Transformer
Sizing Guide.
Work out the kVA your load needs, single-phase or three-phase, then round up to a standard size. Use the calculator for a quick estimate, then check the factors that can push you up a size.
kVA
Calculator.
Enter the load in amps or kW and the system voltage. The calculator shows every step, adds your margin, and rounds up to the next standard size. Switch to “Amps from kVA” to get full-load current for a given rating.
Enter the load to see the recommended size and each step of the math.
1ΦkVA = V × A / 10003ΦkVA = √3 × V × A / 1000kWkVA = kW / PFEstimate onlyThis calculator gives a starting point, not a final design. Final sizing should account for motor starting, harmonics (K-factor), ambient temperature and altitude, and code and utility requirements. Confirm with BPS or your engineer before you order.
The
Formulas.
The same math the calculator uses. Transformers are rated in kVA (apparent power), so everything starts from volts and amps.
kVA = V × A / 1000A = kVA × 1000 / VOn a 120/240 V service, use 240 V and the current on each leg (L1, L2). If the legs differ, use the higher one.
kVA = √3 × VLL × A / 1000A = kVA × 1000 / (√3 × VLL)√3 ≈ 1.732. Use line-to-line voltage (480 on a 480Y/277 system, not 277) and the current per phase.
kVA = kW / PFIf you only know real power, divide by power factor (PF). A lower PF means more kVA for the same kW. If PF is unknown, 0.8 is a conservative assumption.
kVAdesign = kVA × (1 + margin)Add margin first (for 20%, multiply by 1.20), then round up to the next standard size. Never round down.
| Load | Load kVA | Size, no margin | Size, 20% margin |
|---|---|---|---|
| 480 V three-phase, 100 A | √3 × 480 × 100 / 1000 → 83.1 kVA | 112.5 kVA | 83.1 × 1.20 → 99.8 → 112.5 kVA |
| 120/240 V single-phase, 200 A | 240 × 200 / 1000 → 48.0 kVA | 50 kVA | 48.0 × 1.20 → 57.6 → 75 kVA |
| 90 kW at 0.9 PF, 480 V three-phase | 90 / 0.9 → 100.0 kVA | 112.5 kVA | 100.0 × 1.20 → 120.0 → 150 kVA |
Standard
kVA Ratings.
Distribution transformers come in standard kVA ratings. Size your design load up to the next one on the list.
- 10
- 15
- 25
- 37.5
- 50
- 75
- 100
- 167
- 250
- 333
- 500
- 15
- 30
- 45
- 75
- 112.5
- 150
- 225
- 300
- 500
- 750
- 1,000
- 1,500
- 2,000
- 2,500
- 3,000
- 3,750
- 5,000
300 kVA and up: BPS substation transformers start at 300 kVA. Three-phase pad-mount transformers also come in these sizes.
| kVA | Amps @ 240 V |
|---|---|
| 10 | 41.7 |
| 15 | 62.5 |
| 25 | 104.2 |
| 37.5 | 156.3 |
| 50 | 208.3 |
| 75 | 312.5 |
| 100 | 416.7 |
| 167 | 695.8 |
| 250 | 1,041.7 |
| 333 | 1,387.5 |
| 500 | 2,083.3 |
| kVA | 208 V | 240 V | 480 V | 600 V |
|---|---|---|---|---|
| 15 | 41.6 | 36.1 | 18.0 | 14.4 |
| 30 | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 | 124.9 | 108.3 | 54.1 | 43.3 |
| 75 | 208.2 | 180.4 | 90.2 | 72.2 |
| 112.5 | 312.3 | 270.6 | 135.3 | 108.3 |
| 150 | 416.4 | 360.8 | 180.4 | 144.3 |
| 225 | 624.5 | 541.3 | 270.6 | 216.5 |
| 300 | 832.7 | 721.7 | 360.8 | 288.7 |
| 500 | 1,387.9 | 1,202.8 | 601.4 | 481.1 |
| 750 | 2,081.8 | 1,804.2 | 902.1 | 721.7 |
| 1,000 | 2,775.7 | 2,405.6 | 1,202.8 | 962.3 |
| 1,500 | 4,163.6 | 3,608.4 | 1,804.2 | 1,443.4 |
| 2,000 | 5,551.4 | 4,811.3 | 2,405.6 | 1,924.5 |
| 2,500 | 6,939.3 | 6,014.1 | 3,007.0 | 2,405.6 |
| 3,000 | 8,327.2 | 7,216.9 | 3,608.4 | 2,886.8 |
| 3,750 | 10,409.0 | 9,021.1 | 4,510.5 | 3,608.4 |
| 5,000 | 13,878.6 | 12,028.1 | 6,014.1 | 4,811.3 |
Full-load amps are calculated from the formulas above and rounded to 0.1 A. Column voltages: 208 V means 208Y/120 · 240 V means 240 V Δ · 480 V means 480Y/277 or 480 V Δ · 600 V means 600Y/347. The ratings above are common standard sizes, not a stock list. Ask us what is available.
Size It
Right.
