is Different From Running Load
This is another reason generator sizing requires more than adding up amperages.
A motor may require substantially more current while starting than it requires after it is running. Commercial buildings can have large compressors, pumps, exhaust fans, refrigeration equipment, HVAC systems, elevators, and other motor loads.
The generator has to respond when those motors start. A generator might appear large enough when we’re only looking at the normal running amperage, but that doesn’t automatically mean it will handle every motor starting simultaneously without an unacceptable voltage dip.
This is why we evaluate motor starting kVA, starting sequence, voltage dip, and load management where appropriate. Sometimes the better design isn’t simply buying a much larger generator. Sometimes it’s controlling when the larger loads are allowed to start.
Load Shedding Can Make a Generator System Smarter
A properly designed commercial generator system can sometimes prioritize loads to optimize energy distribution.
Instead of allowing several large HVAC compressors or motors to start simultaneously when the transfer occurs, we can design the system so certain loads start in sequence or lower-priority loads remain disconnected until sufficient generator capacity is available.
This is called load management or load shedding.
That can help us design a more efficient standby system without unnecessarily oversizing the generator.
Again, the goal isn’t to sell the largest generator possible. The goal is to install the generator the building actually needs.
Briggs & Stratton Generator Placement & Setbacks
Generator location and site preparation are another major part of the project. A standby generator produces heat, exhaust, noise, and carbon monoxide, and it requires sufficient airflow for cooling and combustion.
Briggs & Stratton emphasizes that generator installations must comply with applicable codes, industry standards, regulations, and the specific installation manual for the model being installed. Briggs also points installers to NFPA 37 for stationary engine installation considerations.
Briggs’ general placement guidance states that standby generators should be kept away from windows, doors, ventilation intakes, and other building openings because exhaust gases contain carbon monoxide. Its published guidance discusses five feet from windows, doors, and wall openings, while noting that certain specifically fire-tested models can have different building clearances. It also gives general guidance of 36 inches at the front and ends and 48 inches overhead from structures, projections, and overhangs.
But there’s an important distinction for a commercial page like this: We do not take the residential clearance dimensions from one Briggs generator and automatically apply them to a 125 kW commercial generator.
The exact generator model’s installation manual, NFPA requirements, fuel-gas requirements, local building and fire requirements, property setbacks, utility requirements, necessary permits, and authority having jurisdiction determine the final location. That’s especially important when we’re talking about a generator that can weigh several thousand pounds.
A 125 kW Generator Is Serious Equipment
Briggs & Stratton’s current 125 kW LP commercial generator specifications list an assembled size of approximately 135 inches long, 52 inches wide, and 86 inches high, with an assembled weight around 5,380 pounds.
That’s more than a box we’re setting beside an electrical meter.
We need to think about the equipment pad, access, service clearances, exhaust direction, fuel piping, underground utilities, electrical conduits, transfer equipment, drainage, vehicle access, landscaping, property lines, building openings, and how a service technician will access the generator years from now.
One thing I’ve learned over the years is that where we put the generator can be just as important as which generator we buy.
NFPA 37, Stationary Engine Installation
NFPA 37 addresses installation and use of stationary combustion engines and gas turbines and is particularly relevant to generator location and fire safety. Briggs specifically references NFPA 37 in its generator placement guidance.
This is another reason generator placement needs to be determined during the planning stage instead of after the equipment arrives.
We have to consider the building, openings, combustible construction, exhaust, ventilation, fuel system, and the manufacturer’s installation requirements together.
NFPA 110, Emergency & Standby Power Systems
For applicable emergency and standby systems, NFPA 110 can also become part of the design.
NFPA 110 addresses energy, emergency, and standby power systems and classifies systems according to their application and performance requirements.
This is particularly important for facilities where loss of electrical power affects functions beyond ordinary business convenience.
As mentioned earlier, Briggs states that the current 125 kW platform may be used for certain NFPA 110 Level 2 applications when equipped and installed appropriately, but Briggs does not authorize that particular generator for NFPA 110 Level 1 applications.
That’s why the facility classification needs to be determined before selecting equipment.
NEC 110.26, Electrical Working Space
We also need to remember the electricians and generator technicians who will eventually maintain this system. NEC 110.26 addresses working space around electrical equipment.
The generator, transfer switch, disconnects, distribution equipment, and associated electrical components need to be located so required electrical working clearances can be maintained.
I don’t want to design something that looks great on a drawing but requires a technician to crawl between a generator and a wall ten years from now just to troubleshoot a transfer switch. We design for installation and future service.
Fuel Supply Can Determine Whether the Generator Actually Produces Rated Power
Another major part of a commercial generator installation is the diesel or other fuel system.
A 125 kW generator requires a substantial amount of fuel when operating under load.
The natural gas or LP system needs to be capable of supplying the generator under the required operating conditions.
That means looking at fuel type, available gas pressure, pipe size, distance, regulator requirements, meter capacity, other gas appliances, and generator fuel consumption.
This is why the gas contractor or utility often becomes another important part of the project.
A perfectly installed electrical generator that can’t get enough fuel isn’t going to deliver its rated output.
Florida Changes the Generator Conversation
Here in Pinellas County, we also have environmental conditions that I want to consider. We deal with extreme heat, humidity, thunderstorms, hurricanes, coastal salt air, flooding concerns, power outages, and extended utility outages.
Briggs & Stratton’s current commercial generators use corrosion-resistant enclosures designed to help withstand outdoor conditions.
But the enclosure doesn’t eliminate the need for good site planning, including thorough site preparation.
