Why standby and emergency power exist
Utility supply is never guaranteed. Building codes, insurers and operational risk all drive the need for on-site backup generation, but not every backed-up load is backed up for the same reason.
Standby power vs emergency (life safety) power
Standby power
Power provided to maintain business continuity and operational convenience when utility supply is lost. It protects revenue, data, comfort and process continuity rather than life safety. Typical loads: IT and server rooms, general small power, non-critical HVAC, retail and process operations, standby lifts.
Whether standby power is required at all, and for how long, is usually a business decision captured in the project brief rather than a life-safety code mandate, though some jurisdictions also treat certain standby categories as "legally required" for specific building types.
Emergency / life safety power
Power required by code to protect occupants and support firefighting operations during an outage. These loads are code-mandated and must-run: fire pumps, emergency and egress lighting, fire alarm and voice evacuation systems, smoke control and stair pressurization fans, and (in many jurisdictions) firefighter-service elevator operation.
Emergency/life-safety circuits are typically wired on dedicated emergency distribution boards, kept electrically separate from normal and standby distribution, and are the first loads guaranteed to run on any generator.
Verify before design: the exact classification of a given load as emergency, legally-required standby, or optional standby, and the minimum run time each class requires, depends on the adopted code edition (commonly referenced frameworks include NFPA 110 generator classifications and NFPA 70/99 style life-safety branch concepts) and the local fire authority having jurisdiction. Always confirm against the current project specification and code edition in force, not on general training material.
How loads are prioritized on a generator load schedule
A typical building generator does not power the entire normal switchboard. Loads are grouped and prioritized so that if generator capacity is limited (or a fault reduces available capacity), the most critical loads are guaranteed to run and lower-priority loads are shed automatically:
- Priority 1 - Life safety / emergency loads - fire pumps, emergency lighting, fire alarm, smoke control. Never shed. Sized and wired first.
- Priority 2 - Legally required standby loads - loads a specific code or authority requires to run, but which are not strictly "life safety" (e.g. some ventilation, communication or process systems depending on building type and jurisdiction).
- Priority 3 - Optional / business-continuity standby loads - IT, general power, comfort HVAC, non-essential process loads. First to be shed by load-shedding relays or priority controllers if generator capacity is tight.
This prioritization is exactly why the Generator Sizing Calculator tab treats the running load total and the worst-case motor starting event separately: a generator must comfortably carry the full priority load it is assigned, plus survive the transient of starting its largest motor, without dropping voltage or frequency low enough to trip protection or stall other running loads.
Where this tool fits
Use the tabs on the left to move through system architecture, sizing, transfer schemes, paralleling and synchronizing, fuel and runtime planning, the automatic mains failure (AMF) control sequence, load bank testing, commissioning, troubleshooting and a scored self-assessment. Each calculator is illustrative and for training only - see the disclaimers in each tab.
System architectures
Four arrangements cover most building-services standby/emergency power projects, from a simple manually-connected set up to fully paralleled plants backing a UPS-protected critical load.
1. Standby generator only (manual connection)Simplest arrangement - no automatic transfer
The generator is connected through a manual transfer switch (MTS) or a manual interlocked changeover, requiring an operator to start the set and select it as the source. Used for small, non-critical facilities, temporary power, or where the cost and complexity of automatic transfer is not justified. Not suitable for code-mandated life-safety loads, which require automatic transfer within a short, code-defined time.
2. Generator + ATS (automatic transfer switch)The standard permanent arrangement for standby/emergency loads
The ATS continuously monitors the utility supply. On loss or unacceptable sag, it signals the generator to start; once the generator reaches rated voltage and frequency, the ATS transfers the load automatically. When utility returns and is stable, the ATS re-transfers back and the generator runs a cool-down period before shutdown. This is the default arrangement for essentially all permanent emergency and standby installations.
3. Multiple generators in parallelUsed for large loads, redundancy (N+1) or modular expansion
Where a single generator cannot economically cover the load, redundancy is required, or the plant needs to grow in modules, two or more generators synchronize onto a common generator bus through paralleling switchgear and share the load between them (see the Paralleling tab). The combined bus then feeds the ATS as if it were one large source.
4. Generator + UPS for critical loadsUsed when even the ATS/generator start gap cannot be tolerated
Some loads (data centers, critical medical or building-control systems) cannot tolerate the several-second to ~10-30 second gap between utility loss and the generator reaching rated voltage/frequency and transferring. A UPS sits downstream of the ATS output specifically for these critical loads, bridging the gap from batteries while the generator starts and stabilizes; the generator then sustains the UPS (recharging its batteries) and any non-critical load for the duration of the outage.
Generator sizing calculator
Build a load list, and the calculator separately checks (a) the continuous running demand and (b) the worst-case single starting event, since a generator that easily carries the running load can still stall or dip badly if it cannot ride through the largest motor starting against an already-running base load.