Four things decide the size: the service type, the real load, the margin you add, and the site conditions that push you up a size.
Single-phase or three-phase?
Match the service you have, or the one the utility will provide.
- Single-phase serves homes, small shops, farms and lighting loads, usually at 120/240 V. It comes from pole-mount and single-phase pad-mount transformers.
- Three-phase serves commercial buildings, plants and anything with three-phase motors: pumps, compressors, large HVAC, conveyors. Common secondaries are 208Y/120, 240 V Δ, 480Y/277, 480 V Δ and 600Y/347.
- Three-phase motors need a three-phase supply. A three-phase transformer can also serve single-phase loads; a single-phase transformer cannot serve three-phase loads.
Read the load
Good sizing starts with good load numbers. Use the best data you have:
- Nameplate amps. Use the full-load amps (FLA) on each nameplate at the voltage you will run. Adding every nameplate gives the worst case, with everything on at once.
- Panel schedules. If the schedule lists amps per phase, put the highest phase into the three-phase formula. If it lists total kVA, use that directly.
- Metered demand. For an existing site, 12 months of utility peak demand is the best real-world number. It is usually in kW, so convert with the power factor.
- Main breaker size is a ceiling, not the load. Sizing to it is conservative and can oversize the transformer.
- Demand factors. The NEC (NFPA 70, Article 220) lets the designer apply demand factors to connected load. Leave that to the engineer of record rather than guessing.
kW vs. kVA. kW is real power. kVA is apparent power: volts × amps. Transformer heating follows current, so transformers are rated in kVA. Convert with kVA = kW / PF. For example, 90 kW at 0.9 PF is 100 kVA.
Add margin on purpose
Margin covers load growth, uncertainty in your load numbers, and keeps the transformer from running at full nameplate all the time. The calculator defaults to 20%. That is a common rule of thumb, not a code requirement. Use what fits your project.
- Plan known growth now. Replacing a transformer later means another outage and another unit.
- Do not overdo it. A transformer much larger than its load costs more up front and has higher no-load (core) losses, which run around the clock whether or not the load is there.
- Apply margin before rounding up. A load that lands exactly on a standard size has no headroom left.
What pushes you up a size
- Motor starting. A motor started across-the-line draws several times its running current for a few seconds, often around 6× full-load amps. If one motor is a large share of the transformer rating, the voltage dip can drop contactors or trip drives. Check the voltage dip, or plan for soft starters or VFDs.
- Nonlinear loads (harmonics / K-factor). VFDs, LED drivers, computers and servers, UPS systems and EV chargers draw distorted current. Harmonics add heating that the kVA math does not show. The fix is a K-factor rated transformer (K-4, K-13 and K-20 are common ratings) or derating a standard unit. Tell us the load mix when you quote.
- High ambient temperature. Ratings assume standard conditions. For liquid-filled distribution transformers that is typically a 30°C average and 40°C maximum ambient. Hot sites, enclosed vaults and poor ventilation reduce usable capacity.
- Altitude. Standard ratings apply up to 3,300 ft (1,000 m). Above that, thinner air cools less and the unit is derated.
- Unbalanced loads. On three-phase, size from the most heavily loaded phase, not the average.
- Code and utility rules. NEC Article 450 covers transformer installation and protection, and your utility may have its own requirements for service transformers.
- Get real load data: amps, kW with power factor, or metered demand.
- Use the right formula: √3 and line-to-line volts for three-phase.
- Add a deliberate margin.
- Round up to the next standard size.
- Check motors, harmonics, heat and altitude, then confirm with BPS or your engineer.
Sizing
Answers.
Send nameplate photos or a panel schedule with your quote request to sales@bearpowersolutions.com, or call the team.
Call 601-912-3512What size transformer for a 200 A, 120/240 V service?
At full rating, 240 V × 200 A / 1000 gives 48 kVA. A 50 kVA unit covers that with no margin; with a 20% margin (57.6 kVA) the next size is 75 kVA. Actual demand on a 200 A service is often well below the breaker rating, so size from metered or calculated load if you have it.
Is kVA the same as kW?
No. kW is real power; kVA is apparent power (volts × amps). They are equal only at a power factor of 1.0. Transformers are rated in kVA, so convert with kVA = kW / PF. 90 kW at 0.9 PF is 100 kVA.
Can a transformer run at 100% of its rating?
Distribution transformers are designed to carry nameplate kVA continuously under standard rating conditions. Running at full load leaves no room for growth, and heat, altitude or harmonics reduce what the unit can safely carry. That is why the calculator adds margin before rounding up.
What do you need from me for a quote?
kVA, single- or three-phase, primary voltage, secondary voltage, type (pad-mount, pole, substation or dry-type), quantity and where it is going. Mention large motors, harmonic loads or listing requirements (UL Listed options available). We have hundreds of units in stock and will tell you what is ready; typical lead time is 12–16 weeks when a unit has to be built.
Have your
kVA number?
Send kVA, phase, primary and secondary voltage, quantity and where it’s going. We’ll check stock and give you a lead time. UL Listed options available when the job needs them.