Generator sizing is the number-one source of specification error on commercial standby installations. Contractors who size on connected kW instead of running kW plus starting kVA either specify a generator too large (wasting $20,000-$100,000) or too small (which stalls at motor start and drops the entire load).

The three questions:

1. What is the running load? Continuous load in kW after all motor-start transients have settled. From measured demand data or NEC 220 calculation.
2. What is the largest single motor? Motor inrush current, expressed as kVA (locked-rotor code letter from NEMA MG 1 or the motor nameplate).
3. What is the acceptable voltage sag? Standby generators allow 20-30% voltage sag during motor start. If sensitive loads (VFDs, computers) require < 15% sag, the generator must be oversized 30-50%.

Use the load calculator to combine running load and starting kVA. Manufacturer sizing software (Generac PowerDesign, Kohler Genset Sizer, Cummins GenSize) is more accurate than rules of thumb.

The automatic transfer switch (ATS) is often more scrutinized than the generator itself. Three-position ATS (Normal, Emergency, Off), open transition or closed transition (paralleled), rated at the generator output kVA. Selection factors:

• Transition type — Open-transition (100 ms outage during transfer) fine for most commercial. Closed-transition (paralleled with utility for 100 ms, then utility source disconnects) required for data centers and mission-critical.
• Withstand rating — UL 1008 (2016) requires ATS withstand rating ≥ available fault current at the ATS location. Typically 22-42 kAIC for commercial, 65-100 kAIC for industrial.
• Bypass isolation — Larger ATS (typically ≥ 600 A) offer bypass-isolation feature so the ATS can be serviced without interrupting load. Adds ~15-25% to ATS cost but pays back in maintenance downtime.
• Service entrance rated — ATS can serve as the service entrance disconnect (per NEC 230). Adds ~20% cost but eliminates a separate service disconnect.

1. 1

   Step 1 — Site survey and load study

   Measure existing load (30 days recorded data preferred), identify largest motor, plan ATS location within 25 ft of service disconnect (NEC 230.72(A)).

2. 2

   Step 2 — Generator sizing calculation

   Manufacturer software: input all motors with locked-rotor codes, all resistive loads, and voltage sag tolerance. Result is genset kW rating.

3. 3

   Step 3 — Fuel selection

   Natural gas (cheapest, unlimited runtime, no on-site storage), LP (2 hr on-site tank standard, quick deploy), diesel (highest kW density, 72-hr runtime standard on 500 gal tank).

4. 4

   Step 4 — Pad and structural

   Concrete pad sized per genset weight (typically 6" thick, 12" wider than skid). Level within 1" over the pad. Structural review for > 100 kW gensets.

5. 5

   Step 5 — Fuel line sizing

   Natural gas: 5 psi max at meter, pipe sized per genset manufacturer flow at max load. LP: 11-14" WC at generator inlet. Diesel: 5/8" supply and 3/8" return lines standard.

6. 6

   Step 6 — Exhaust routing

   Manufacturer-supplied silencer or aftermarket critical-grade for noise-sensitive locations. 10 ft minimum clearance from air intakes and 3 ft from combustible surfaces.

7. 7

   Step 7 — Genset electrical connections

   AC output cables sized per NEC 445.13 (115% of nameplate). Neutral configuration: 3-pole ATS with neutral bond in generator (separately derived), or 4-pole ATS switching neutral (unbonded generator).

8. 8

   Step 8 — ATS installation and wiring

   Mount within 25 ft of service disconnect. Wire utility-side, load-side, and generator-side feeders. Program timers: engine start delay, transfer delay, cool-down delay, retransfer delay.

9. 9

   Step 9 — Commissioning and load bank test

   Full-load test with resistive load bank for 30 min per NFPA 110 Level 1 systems. Verify voltage regulation, frequency stability, load acceptance response.

10. 10

    Step 10 — Documentation and turnover

    Deliver O&M manuals, factory test reports, load bank certification, ATS programming sheet, and warranty registration to owner. Schedule 30-day and 6-month follow-up.

Fuel selection drives long-term operational cost and generator specification. The three practical fuels for commercial standby:

• Natural gas — Cheapest per hour of runtime (~$0.10-$0.15 per kWh at $8/MMBtu). Unlimited runtime (as long as gas main pressure holds). Best for utility outages driven by grid events. Weakness: gas mains can lose pressure in extended outages (hurricane, ice storm), so backup fuel plan recommended for critical loads.
• LP (propane) — Middle cost (~$0.20-$0.35 per kWh). Standard 500-1000 gal tank gives 24-72 hrs runtime. Best for residential and small commercial where natural gas is not available.
• Diesel — Highest fuel cost ($0.30-$0.50 per kWh) but highest kW density. Standard 500 gal sub-base tank gives 72 hrs at full load; larger day tanks and bulk storage for longer runtime. Best for hospitals, data centers, and any load requiring > 24 hr on-site fuel storage.