I want the generator positioned where it can operate safely, where water isn’t being directed toward it, where technicians can service it, where exhaust isn’t being pulled into the building, and where the surrounding environment isn’t working against us.
Surge Protection & Generator Systems
Commercial generators also contain sensitive electronic controls, and the building itself may contain thousands or hundreds of thousands of dollars in electronic equipment.
That’s why surge protection should be part of the conversation.
A standby generator doesn’t replace a properly designed surge protection system.
The utility source, generator source, transfer equipment, distribution system, and sensitive downstream equipment all need to be evaluated as part of the overall electrical protection strategy.
Here in Florida, where lightning and utility disturbances are common concerns, I believe in taking a layered approach to electrical protection.
Commercial Generator Maintenance Matters
A generator can sit quietly for months. Then a hurricane comes through Pinellas County, and suddenly that generator may be expected to operate for hours or days. That’s not the time to discover a dead starting battery, charger problem, fuel issue, transfer switch problem, control fault, coolant problem, or maintenance issue.
Commercial standby generators need regular exercising, inspection, and maintenance according to the manufacturer’s requirements. Briggs also offers remote monitoring capabilities on current commercial equipment, allowing generator status information and certain diagnostic capabilities to be accessed remotely.
A standby generator is only valuable if it’s ready when the utility fails.
NEC & NFPA References for Commercial Standby Generators
Depending on the particular installation and locally adopted code edition, the major standards and NEC provisions we evaluate can include NEC Article 110 for general electrical requirements and working clearances, NEC 110.26 for electrical working space, NEC Article 220 for load calculations, NEC Article 225 where applicable to outside feeders and branch circuits, NEC Article 230 where the installation interacts with service equipment, NEC Article 240 for overcurrent protection, NEC Article 250 for grounding and bonding, NEC Article 310 for conductors, NEC Article 445 for generators, NEC Article 700 for emergency systems, NEC Article 701 for legally required standby systems, and NEC Article 702 for optional standby systems.
Depending on the facility, we may also need to consider NFPA 37 for stationary combustion engines, NFPA 110 for emergency and standby power systems, UL 2200 for stationary engine generator assemblies, the generator manufacturer’s installation instructions, fuel-gas requirements, local building and fire codes, zoning requirements, necessary permits, and the requirements of the authority having jurisdiction.
Briggs & Stratton identifies its current 125 kW commercial units as UL 2200 listed.
The exact requirements depend upon what type of facility we’re protecting and what the generator is expected to power.
Commercial Standby Generator FAQs
Is a 125 kW generator large enough for a 300 amp commercial building?
It can be for many applications, but we need to know the voltage and actual calculated load. At 120/208V three-phase, Briggs & Stratton’s current 125 kW generator is rated approximately 434 amps. That does not automatically mean every 300-amp building should receive a 125 kW generator. We evaluate the actual loads, motor starting requirements, transfer strategy, and whether the entire building or only selected loads will be backed up.
Can Pinellas County Electric install generators larger than 125 kW?
Yes. 125 kW is one commercial size we work with, not our maximum capability. Larger buildings and facilities may require larger generators or a different standby power design. We size the system around the facility rather than trying to make the facility fit one generator.
How far does a commercial generator need to be from the building?
There isn’t one universal dimension that should be applied to every commercial generator. Generator placement depends on the specific manufacturer’s installation instructions, generator model, NFPA 37, building openings, exhaust, fire separation, fuel system, property conditions, local code requirements, and the authority having jurisdiction. Briggs publishes general placement guidance, but the model-specific installation manual controls the installation details.
Does a commercial standby generator automatically start during an outage?
When properly designed with automatic transfer equipment, yes. The system can detect loss of normal utility power, start the generator, verify generator power, transfer the designated loads, and eventually transfer back to utility power when the normal source returns.
Should I call Pinellas County Electric before buying the generator?
Absolutely. This is one of those projects where I want to be involved before equipment is ordered. We can evaluate the building voltage, phase, electrical load, service equipment, transfer switch requirements, generator location, fuel source, motor loads, conductor routes, grounding and bonding, and applicable code requirements before thousands of dollars of equipment arrives on site.
Service Areas
- Pinellas County Electric provides commercial standby generator installation
- Three-phase generator systems
- Automatic transfer switches
- Commercial generator electrical connections
- Commercial generator electrical connections
- Generator service upgrades
- and commercial backup power solutions throughout Clearwater
- Clearwater Beach
- St. Petersburg
- Largo
- Pinellas Park
- Safety Harbor
- Dunedin
- Palm Harbor
- Tarpon Springs
- Seminole
- Belleair
- Belleair Beach
- Indian Rocks Beach
- Indian Shores
- Madeira Beach
- Redington Beach
- Redington Shores
- North Redington Beach
- Treasure Island
- Gulfport
- Oldsmar
and surrounding communities throughout Pinellas County, Florida.
Planning a Commercial Standby Generator?
My recommendation is simple: Call us before you order the generator.
At Pinellas County Electric, The Panel Pros™, we want to see the building first.
We’ll look at the service voltage, phase, actual electrical load, motor loads, transfer equipment, grounding and bonding, available generator location, fuel requirements, conductor routes, distribution equipment, and what you actually need operating during an outage.
Then we can determine whether your project needs a 125 kW three-phase generator, something smaller, something larger, or a system with load management.
The goal isn’t to sell you the biggest generator we can.
The goal is to design a standby power system that starts, transfers, and carries the loads your business actually needs when the utility goes down.
Pinellas County Electric, The Panel Pros™ Commercial Standby Generators & Automatic Transfer Systems Clearwater & Pinellas County, Florida 727-269-1982
