Get in touch with us
Leave a message
Backup Power Planning
Sizing the Backup Generator for Your Transformer
Here is where transformer sizing and generator sizing meet — and where most standalone calculators stop.
kW
Usable Power
The real electrical power your connected equipment consumes while operating — the same load that sizes your transformer.
kVA
Generator Capacity
The total apparent power the generator must provide, directly derived from your transformer’s kVA requirements and load factor.
Amps
System Current
The current at your selected voltage, which determines cable sizing, switchgear, and distribution — for both transformer and generator.
From Transformer to Generator – A Direct Path
A transformer steps voltage down to the load; it does not keep the load running when the utility fails. For any facility where downtime is costly, the same load list that sized your transformer is the starting point for your backup generator. The relationship is direct: total connected load, less demand factor, divided by power factor, plus margin — the exact figures you just computed.
Integrated Approach
Why Sizing Both Together Matters
01
One Load List, Two Applications
The same connected‑load calculation that sizes the transformer directly feeds the generator sizing process — no need to start over.
02
Coordinated Capacity
Generator kVA must match or exceed transformer kVA (after demand factor and power factor) to ensure full backup capability without overload.
03
Transient Performance
Motor starting surges that affect transformer sizing also affect generator response — both must handle the largest startup current.
04
From Size to Quote
Shandong Huali publishes both tools together. Size your transformer here, then move your load numbers into our kVA calculator to size the diesel generator set that protects it.
Transformer Sizing Tool
How to Calculate Transformer kVA
Transformer capacity is measured in kilovolt-amperes (kVA), not kilowatts, because a transformer must supply apparent power — the total of real power (kW) and reactive power (kVAR). The formula depends on your supply type.
Enter Voltage and Current
For single‑phase: kVA = V × A ÷ 1000
For three‑phase: kVA = V × A × √3 ÷ 1000
Used only for kW display
Enter Load in kW and Power Factor
Convert your load from kilowatts to kVA using: kVA = kW ÷ PF
Optionally enter voltage to also compute current.
Decimal value (e.g. 0.85)
Leave 0 to skip current calculation
Transformer Sizing Guide
Transformer Sizing: What Size Transformer Do You Need?
Knowing the formula is not the same as knowing the size. Real facilities rarely run every load at once, which is why correct sizing follows a four‑step process rather than a single multiplication.
STEP 01
List Every Connected Load
Motors, lighting, HVAC, UPS, and any planned future equipment — record each load in kW.
STEP 02
Apply a Demand or Diversity Factor
Not everything runs simultaneously. Use 0.4–0.6 for residential, 0.6–0.8 for commercial, 0.7–0.9 for industrial, and 0.9–1.0 for data centres.
STEP 03
Convert Demand to kVA
Divide the demand by the expected power factor. Typical values are 0.85 for commercial loads and 0.90–0.95 for corrected industrial or data‑centre systems.
STEP 04
Add a Growth Margin & Select Standard Size
Add 15–30% for future expansion, then round up to the next standard kVA rating. Aim to operate the transformer at 60–80% of its rating for the best balance of efficiency and headroom.
Worked Example
A building has 3,000 kW of connected load with an industrial demand factor of 0.7, giving 2,100 kW of maximum demand. At a 0.85 power factor, that is 2,471 kVA. Add a 20% growth margin and you need roughly 2,965 kVA — so you specify a 3,150 kVA unit, or split the load across two 1,600 kVA transformers for N+1 redundancy.
Transformer Protection & Derating
Transformer Derating & Overcurrent Protection (NEC 450.3(B))
Ambient temperature and altitude reduce transformer capacity, while overcurrent protection must be sized according to the NEC rules to safely clear faults. These two factors are often overlooked — and both can change the final specification.
Derating: Temperature & Altitude
A transformer rated at standard conditions loses capacity in a hot or high‑altitude environment. Derate approximately 1% per degree Celsius above 40°C ambient, and 3% per 1000 metres above 1000 m of elevation.
A 500 kVA unit at 50°C and 2000 m elevation, for instance, effectively becomes a smaller unit — factor this into the transformer sizing calculator before you commit to a rating.
Overcurrent Protection – NEC 450.3(B)
Overcurrent protection prevents a transformer from carrying a damaging fault current. Under NEC Table 450.3(B), primary‑only protection for a transformer rated 9 amps or more is limited to 125% of the primary full‑load current, with the next standard size up permitted. Ratings between 2 and 9 amps allow 167%, and under 2 amps allow 300%.
When both primary and secondary protection are used, the primary limit rises to 250% while the secondary stays at 125% (or 167% below 9 amps).
| Primary Current Rating | Primary‑Only Protection | Primary (with Secondary) | Secondary Protection |
|---|---|---|---|
| ≥ 9 A | 125% | 250% | 125% |
| 2 A – 9 A | 167% | 250% | 167% |
| < 2 A | 300% | 250% | 167% |
Based on NEC 450.3(B) – next standard size permitted.
About the Manufacturer
Why Choose Shandong Huali for Your Power System
A transformer is only one link in a power system. The generator behind it is the link where Shandong Huali manufactures everything in‑house.
01
One‑Stop Power‑Systems Manufacturer
We produce diesel generator sets from 8 kVA to 4,000 kVA across silent, open, trailer, container, and high‑voltage configurations — alongside dust‑suppression and industrial automation equipment.
02
Factory‑Direct Pricing
Buying direct removes intermediaries, so you pay for engineering and materials rather than markup.
03
OEM/ODM Customization
Our team of 80+ engineers designs to your specification — voltage, frequency, enclosure, control system, and engine choice — so the generator integrates with the rest of your electrical design.
04
Certified, Tested Quality
Every unit is 100% load‑tested in our national‑standard testing centre before delivery, under ISO9001, CE, CCC, and ISO14001 systems. With 25+ years of manufacturing experience and exports to 20+ countries, we stand behind every generator we ship.
05
Proven Component Partners
We integrate Cummins, Perkins, Weichai, and Yuchai engines with Stamford alternators, matched to your duty cycle and budget.
Let's talk
Get a Factory-Direct Generator Quote
Tell us your required kVA, application, and location, and a power-systems engineer will review your load and respond with a tailored specification — no obligation, no hard sell.
Frequently Ask Questions
Q: How do I calculate transformer kVA?
A: For single-phase, use kVA = (Volts × Amps) ÷ 1000. For three-phase, use kVA = (Volts × Amps × √3) ÷ 1000. If you know load in kW, divide by the power factor: kVA = kW ÷ PF.
Q: What size transformer do I need?
A: List all connected loads, apply a demand factor (0.4–1.0 depending on sector), convert kW to kVA by dividing by power factor, add a 15–30% growth margin, and select the next standard kVA rating. This transformer sizing calculator automates the steps.
Q: How many amps does a transformer draw?
A: For three-phase, Amps = (kVA × 1000) ÷ (Volts × √3). A 100 kVA transformer at 415 V draws about 139 amps. For single-phase, drop the √3 term.
Q: What are the standard transformer kVA sizes?
A: Common three-phase ratings are 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA, with larger IEC 60076 sizes at 1250, 1600, 2000, 2500, and 3150 kVA.
Q: What is transformer power factor?
A: Power factor is the ratio of real power (kW) to apparent power (kVA). It is determined by the connected load, not the transformer. Typical values are 0.85 for commercial loads and 0.90–0.95 for corrected industrial and data-centre systems.
Q: Do I need a generator behind my transformer?
A: If downtime is costly — a data centre, hospital, factory, or mine — yes. A transformer steps voltage down but cannot supply power during an outage. Size the generator from the same load list using our kVA calculator.