NFPA 110 Level 1 systems require 96-hr fuel supply for Class X, 4-hr for Class 4, decreasing to none for Class 0. Verify class in owner's specification before quoting.

Diesel gensets > 50 hp fall under EPA Tier 4 emissions requirements (Tier 4 Final for new installations since 2015). Louisville-metro installations also need Kentucky Division for Air Quality permits for gensets over specific thresholds — typically 100 kW+ for stationary compression-ignition (CI) engines running > 100 hrs/year.

Emergency-only gensets (limited to 100 hrs/year non-emergency operation) get streamlined permitting. Prime-power gensets require full permitting. Coordinate with the owner's environmental consultant before ordering.

Diesel gensets typically come with a sub-base fuel tank sized for 24-72 hrs runtime at full load. Tank options and code:

• UL 142 double-wall sub-base — Steel double-wall with interstitial monitoring. Standard on 100-2000 kW gensets.
• UL 2085 protected aboveground — Insulated and fire-rated tank; required in some jurisdictions for above-ground > 60 gal. Not common on rear-yard commercial installations.
• Day tanks — Small day tank (55-500 gal) at the genset with bulk storage remote. Fuel-transfer pump moves fuel between. Standard for large gensets and long-runtime installations.

Kentucky UST (underground storage tank) regulations (401 KAR 42) apply if fuel storage is buried. Louisville Metro fire marshal permit required for tanks > 660 gal aboveground. Coordinate with owner's fire and environmental consultants early in bid.

Every diesel genset needs to dissipate ~40-60% of fuel input energy as coolant heat. Cooling options:

• Belt-driven radiator (standard) — Integrated with the genset skid. Requires 2-4 ft clearance in front of the radiator for airflow. Ambient max typically 40°C for full-power output.
• Remote radiator — Radiator relocated 20-100 ft away with piping between genset and radiator. Required when genset is indoors with limited air-flow, or when radiator noise/heat cannot be near sensitive equipment. Adds $5,000-$25,000 to installation.
• City-water cooling — Heat exchanger cooled by potable or process water. Rarely used; typically only in specialized installations where radiator is not feasible.

Verify site air-flow calculations: the manufacturer publishes minimum air-flow (CFM) and maximum ambient temperature. Louisville summer highs of 95°F+ at rooftop are common; specify a genset rated for 50°C ambient or accept derated output at peak summer.

Diesel exhaust must be routed clear of building air intakes and combustible surfaces. Requirements:

• Silencer — Industrial-grade (10-15 dBA reduction), residential-grade (18-25 dBA), or critical-grade (30-35 dBA). Cost scales roughly linearly with dBA reduction.
• Exhaust routing — 10 ft minimum from any air intake; 3 ft from combustible siding; vertical stack recommended (heat rise carries away from building).
• Rain cap and thimble — Rain cap on the terminal end; roof or wall thimble sized 2" larger than exhaust OD for clearance and heat dissipation.
• Insulation — Ceramic-fiber blanket over indoor exhaust runs; keeps room temperature down and reduces burn hazard.
• Condensate drip leg — Low point in the exhaust run with a drain valve, particularly on long horizontal runs.

NFPA 110 Level 1 systems (hospitals, life safety) require weekly no-load exercise (5 min), monthly load exercise (30 min at ≥ 30% rating), and annual load-bank test at 100% rating for 4 hrs (or at 30% rating with augmented load per 8.4.2).

Level 2 systems (non-critical standby) require monthly exercise only; annual load bank recommended but not mandatory.

Contractor deliverables at turnover:

1. O&M manuals (engine, alternator, controller)
2. Commissioning report with load-bank data
3. Factory test certificate
4. ATS programming sheet
5. Fuel-system schematics
6. Exhaust routing single-line
7. Warranty registrations
8. Recommended maintenance schedule
9. Local service-provider list (Generac, Kohler, Cummins dealer contacts)

Portlandia sells and drop-ships gensets from Generac, Kohler, Cummins, and Caterpillar. Load-bank testing service coordinated through PES service partners.

Fuel choice is often decided by utility infrastructure and owner preference more than by engineering. Key considerations:

Natural gas is preferred for buildings with existing gas service and where extended outages are unlikely. Advantages: unlimited runtime (as long as gas main pressure holds), no on-site fuel storage, cleanest emissions. Disadvantages: susceptible to gas-main pressure loss during widespread outages; lower kW density (typically 10-15% derated from diesel equivalent).

