Commercial Generator Installation FAQ
A commercial generator protects your business from power outages, data loss, and revenue disruption. Whether you need emergency backup for life-safety systems or standby power for your entire building, this FAQ covers the key questions about commercial generator installation, sizing, fuel types, and ongoing maintenance requirements.
For a full overview of our generator services, visit our commercial generator installation page.
Sizing and Selection
How do I determine what size generator my building needs?
Generator sizing starts with a load calculation that identifies all the equipment you need to power during an outage. This includes HVAC, lighting, elevators, IT systems, refrigeration, and any life-safety systems. We calculate the running watts and starting watts (motors need 2 to 3 times their running watts to start) and add a 20% to 25% safety margin. Undersizing causes overload failures, while oversizing wastes fuel and capital.
| Building Type | Typical Generator Size | Notes |
|---|---|---|
| Small office (under 5,000 sq ft) | 20 - 50 kW | Lights, HVAC, computers |
| Mid-size office / retail | 50 - 150 kW | Full building backup |
| Restaurant / food service | 75 - 200 kW | Refrigeration, cooking, HVAC |
| Warehouse / light industrial | 100 - 500 kW | Equipment, lighting, dock doors |
| Multi-story / large commercial | 250 - 2,000+ kW | Full building or priority loads |
What is the difference between natural gas, diesel, and propane generators?
Natural gas generators connect to the utility gas line and provide unlimited fuel supply, making them ideal for extended outages. Diesel generators deliver more power per gallon and are the standard for large commercial applications, but require on-site fuel storage and have emissions regulations. Propane generators offer long shelf life for stored fuel and lower emissions than diesel. Most commercial installations in the Seattle-Lynnwood area use natural gas due to reliable gas service and minimal storage requirements.
What is the difference between emergency, standby, and optional standby power?
NEC defines three categories. Emergency power (NEC Article 700) is legally required for life-safety systems like exit lighting, fire alarms, and smoke evacuation - it must start within 10 seconds. Standby power (Article 701) covers systems like elevators and ventilation that aren’t life-safety but are needed for safe building evacuation - it must start within 60 seconds. Optional standby (Article 702) covers everything else you choose to back up. Each category has different transfer switch, wiring, and testing requirements.
How much does a commercial generator cost?
Total installed cost ranges from $15,000 for a small natural gas unit to $200,000+ for large diesel systems. The generator unit itself accounts for roughly 40% to 60% of the total cost, with the balance going to the transfer switch, concrete pad, fuel connections, electrical connections, permitting, and startup. Natural gas generators typically cost 10% to 20% more than diesel for the same kW rating but have lower fuel and maintenance costs over time.
Installation and Permitting
What is an automatic transfer switch, and do I need one?
An automatic transfer switch (ATS) monitors utility power and automatically starts the generator and transfers the load when an outage is detected. When utility power returns, the ATS transfers back and shuts down the generator after a cool-down period. Every commercial generator installation requires a transfer switch - manual transfer is allowed only in limited circumstances. The ATS is a critical safety device that prevents backfeeding power into utility lines.
What permits are required for commercial generator installation?
Commercial generator installations require electrical permits (for the transfer switch and power connections), mechanical permits (for fuel connections and exhaust), and sometimes building permits (for the concrete pad and enclosure). In some jurisdictions, a noise variance may be required. Fire department review is needed for diesel fuel storage. We handle all permitting as part of the installation.
How long does a commercial generator installation take?
The typical timeline is 4 to 8 weeks from contract to commissioning. This includes equipment procurement (2 to 4 weeks lead time), site preparation (concrete pad, fuel line), electrical and mechanical installation (3 to 7 days), permitting and inspections, and startup/commissioning testing. Rush installations are possible but may incur premium charges. Utility gas service upgrades can extend the timeline.
Can a generator connect to my existing electrical panel?
The generator connects through a transfer switch, which is installed between the utility service and your main panel (or a dedicated sub-panel for priority loads). If your panel is at capacity, you may need a panel upgrade to accommodate the transfer switch. In some cases, a separate critical loads panel is installed next to the main panel, and only priority circuits are transferred to generator backup. See our panel upgrade FAQ for capacity questions.
What are the noise restrictions for commercial generators?