Load list
Enter each load's real power (kW) and power factor. If power factor is left blank, 0.8 is assumed. For motor loads, pick the starting method - this sets the starting current multiplier used to estimate the starting kVA surge (DOL ≈7×, Star-Delta ≈2.5×, Soft starter ≈3×, VFD ≈1.1×, matching this site's other starting-method tools). Non-motor loads should stay on "Non-motor / constant".
| Load name | kW | PF | Starting method | Running kVA | Starting kVA |
|---|
Sizing formulas
Verify before selection: this is a training-level approximation using a single dominant-motor starting assumption. Final generator selection must use the manufacturer's genset performance data (transient voltage-dip and recovery curves at the applicable step load), a full sequential-starting study when more than one large motor can start close together, altitude/temperature derating, and the project's actual load schedule and diversity - not this simplified tool.
ATS & transfer schemes
How the transfer switch actually moves the load between utility and generator differs mainly in whether, and how, the two sources are briefly allowed to overlap.
Open transition (break-before-make)The default, lowest-cost scheme
The switch breaks the connection to the outgoing source completely before making the connection to the incoming source. There is a brief, momentary interruption of supply (typically well under a second on retransfer, and the full engine-start time on initial failure). This is acceptable for the vast majority of standby and emergency loads, which tolerate a short break. It is the simplest and most common scheme and does not require the two sources to ever be synchronized.
Closed transition (make-before-break)Momentary parallel for a "no-break" transfer
Both sources are briefly closed together (typically well under 100 ms) so the load never sees an interruption. This requires the incoming and outgoing sources to be synchronized (voltage, frequency and phase) at the instant of the brief overlap, plus protection and interlocking to guarantee the parallel operation cannot be sustained if something goes wrong. Used where even a momentary loss is unacceptable, and commonly used for periodic no-break testing of the generator against live utility.
Soft-loading / soft transferA closed-transition variant that ramps the load between sources
A more advanced closed-transition scheme where, during the brief overlap, the load current is ramped smoothly from the outgoing source to the incoming source (rather than an instantaneous switch), reducing mechanical and electrical shock on the transfer. This is common when transferring back to utility after a test, or in installations sensitive to step changes in load or torque.
Choosing a scheme
| Scheme | Break in supply | Needs synchronizing? | Typical use |
|---|---|---|---|
| Open transition | Yes, brief | No | Most standby/emergency loads - default choice |
| Closed transition | No (momentary parallel) | Yes | Loads intolerant of any interruption; no-break test transfers |
| Soft-loading transfer | No (ramped parallel) | Yes | Sensitive mechanical/process loads; smooth no-break testing |
Confirm the required transition type against the project specification - some life-safety codes and utility interconnection rules restrict or specifically require particular transfer schemes and paralleling durations with the utility.
Paralleling & load sharing
When two or more generators run on a common bus, their governors must share load sensibly, and any breaker that joins a generator to a live bus must only close once the incoming machine matches the bus.
Droop vs isochronous governor control
Droop governor
Frequency is allowed to fall slightly as load increases (a small, deliberate "droop", commonly a few percent from no-load to full-load). Because every paralleled set droops the same way, they naturally share load in proportion to their rating without any communication link between them - simple and robust. The trade-off is that overall bus frequency sags a little as total load rises.
Isochronous governor
The governor holds frequency constant regardless of load. A single isochronous set can hold tight frequency on its own. When multiple sets must all run isochronous together, they need a load-sharing communication line between the governors so they agree on how to split load while every set still holds near-nominal frequency; without that coordination, isochronous sets fight each other for load.
Why synchronizing matters before closing a paralleling breaker
Closing a breaker to join a generator onto a live bus (or onto the utility, for closed transition) while out of step causes a large inrush and circulating current between the two sources, a sudden mechanical shock/torque spike on the shaft and coupling, and can trip protection or damage the alternator and prime mover. Before closing, the incoming machine must match the bus in three respects:
- Voltage magnitude - equal (or within a small tolerance) on both sides of the breaker.
- Frequency - equal, or the incoming machine's frequency deliberately set very slightly higher so it "slips in" gently and picks up a small amount of load on closing.
- Phase angle / sequence - the incoming voltage waveform must be in phase with the bus, and phase rotation (sequence) must match; sequence is normally checked and fixed once at commissioning, while voltage, frequency and phase angle are checked every time before closing.
Synchroscope (illustrative)Adjust the frequency difference and try closing the breaker only when in sync
This is a simplified training visual, not a precision simulator. The needle rotates clockwise when the incoming generator runs faster than the bus, and counter-clockwise when slower. Pointing straight up (12 o'clock) with near-zero rotation speed means voltage, frequency and phase are matched - the safe window to close.
Fuel systems & runtime
How fuel is stored and delivered, and a calculator to estimate achievable runtime against a required minimum.
Day tank vs bulk tank
Day tank
A small tank mounted at or near the generator, feeding the engine directly and automatically refilled from the bulk tank by transfer pumps with level switches. It acts as a buffer and containment point close to the set, and is typically sized only for a short duration (often on the order of an hour), not as the project's primary fuel reserve.
