How to Choose the Best Power Generators for Your Needs

Selection Dimension	Recommended Evaluation	Transfer-Switch Requirement	Grounding and Bonding Check	NEC 702 Standby-Power Focus	Verification Record
Essential Load Coverage	List only the loads that must operate during an outage, such as life-safety support equipment, refrigeration, communications, pumps, security systems, and selected lighting.	Use a transfer switch that supplies the designated standby load and is rated for the system voltage, phase, ampere rating, and available fault current.	Confirm that equipment grounding conductors remain continuous to the transferred loads and that bonding follows the system design.	Optional standby systems should serve loads selected by the owner or designer and installed in accordance with the applicable NEC edition and local authority requirements.	Signed load schedule identifying connected load, demand load, and designated standby circuits.
Generator Capacity	Size the generator from a calculated load, not from the sum of nameplate ratings alone. Include continuous loads, demand factors, motor starting current, and future expansion.	The switch rating must not be less than the calculated feeder or service load it controls. Confirm compatibility with the generator’s short-circuit and switching characteristics.	Verify that the generator frame, enclosure, transfer equipment, and connected equipment grounding conductors are bonded as required by the approved design.	NEC 702 requires the system to have adequate capacity for the intended optional standby loads and appropriate overcurrent protection.	Generator rating in kW or kVA, voltage, phase, frequency, power factor, and calculated peak demand.
Motor and Compressor Starting	Identify the largest motor-starting event. Pumps, compressors, air-conditioning equipment, and elevator motors may require substantially more starting power than their running ratings.	Consider staged loading, delayed-start controls, or priority load management when several motors may start at the same time.	Check that control circuits and metallic raceways are grounded and bonded, particularly where motor starters or variable-frequency drives are used.	The standby system must perform as intended under the connected load; verify operation through testing rather than relying only on the nameplate rating.	Starting-current calculation, voltage-dip review, load-sequencing plan, and witnessed functional test.
Transfer Switching Method	Choose open-transition switching for a break-before-make transfer in most standard applications. Use closed-transition equipment only when the design, utility rules, and equipment ratings permit momentary paralleling.	Transfer equipment must prevent an unintended connection between the normal source and the standby source. Confirm automatic or manual operation, transition type, time delay, and bypass features.	Determine whether the transfer switch switches the neutral. This affects whether the generator is treated as a separately derived system and where the system bonding jumper is installed.	NEC 702.6 addresses transfer equipment and requires equipment suitable for the intended use and arranged to prevent inadvertent interconnection of sources.	Transfer-switch wiring diagram, source ratings, neutral configuration, interlocking method, and commissioning results.
Grounding Electrode System	Inspect the grounding electrode system available at the generator location, including grounding electrode conductor routing, termination, protection, and accessibility.	Coordinate the transfer-switch neutral arrangement with the grounding design. Avoid creating parallel neutral-to-ground paths.	A separately derived generator system generally requires a system bonding jumper and connection to the grounding electrode system. A non-separately derived system generally relies on the premises grounding electrode system through the equipment grounding conductor.	Apply NEC grounding and bonding provisions, including the rules applicable to separately derived systems, generator installations, and the adopted local code.	Grounding diagram, conductor sizes, bonding-point location, continuity test, and inspection approval.
Fuel and Operating Duration	Compare fuel availability, storage limitations, ambient conditions, refueling access, and the expected outage duration. Actual runtime varies with load and fuel-tank size.	Provide controls for automatic start, transfer, retransfer, cooldown, and fault shutdown. Ensure the control wiring is protected and clearly identified.	Bond fuel-system components and metallic piping only where required by the applicable electrical and fuel-system codes; coordinate with the mechanical design.	NEC 702 focuses on optional standby electrical installation requirements; fuel storage, emissions, fire separation, and location requirements may also be governed by other codes and the local authority.	Fuel type, tank capacity, estimated runtime at selected loads, ventilation plan, and maintenance schedule.
Voltage, Phase, and Frequency	Match the generator output to the connected equipment, such as single-phase or three-phase loads, line-to-line voltage, line-to-neutral voltage, and operating frequency.	Confirm that the transfer switch is rated for the same voltage system, phase configuration, frequency, neutral arrangement, and service conditions.	Verify neutral identification, equipment grounding conductor termination, enclosure bonding, and phase rotation where applicable.	Install and label the system according to the adopted NEC requirements, equipment instructions, and inspection authority requirements.	One-line diagram, voltage and phase readings, phase-rotation test, frequency test, and load-bank test results.
Code, Labels, and Inspection	Confirm the design uses the NEC edition adopted by the jurisdiction and satisfies applicable building, fire, fuel, environmental, and utility requirements.	Label the normal and standby sources, identify the supplied loads, provide operating instructions where required, and ensure the switch is accessible for inspection and service.	Document grounding and bonding details before energization. Correct any neutral-to-ground connections that create objectionable current paths.	Review NEC Article 702, especially provisions concerning capacity, transfer equipment, installation, and optional standby system identification. The authority having jurisdiction has final approval.	Approved drawings, permits, inspection reports, equipment manuals, test certificates, and maintenance log.