Most jurisdictions limit generator noise to 75 to 85 decibels at the property line during testing and 70 to 80 dB during emergency operation. Sound-attenuated enclosures reduce noise by 15 to 25 dB. Location matters - placing the generator away from property lines and neighboring buildings reduces noise impact. Some areas require a noise study before permitting. We factor noise requirements into every site plan.
Code Requirements
What NEC code requirements apply to commercial generators?
NEC Articles 700 (Emergency), 701 (Legally Required Standby), and 702 (Optional Standby) define the requirements. Key provisions include: transfer equipment must prevent paralleling with utility, emergency circuits must be separated from other wiring, automatic startup must meet specified time limits, and fuel supply must support a minimum run time (typically 2 hours for emergency systems). Local amendments may add additional requirements.
What are the fuel storage requirements?
Diesel generators require on-site fuel tanks, which are regulated by fire code, EPA, and local environmental rules. Above-ground tanks over 660 gallons require secondary containment (double-wall tank or containment berm). Indoor fuel storage is limited and requires fire-rated enclosures. Natural gas generators avoid fuel storage issues entirely since they connect to the utility gas line. Propane tanks have their own setback and placement requirements.
Where can the generator be installed - indoor vs outdoor?
Most commercial generators are installed outdoors on a concrete pad with a weather-protective enclosure. Indoor installation is possible in properly ventilated mechanical rooms with adequate exhaust routing, combustion air intake, and fire-rated separation. Indoor installations cost more due to ventilation requirements and structural considerations. Outdoor placement is simpler and less expensive but must meet setback requirements from property lines, air intakes, and operable windows.
Maintenance and Testing
What maintenance does a commercial generator require?
Commercial generators need regular maintenance including oil and filter changes (every 200 to 500 hours or annually), coolant checks, battery testing, belt and hose inspection, fuel system checks (diesel: fuel polishing and tank inspection), air filter replacement, and load bank testing. A comprehensive maintenance plan extends generator life to 20 to 30 years. Neglecting maintenance leads to startup failures when you need the generator most.
How often does a commercial generator need to be tested?
NFPA 110 requires weekly no-load testing (run for at least 30 minutes) and annual load testing at a minimum of 30% rated load for at least 30 minutes. Many authorities having jurisdiction (AHJs) require monthly testing with documentation. Emergency generators (NEC 700) have the strictest testing requirements. We offer maintenance contracts that include all required testing with documented records for your files. See our electrical maintenance page for service plans.
What is load bank testing, and why is it important?
Load bank testing applies an artificial load to the generator to verify it can actually deliver its rated capacity. Generators that only run at low loads during weekly testing can develop “wet stacking” (unburned fuel buildup in diesel exhaust systems), which degrades performance and reliability. Annual load bank testing at 75% to 100% of rated capacity confirms the generator will perform when you need it. This test is required by NFPA 110 for emergency and standby systems.
What is the expected runtime for a commercial generator?
Runtime depends on fuel type and tank size. Natural gas generators run indefinitely as long as gas service is available. Diesel generators run for 24 to 72+ hours on a full tank depending on load and tank capacity. Most commercial diesel generators are sized for 24 to 48 hours of runtime at 75% load. Extended outages may require fuel delivery coordination. We can design the fuel system for your specific runtime requirements.
Planning and ROI
How do I calculate the cost of downtime to justify a generator?
Calculate your hourly revenue, add the cost of employee downtime, spoiled inventory (if applicable), IT recovery costs, and reputational impact. A restaurant losing $500/hour in revenue plus $2,000 in spoiled food per outage can justify a generator with just a few outages. Data centers, medical facilities, and manufacturing operations typically have the highest downtime costs and the shortest payback periods for generator investments.
Can I add a generator to an older building?
Yes, generators can be installed on virtually any commercial building. Older buildings may need panel upgrades to accommodate the transfer switch, and fuel line installation may require additional work. The key constraint is usually available space for the outdoor generator pad and clearances from building openings. We assess the building’s electrical system, available space, and fuel options during the site survey.
Next Steps
Need backup power for your commercial building? Electrical Support Company provides free site assessments and detailed generator sizing proposals for businesses throughout the Seattle-Lynnwood area.
Call (425) 583-4869 or contact us online to discuss your generator needs.
Explore all of our commercial electrical services to see the full range of support we provide for commercial properties.