Bulk tank
The main fuel reserve (above-ground or underground/sub-base), sized to meet the project's required runtime at the expected operating load. Fitted with high-high/low-low level alarms, leak detection/containment as required, and feeds the day tank via duty/standby transfer pumps.
Fuel consumption varies with load
A generator does not consume fuel in direct proportion to kW alone at very light loads; consumption as a fraction of the full-load rate typically follows a curve like this (indicative training figures only - always use the manufacturer's fuel curve for the actual model):
| Load (%) | 0 | 25 | 50 | 75 | 100 |
|---|---|---|---|---|---|
| Fuel consumption (% of full-load rate) | ~10 | ~30 | ~55 | ~80 | 100 |
Runtime calculator
Verify before design: the actual required minimum runtime for a given generator/load classification is a project-specific and code-specific figure (commonly referenced against NFPA 110-style generator classification hours or the local fire authority's requirement) and must be confirmed against the current project specification, not assumed from this training tool. Consumption-vs-load figures above are indicative; use the manufacturer's certified fuel curve for the actual engine/alternator combination and altitude/temperature conditions.
Control & monitoring
Step through the automatic mains failure (AMF) panel's sequence of operation, and see the monitoring points typically brought back to the BMS.
AMF sequence stepper
Click Start to begin from Normal, then Next Step to walk through the full failure-to-restore cycle. The three source nodes below reflect utility, generator and ATS/load state at each step.
Press Start to begin the sequence.
Typical BMS / monitoring points
- Run status - generator running/stopped
- Common fault / alarm - summary trip
- Fuel level - day tank and bulk tank
- Battery voltage / charger fail
- In-generator-mode - ATS position / generator supplying load
- Low oil pressure / high coolant temperature
- Overspeed trip
- Load % / kW output
- Engine running hours
Load bank testing
Why gensets are periodically tested against an artificial load, and the two basic load bank types.
Why load bank test
Many standby generators spend most of their life either idle or lightly loaded during brief tests. Running an engine for long periods at light load can prevent it from ever reaching full operating temperature, leading to incomplete fuel combustion and a build-up of unburned fuel and carbon deposits in the exhaust system, commonly called wet-stacking. Wet-stacking reduces engine efficiency, increases the risk of exhaust fires, and masks whether the set can actually deliver its rated output when it is genuinely needed.
Load bank testing exercises the generator at a realistic or rated load using a temporary, controllable artificial load, independent of whatever the building's actual demand happens to be that day. It is used both for commissioning acceptance testing (proving the set meets its rated performance) and for periodic maintenance testing.
Resistive vs reactive load banks
Resistive load bank
Purely resistive elements (large heater grids), loading the generator at unity power factor. Tests real power (kW) delivery capability and is the simplest, most common load bank type.
Reactive load bank
Combines resistive elements with inductive (and sometimes capacitive) elements to load the generator at a realistic lagging power factor, e.g. around 0.8, closer to how real building loads actually behave. This is a more representative test of the alternator's voltage regulation and reactive capability than a unity-PF resistive test alone.
Commissioning & testing checklist
Tick items off as they are completed and verified on site. This is a training-level checklist, not a substitute for the project's approved commissioning procedure.
Troubleshooting reference
Common symptoms, likely causes and first checks. Always follow the manufacturer's fault-finding procedure and lock-out/tag-out rules before working on a genset or its control panel.
| Symptom | Likely causes | First checks |
|---|---|---|
| Fails to start / no crank | Flat or faulty starting battery; charger fault; blown control fuse; emergency stop engaged; low fuel in day tank; starter motor/solenoid fault | Check battery voltage and charger status; check E-stop and control fuses; confirm fuel level and control panel fault log |
| Cranks but fails to reach rated speed / voltage | Fuel supply restricted or air in fuel lines; governor fault; AVR fault; excitation fault; cold-start/glow issue | Check fuel supply and bleed air; review governor/AVR fault codes; check excitation and sensing wiring |
| ATS fails to transfer | ATS control fuse or control circuit fault; generator voltage/frequency window not satisfied; interlock or mechanical jam; incorrect time-delay settings | Verify generator output is within the ATS transfer window; check ATS control power and interlocks; review programmed time delays |
| Nuisance shutdowns during running | Low oil pressure or high coolant temperature trip; loose sensor wiring; overspeed/underspeed trip; fuel starvation under load | Review the event/fault log for the specific trip; check sensor wiring and calibration; check fuel supply under load |
| Wet-stacking (black/wet exhaust, poor performance) | Prolonged light-load or no-load running; undersized load relative to engine; infrequent load bank testing | Schedule a load bank test at or near rated load; review typical operating load history; check exhaust and turbo condition |
This table lists common, widely-taught symptoms only. Always diagnose against the specific manufacturer's fault codes and troubleshooting manual for the installed generator and control panel.
Assessment
15 questions covering the whole tool. Select an answer for each question, then press Check Answers.