Calculator Inputs
Example Data Table
| Rating | Phase | Voltage | Power Factor | Load | Approximate FLA |
|---|---|---|---|---|---|
| 50 kVA | Three phase | 480 V | 0.80 | 100% | 60.14 A |
| 100 kW | Three phase | 400 V | 0.80 | 100% | 180.42 A |
| 25 kVA | Single phase | 240 V | 0.90 | 80% | 83.33 A |
Formula Used
When generator rating is in kVA:
Adjusted kVA = Generator kVA × Load percentage ÷ 100
When generator rating is in kW:
Base kVA = kW ÷ Power factor
Single phase full load amps:
Amps = kVA × 1000 ÷ Voltage
Three phase full load amps:
Amps = kVA × 1000 ÷ (1.732 × Voltage)
Derated capacity:
Available kVA = Base kVA × (1 − Total derating percentage ÷ 100)
Reserve amps:
Reserve amps = Full load amps × (1 + Reserve percentage ÷ 100)
How To Use This Calculator
- Enter the generator nameplate rating.
- Select whether the rating is listed in kVA or kW.
- Choose single phase or three phase service.
- Enter the line voltage used on the construction site.
- Add the expected power factor and load percentage.
- Enter derating values for site conditions.
- Add a reserve margin for safer planning.
- Press the calculate button and review the result above the form.
- Use the CSV or PDF button to save the calculation.
Generator Amp Planning
A generator full load amps estimate helps crews match machines, conductors, breakers, and temporary panels. It turns rated power into current. That current changes with phase, voltage, load percentage, and power factor. A three phase unit usually carries less current per line than a single phase unit at the same kVA.
Why Full Load Amps Matters
Construction sites rarely run one simple tool. They may power welders, pumps, lifts, lighting, heaters, compressors, and office trailers. Each load adds heat to cables and stress to protective devices. A low estimate can cause nuisance trips, overheated feeders, and poor voltage performance. A high estimate can oversize gear and raise cost. Balanced planning keeps the temporary system safer and easier to inspect.
Practical Generator Sizing
Start with the generator rating from the nameplate. Choose kVA when it is available. Choose kW when only real power is listed. Then enter the expected power factor. Motors, transformers, and welders often reduce power factor. Resistive heaters stay near one. Voltage must match the site distribution. A small voltage error can create a large amp error.
Derating and Reserve
Sites also need room for conditions. Altitude, heat, dust, age, and long runtime can reduce useful capacity. This calculator lets you apply a derating percentage before reviewing available kVA. A reserve margin adds planning space above the calculated load. Reserve is useful when motors start, loads cycle, or future circuits are likely.
Using The Result
The calculated full load amps are an estimate for planning. Compare them with manufacturer data, local code, breaker curves, and cable ampacity tables. For motor-heavy sites, check starting current separately. For parallel generators, divide current by the number of equal units only when load sharing is confirmed. Keep a written record. Exports help supervisors, electricians, and inspectors discuss the same numbers.
Record Assumptions
Write down the selected phase, voltage, power factor, load level, and derating values. These assumptions explain the result later. They also help when a rented generator is changed onsite. Clear notes reduce confusion between estimators, field crews, and maintenance teams before final cable sizes are approved onsite.
Good electrical planning is conservative, documented, and field checked. Use this tool early. Then confirm details before energizing the system.
FAQs
What is generator full load amps?
Generator full load amps are the line current expected when the generator supplies its selected rated load at the entered voltage, phase, and power factor.
Should I use kW or kVA?
Use kVA when the generator nameplate lists it. Use kW when kVA is unavailable. The calculator converts kW to kVA by using the power factor value.
Why does phase change the amp result?
Three phase power spreads current across three conductors. For the same kVA and voltage, three phase line current is lower than single phase current.
What power factor should I enter?
Use the value from equipment data when available. Many generator planning examples use 0.80. Resistive loads can be closer to 1.00.
What does derating mean?
Derating reduces usable generator capacity for site conditions. Heat, altitude, dust, age, and continuous operation can reduce practical output.
Why add a reserve margin?
A reserve margin gives extra room for load changes, cycling equipment, voltage drop, and future circuits. It supports safer construction planning.
Can this size generator breakers?
This tool supports estimating. Breaker sizing must also follow equipment data, cable ampacity, local rules, fault current, and professional judgment.
Does motor starting current matter?
Yes. Motor starting current can be several times running current. Enter the largest motor and multiplier to estimate added starting demand.