A correct commercial generator spec rests on two numbers: running kW and starting kVA. Motor inrush and, where applicable, NFPA fire pump rules often push the required size well above a simple sum of running loads. Before requesting quotes, build a full load inventory and confirm the starting method for every motor and the status of any life-safety equipment.
TL;DR:
- Motor starting inrush can be six to eight times the full-load amps, significantly impacting the required starting kVA beyond the steady-state load.
- Fire pumps may need generator sizes of 125 to 150 percent of motor horsepower when starting across the line, requiring careful voltage dip studies.
- Site factors like elevation and ambient temperature necessitate derating the generator’s rated capacity, especially above 1,000 feet or at high temperatures.
- Accurate load inventory, including motor data and site conditions, is critical; skipping this step risks selecting an undersized generator.
- Contractor verification, acceptance testing, and proper documentation are essential to ensure compliance, safety, and the reliability of the generator system.
Table of Contents
- Quick sizing checklist: required data and ordered steps
- How to calculate running load and convert amps to kW and kVA
- Motor starting and inrush: estimating starting kVA
- Fire pump and life-safety loads: NFPA rules and sizing exceptions
- Derating and site factors: altitude, temperature, fuel, and accessories
- Using sizing tools and calculators to verify your numbers
- Acceptance testing and maintenance under NFPA 110
- Worked example: from load inventory to a recommended rating
- Why contractor verification reduces procurement and compliance risk
- How Guardian Electrical Contractors helps with generator connections and commissioning
- FAQ
- Sources
Quick sizing checklist: required data and ordered steps
Before any calculation makes sense, you need accurate inputs. Skipping this step is the most common reason a generator arrives undersized.
- Inventory every circuit by voltage, phase, amperage, and duty (continuous or intermittent).
- Flag which loads are critical and which could be shed temporarily during generator operation.
- Pull motor nameplate data: full-load amps (FLA), service factor, rated horsepower, and starting method (across-the-line, soft-start, or VFD).
- Document site conditions: altitude, typical ambient temperature, fuel type, and whether you are working from a prime or standby nameplate rating.
- Plan how acceptance testing will happen: who supplies the load bank and what parameters get logged.
Skipping any one of these steps means your sizing calculation starts from a guess rather than a measurement.
How to calculate running load and convert amps to kW and kVA
Running load math is straightforward once you have real amperage readings instead of assumptions. For single-phase circuits, kW = (V × A × PF) / 1,000. For three-phase circuits, kW = (√3 × V × A × PF) / 1,000.
Power factor (PF) determines how much of that apparent power actually does work. To convert kW to kVA, divide by PF: kVA = kW / PF. If you do not have a measured PF, using 0.8 to 0.9 is common practice, but understand the risk: guessing low inflates your generator size unnecessarily, and guessing high can leave you short on starting capacity.
A few worked conversions make this concrete:
- A 120V single-phase lighting panel drawing 40A at PF 0.95 computes to (120 × 40 × 0.95) / 1,000, or 4.56 kW.
- A 480V three-phase motor circuit drawing 60A at PF 0.85 computes to (1.732 × 480 × 60 × 0.85) / 1,000, or about 42.4 kW.
- A UPS load rated at 20 kVA with a 0.9 PF contributes 18 kW of running load to your total.
Motor starting current commonly runs 6 to 8 times full-load amps during startup, which is why running kW alone never tells the full sizing story, according to industrial motor load guidance. The most frequent mistakes at this stage are ignoring PF entirely, counting a motor's inrush twice (once in the running sum and again in the starting calculation), and mixing standby and prime nameplate figures within the same worksheet.
Motor starting and inrush: estimating starting kVA
Running kW tells you what the generator must sustain. Starting kVA tells you what it must survive for a few seconds without collapsing voltage or tripping protection.