Diesel is standard for hospitals, data centers, and critical facilities requiring 72+ hr on-site fuel. Advantages: highest kW density, self-contained fuel supply. Disadvantages: monthly fuel exercise required (diesel deteriorates in storage); EPA Tier 4 emissions equipment required (expensive on larger units); air-quality permitting required.

Propane / LP is standard for residential and small commercial without gas service. Advantages: self-contained (500-1000 gal tank is standard), long shelf life (no deterioration). Disadvantages: highest fuel cost per kWh; tank installation adds $3,000-$8,000 to project.

Dual-fuel gensets (natural gas primary, propane backup, or propane primary with tank-swap capability) are available from all major OEMs for locations where fuel-diversity matters.

Louisville Metro noise ordinance limits sound to 60 dBA at property line during nighttime hours (10 PM-7 AM). Generators typically produce 80-95 dBA at 1 m without silencing. Mitigation options:

• Critical-grade silencer — 30-35 dBA reduction. Manufacturer option or aftermarket (Universal Silencer, Nelson Global).
• Level-4 enclosure — Weatherproof enclosure with sound-dampening insulation. Rated to 65 dBA at 7 m typical. Standard on new commercial gensets.
• Level-1 sound-attenuated enclosure — Rated to 60 dBA at 23 ft. Premium option for noise-sensitive locations (hospitals, schools, hotels).
• Site orientation — Face genset radiator toward building; genset acoustic footprint is asymmetric.
• Berms and walls — Sound barriers between genset and property line. Effective 5-15 dBA reduction depending on height and mass.

Verify site sound levels post-installation. Use SLM (sound level meter) at property line during full-load exercise. Louisville Metro Board of Health can require post-installation mitigation if complaints exceed the ordinance.

Data centers, hospitals, and mission-critical facilities require redundancy (N+1 or 2N configuration). Multiple gensets paralleled through a paralleling switchgear cabinet. Key features:

• Paralleling switchgear — Automatic synchronization (voltage, frequency, phase) before parallel breaker closes. Load-share on kW and kvar. Auto-start / auto-stop based on load demand.
• Common bus — All paralleled gensets connect to a common paralleling bus, then feed a distribution switchboard or MCC.
• Automatic load shed — On loss of one genset, non-essential loads shed automatically to keep essential loads powered.
• Utility parallel — Base-load or peak-shave operation where gensets run parallel with utility. Requires utility interconnection agreement, protective relays (25, 27, 32, 47, 59, 81), and reverse-power protection.

Portlandia works with ASCO, Cummins, and Kohler paralleling switchgear designs for > 1 MW installations.

NFPA 110 (Standard for Emergency and Standby Power Systems) governs testing and maintenance of Level 1 and Level 2 systems. Testing requirements:

• Level 1 initial acceptance — 4-hour continuous full-load test. Verify voltage regulation < ±3%, frequency regulation < ±0.5%.
• Level 1 monthly exercise — 30 min at ≥ 30% of nameplate rating. Automatic transfer switch cycled.
• Level 1 annual load bank — 4-hour test at rated load. Sequential load levels: 25%, 50%, 75%, 100%, each held 30 min. Actual building load can be augmented with resistive load bank.
• Level 2 monthly exercise — 30 min minimum. Load requirement less stringent.
• Level 2 annual load bank — recommended but not mandatory.

Documentation retention: test records for the life of the equipment. Facility manager keeps hard copy in the emergency generator room; contractor keeps electronic copy for warranty support.

Modern gensets integrate with the building automation system (BAS) via Modbus TCP, BACnet/IP, or Ethernet/IP. Data points typically exchanged:

• Genset status (running / stopped / fault)
• Fuel level (diesel or LP)
• Engine oil pressure, coolant temperature
• Alternator voltage, frequency, phase currents
• ATS position (utility / genset)
• Run hours, start count
• Fault codes and alarms

Integration allows the facility manager to monitor genset health from the BAS front-end, receive alerts on faults, and correlate genset operation with utility outages. Cost: $500-$2,000 for the communication module and $2,000-$8,000 for BAS integration engineering.

Every commercial genset turnover should include hands-on training for the owner's facility staff:

• Monthly exercise procedure — how to initiate, what to observe, what to record.
• Fuel level check and refill procedure.
• ATS manual operation — how to transfer to genset manually, and back.
• Emergency shutdown — where the e-stop is, when to use it.
• Alarm response — what each fault code means, who to call.
• Service log — where to record hours, alarms, and service events.

Deliver: quick-reference laminated card, phone-numbers of OEM service and authorized dealer, PES service manager direct line, and a 30-day follow-up visit at no cost to verify the owner is comfortable operating the system.