A generator can produce correct voltage and still leave the installation unsafe or unable to clear a fault. Neutral bonding, earthing, cable routes and breaker settings must match the operating modes and transfer arrangement.
Engineering decisions covered
Electrical designers, contractors and owners preparing generator connection and acceptance.
Cable size and breaker rating are not independent choices; both depend on load, route, fault level, neutral treatment and operating mode.
A documented protection scheme that works in utility, generator and transfer conditions.
Cable size and breaker rating are not independent choices; both depend on load, route, fault level, neutral treatment and operating mode.
Decision 01
What changes when the system transfers to generator power?
The neutral may remain solidly connected or be switched, depending on system design and local rules. That choice affects bonding, earth-fault paths and protection behavior.
Show every pole of the transfer device and every neutral-to-earth connection on the single line.
- Identify switched or solid neutral
- Locate the system bonding point
- Avoid multiple unintended bonds
- Review earth-fault behavior in both modes
Decision 02
How should generator cables be sized?
Ampacity is only one limit. Route length, grouping, ambient temperature, harmonics, starting current and acceptable voltage drop can require a larger conductor.
Prepare cable length, installation method, grouping and load-current data.
- Check continuous current and derating
- Calculate steady voltage drop
- Check motor-starting voltage dip
- Confirm neutral and protective conductor size
Decision 03
Can a utility breaker setting be reused on generator supply?
Generator short-circuit current is often lower and decays differently from utility fault current. A setting that clears quickly on utility may fail to discriminate or clear correctly on generator power.
Review time-current coordination using generator-specific fault data.
- Obtain alternator fault-current data
- Check breaker trip curves
- Coordinate downstream critical circuits
- Verify earth-fault detection
Decision 04
What tests belong before energization?
Continuity, insulation, phase sequence, polarity, protection settings and transfer logic should be verified and documented. Functional testing must include the real operating modes.
Use a signed test sheet tied to the approved single-line revision.
- Measure protective-conductor continuity
- Record insulation-test results
- Confirm phase sequence and rotation
- Test trips and transfer logic safely
Grounding, insulation testing, protection settings and energization involve lethal hazards. They must be completed by qualified electrical personnel under an approved isolation procedure.
Decision record
Separate Site Evidence from Engineering Confirmation
Use traceable site data and a named engineering check to support each decision; specialist calculations remain with the qualified design or service team.
| Decision Point | Prepare on Site | Confirm in Design or Service |
|---|---|---|
| What changes when the system transfers to generator power? | Identify switched or solid neutralLocate the system bonding point | Avoid multiple unintended bondsReview earth-fault behavior in both modes |
| How should generator cables be sized? | Check continuous current and deratingCalculate steady voltage drop | Check motor-starting voltage dipConfirm neutral and protective conductor size |
| Can a utility breaker setting be reused on generator supply? | Obtain alternator fault-current dataCheck breaker trip curves | Coordinate downstream critical circuitsVerify earth-fault detection |
| What tests belong before energization? | Measure protective-conductor continuityRecord insulation-test results | Confirm phase sequence and rotationTest trips and transfer logic safely |
Project record
The Review Should Leave a Usable Project Record
Record the verified condition, engineering conclusion, responsible party and retest or acceptance result for each topic.
What changes when the system transfers to generator power?
Show every pole of the transfer device and every neutral-to-earth connection on the single line.
How should generator cables be sized?
Prepare cable length, installation method, grouping and load-current data.
Can a utility breaker setting be reused on generator supply?
Review time-current coordination using generator-specific fault data.
What tests belong before energization?
Use a signed test sheet tied to the approved single-line revision.