Induction motors typically draw 6 to 8 times their full-load amps on startup, and that locked-rotor current translates directly into a starting kVA spike that a steady-state sum will never capture, according to the same industrial motor load analysis. Run the transient analysis separately from your running-load sum:
- Identify the single largest motor start, since that event usually drives your worst-case kVA requirement.
- Add any motors likely to start simultaneously with it, since concurrent starts stack their inrush.
- Compare that total against the generator's published starting kVA curve, not just its running kW rating.
- Check whether soft starters, VFDs, or reduced-voltage starters are already specified, since each lowers the required starting kVA substantially.
Soft starters and VFDs reduce inrush by ramping current rather than applying it instantly, but the exact reduction depends on the manufacturer's data sheet for that specific drive and can be complemented by a mobile PV-BESS power station to manage load sharing and supplemental power during startup.
Pro Tip: Stagger motor starts in your transfer switch logic whenever two or more large motors could start at once, and the generator you need gets noticeably smaller.
Fire pump and life-safety loads: NFPA rules and sizing exceptions
Fire pumps change the sizing math because the consequences of undersizing are regulatory, not just operational.
- NFPA 20 permits up to a 15% voltage dip for electric starting of fire pumps, but mechanical starting is treated as an exception to that rule, according to NFPA 20 fire pump sizing guidance.
- Across-the-line (DOL) starts on fire pump motors commonly require generator sizing at roughly 125% to 150% of motor horsepower, per the same source.
- As an example, a 75 HP fire pump motor starting across the line could reasonably require a generator sized near 94 to 113 HP equivalent capacity to stay within that range.
- When a single generator serves both the fire pump and general building loads, the shared-load approach needs a voltage-dip study, ideally run in sizing software or verified with the manufacturer.
- Fire pump service requires listed equipment, and every sizing assumption should be documented for the authority having jurisdiction before installation begins.
Derating and site factors: altitude, temperature, fuel, and accessories
A generator's nameplate rating assumes ideal conditions that your site probably does not have. Altitude and ambient temperature both reduce available power, and the correction can become material above 1,000 feet of elevation or at high ambient temperatures, per NFPA 110's derating guidance.
- Consult the manufacturer's derate tables for your specific engine and altitude, since generic percentages vary by model.
- Natural gas installations often need a higher kW rating than diesel for the same mechanical output, so confirm fuel-specific data before finalizing a spec.
- Add accessory loads such as battery chargers and cooling systems to your running total, plus a modest reserve margin for future growth.
- Keep prime and standby ratings separate when calculating the 30% monthly exercise target, since mixing them produces an inaccurate test baseline.
Using sizing tools and calculators to verify your numbers
Vendor sizing calculators can speed up the process, but they are only as good as what you feed them. Expect to provide a detailed load list, motor data including starting mode, site corrections for altitude and temperature, and your desired runtime and redundancy requirements, as described in Caterpillar's genset sizing resources.
- Confirm the running kW output matches your manual sum of loads within a reasonable margin.
- Check the tool's starting-kVA result against your worst-case motor start and any NFPA constraints.
- Treat a calculator result as preliminary whenever the facility includes fire pumps, hospitals, or other life-safety equipment.
- Call a manufacturer or contractor for a voltage-dip study before finalizing any life-safety spec.
Online calculators are a useful first pass, but they are not a substitute for AHJ or manufacturer verification on systems where a mistake has safety consequences.
Acceptance testing and maintenance under NFPA 110
Sizing is only half the job. NFPA 110 requires monthly exercise of at least 30 minutes, and diesel units need at least 30% of nameplate kW during that run to avoid wet stacking, according to the NFPA 110 standard. A practical maintenance routine looks like this:
- Run weekly visual inspections of the unit, enclosure, and fuel system.
- Conduct the monthly operational test, including ATS exercise, and log the results.
- Schedule an annual load bank test if monthly runs consistently fall short of 30% nameplate loading.
- Plan for a triennial 4-hour test on Level 1 systems, and retain records for 36 months, per Electrical Trader's NFPA 110 compliance checklist.
