A generator must be able to carry the load that will run after transfer and handle the highest load imposed while motors or transformers are energized. Adding connected nameplate loads alone does not answer the sizing question, because a motor’s starting current and a transformer’s inrush can be much higher than their normal operating current.
Start by separating the installation into three conditions:
- Running load: The electrical demand after equipment is operating normally.
- Starting load: The temporary demand while a motor starts or a transformer energizes.
- Transfer load: The load that the transfer equipment actually connects to the generator.
The generator rating must suit the governing condition, not merely the total of every connected circuit.
Start With the Actual Transfer Load
Connected load is the total of equipment connected to a panel, feeder, or system. It is not automatically the load that will be transferred to generator power.
Transfer load is narrower: it is the equipment served by the selected emergency, standby, or backup circuits when the transfer equipment operates. A residence may have a large service or panel rating while only selected loads are connected to a generator transfer switch. The same separation applies to commercial and industrial systems with a dedicated emergency panel or generator-backed distribution section.
Use the Generator Transfer Load Calculator to organize the loads intended to transfer. For dwelling applications, the Residential Generator Load Calculator can help identify the expected residential loads before evaluating motor starting.
Do not substitute a panel’s main-breaker rating, service rating, or total installed breaker ratings for the actual load expected to run on the generator. A panel rating describes equipment capability; it does not establish the operating demand. For the distinction between panel load and service capacity, see panel load vs. service size.
Separate Running Load From Starting Load
Running load is the demand after motors, compressors, pumps, fans, and other equipment have reached normal operating condition. It may be expressed as watts, kilowatts, amps, or kVA, depending on the available information and the calculator or equipment data being used.
Starting load is transient. Motor starting current can be several times normal full-load current, and transformer energization can produce a magnetizing inrush much higher than normal operating current.
That temporary event can govern generator selection even where the normal transferred load is comfortably below the generator’s continuous rating.
| Load condition | What it represents | Why it affects generator selection |
|---|---|---|
| Connected load | Equipment electrically connected to the system | Defines the possible load inventory, but may include loads that never transfer or never run together |
| Running load | Load operating after normal acceleration and stabilization | Establishes the sustained demand the generator must carry |
| Starting load | Temporary demand during motor starting or transformer energization | Can produce the highest short-duration demand and affect generator voltage response |
| Transfer load | Load connected by the transfer equipment during generator operation | Defines the real backup load that must be evaluated |
The starting condition depends on the specific motor, driven equipment, starting method, and the loads already operating. Nameplate full-load current is useful input data, but it is not a substitute for motor-starting information when that information is available.
For motor terminology and the relationship between full-load current and starting current, see Motor Current, Starting Current, and FLC.
Build the Generator Load Picture
A practical sizing review begins with a load schedule rather than a single connected-load total. Record the information needed to show both steady operation and the highest anticipated starting event.
Include:
- Equipment description and whether it transfers to generator power.
- Voltage and phase.
- Running electrical data, such as watts, kW, amps, or kVA.
- Motor full-load current and starting information where applicable.
- Starting method, if known.
- Whether the load is already running when another motor starts.
- Whether multiple loads can start at the same time.
- Transformer ratings and any available energization or inrush information.
- The proposed generator’s voltage, phase, frequency, and rating basis.
The core planning sequence is straightforward:
1. Identify the loads that will actually transfer.
2. Establish the maximum normal running load expected at one time.
- Identify the motor or transformer energization event that creates the highest temporary demand.
4. Evaluate that event with the other loads expected to remain online.
- Compare the resulting operating conditions with the generator manufacturer’s ratings and application guidance.
The Generator Size Calculator is the appropriate starting point when connected running load and starting load need to be considered together. The Motor Starting Current Calculator can help develop the motor-starting input rather than treating every motor as a simple running-watt load.
Motor Starting Can Set the Generator Size
A generator may have enough capacity to operate the final running load but still struggle when a large motor starts. During that event, the generator must supply the motor’s starting demand while also supporting loads already connected to the transfer system.
For example, consider a transferred load that includes lighting, receptacle loads, and a pump. Once the pump is running, the total load may be modest. But if the pump starts after the other loads are already energized, the generator sees the existing running load plus the pump’s higher starting demand. That starting condition—not the stabilized running total—may control the selection.
Avoid assuming that only the physically largest motor matters. The governing motor is the one with the highest starting demand under the actual operating sequence. A smaller motor can become the critical event if it starts while more load is online, starts across the line, or has different starting characteristics than other equipment.
Where a starting method is part of the system design, use the manufacturer’s data for the motor and starter. Eaton notes that soft-starter application information must account for the generator and the motor’s starting requirements rather than relying solely on normal motor current.
Coordinate Transformers and Generators
Transformers require a separate handoff in the review. Transformer kVA and normal load current establish the ongoing load relationship, while transformer energization can introduce a short-duration inrush condition that differs from normal operating current.
First confirm the transformer’s voltage, phase, and kVA relationship to the generator system. The transformer kVA-to-amps conversion guide is useful when translating transformer ratings into current values for the connected system.
Then determine whether the transformer will be energized by the generator during transfer, restarted after the generator is already loaded, or remain energized through the sequence. Those conditions can produce different generator loading events. Do not treat transformer inrush as equivalent to motor starting current or assume a generic multiplier where manufacturer or engineering data are available.
Inputs That Commonly Get Misapplied
Generator sizing errors often come from mixing ratings and units that describe different conditions.
- Combining single-phase and three-phase values without preserving the correct voltage and phase basis.
- Treating amps as interchangeable with kW or kVA without the supporting electrical data.
- Adding every connected circuit even when only selected circuits transfer.
- Using normal motor current as the motor-starting value.
- Ignoring other loads that remain online when the largest motor starts.
- Assuming all loads start one at a time without confirming the transfer and control sequence.
- Treating a transformer’s normal kVA rating as its energization demand.
- Confusing a generator’s continuous rating with its capability during a transient starting event.
- Using panel, service, breaker, conductor, or transfer-switch ratings as if they prove generator capacity.
Temperature, conductor count, conductor ampacity, branch-circuit protection, motor overload protection, and disconnecting means remain installation and equipment-coordination issues. They should not be inferred from a generator load calculation.
Use Calculator Results as a Planning Input
A generator sizing connected starting loads calculator is useful for organizing the load schedule and identifying the condition most likely to govern. It does not by itself establish that the generator, transfer equipment, conductors, overcurrent protection, fuel supply, grounding arrangement, or installation is suitable.
Final selection and installation require review of the actual generator data, motor and transformer characteristics, transfer equipment, manufacturer instructions, adopted electrical requirements, local permitting, and the authority having jurisdiction.