Fuel quality and battery condition checks reduce no-start risk more than any other single maintenance habit.
Worked example: from load inventory to a recommended rating
Take a small mixed-use facility: a lighting panel at 4.56 kW, an HVAC compressor motor rated 42.4 kW running with 225A locked-rotor current, and a UPS load of 18 kW.
- Sum the running loads: 4.56 + 42.4 + 18 = 64.96 kW, call it 65 kW running.
- Estimate starting kVA for the compressor motor at 7 times FLA, producing a transient spike well above its running draw, consistent with typical motor inrush multipliers.
- Apply a modest derate for a site at 2,000 feet elevation and warm ambient temperatures, per manufacturer tables.
- Select a standby unit rated to cover both the 65 kW running figure and the motor's starting kVA with margin, then confirm against the vendor's starting curve.
Hand the contractor your load list, nameplate photos, and a one-line diagram for final verification.
Why contractor verification reduces procurement and compliance risk
Sizing math gets you close, but a contractor can confirm starting modes in the field, run acceptance tests, and assemble the documentation an AHJ expects, often faster than an internal team without dedicated electrical staff. Load-bank logistics and ongoing recordkeeping are easier to manage with a crew that does this work regularly. Generator connections benefit from detailed consultations, often built around photos clients share of their panels and equipment, paired with a community-based, professional approach to the work.
— Logan
How Guardian Electrical Contractors helps with generator connections and commissioning
Once your load list and motor data are in hand, the next step is getting the electrical connection done right. A qualified electrical contractor can handle the work between a sizing calculation and a working, code-compliant installation for homes and businesses in the relevant area.
That includes a site load audit, the generator electrical connection itself, automatic transfer switch (ATS) integration, panel upgrades where existing distribution needs more capacity, and ongoing acceptance testing or maintenance arrangements.
- Bring a load list with voltage, phase, and amperage for each circuit.
- Bring photos of motor and panel nameplates for accurate data capture.
- Bring any single-line or distribution drawings you already have.
- Note your fuel type and tank or supply details on site.
| What you provide | What Guardian verifies |
|---|---|
| Load list with amps and phase | Running kW totals match field readings |
| Motor nameplate photos | Starting method and inrush expectations |
| Site fuel and panel details | Panel capacity for the new connection |
Guardian's panel upgrade services often come into play when existing distribution needs more room before a generator can be added. For the generator work itself, visit the generator installation page to schedule an on-site consultation.
FAQ
What is the 20/20 rule for generators?
There is no single, widely recognized "20/20 rule" documented in generator sizing standards. If you have seen the term used informally, it is worth asking the source to clarify whether they mean a load percentage, a runtime figure, or something specific to their product line before applying it to your project.
What is the 80% rule for generators?
This typically refers to a conservative practice of sizing a generator so that expected running load stays at or below roughly 80% of its rated capacity, leaving headroom for motor starting and future additions. It is a common rule of thumb in the field rather than a fixed NFPA requirement, so always confirm specifics against your actual load calculation and manufacturer guidance.
How big of a generator do I need to run a 2,000 sq ft house?
Generator size for a home depends on which circuits you want powered, not square footage alone, since a furnace blower motor or well pump can demand far more starting capacity than lighting circuits. A full load inventory, including motor starting data for any pumps or HVAC compressors, is the only reliable way to size a unit for a specific house.
How long will a 5,000 W generator run on 5 gallons of gas?
Runtime on a given fuel quantity depends on the generator's specific fuel consumption rate at that load level, which varies by model and manufacturer. Check the specific unit's fuel consumption chart at the load percentage you plan to run, since published runtime figures assume a particular load and tank size.
Sources
- NFPA 110 authoritative statements (2021/2025 edits)
- NFPA 110 compliance checklist 2026 — Electrical Trader
- Generator sizing for industrial motor loads — Industrial Monitor Direct