GuideCalculators and sizing9 min read
How to size a transformer
A transformer is sized from the load it will carry: add up the load in kW, divide by the power factor to get kVA, apply the demand factor and an allowance for growth, and choose the next standard size up, so that the unit runs at about 60 to 80 percent of its rating. A 250 kW load at 0.85 power factor with 25% growth needs 368 kVA, which is a 500 kVA transformer. The calculator below does the arithmetic; the checks that follow catch the cases it can’t.
The short version
- kVA needed = kW ÷ power factor × (1 + growth). Motors and mixed loads run at 0.8 to 0.9 power factor; heating and lighting near 1.0.
- Use the demand load, not the connected load: NEC Article 220 and a year of utility bills are the two sources.
- Pick the next standard size up and aim for 60 to 80% loading at peak. Standard three-phase sizes run 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000, 1,500, 2,000, 2,500, 3,000, 3,750 and 5,000 kVA.
- Check the largest motor: starting it across the line dips the voltage by about %Z × (motor starting kVA ÷ transformer kVA).
- Fans (a 750/1,000 kVA ONAF rating) buy a third more capacity for peaks; a lower temperature rise on a dry-type buys 15 to 30%.
Calculator
Enter the load in kW (after demand factors), the power factor and the growth you want to allow for, and the calculator gives the kVA, the next standard size and how hard it would run.
500kVA, standard size
250 kW ÷ 0.85 × 1.25 = 368 kVA needed; a 500 kVA unit would run at 74% of its rating and 601 A at 480 V
The five steps
Find the load in kW
For a new building, the NEC Article 220 load calculation, which applies demand factors to each kind of load. For an existing one, the peak demand on a year of utility bills, which is what the building actually draws. For a plant, the sum of the equipment that runs at once, with motors at their running load rather than their nameplate.
Convert to kVA
kVA = kW ÷ power factor. Motors and drives run at 0.8 to 0.9; resistive heating and most lighting near 1.0; a mixed commercial building about 0.9. If the bill gives kVA or the power factor directly, use that.
Allow for growth
Twenty to 25% is the usual allowance, more for a plant that expects to add lines. A transformer lasts thirty years; the load it was bought for rarely does.
Choose the next standard size up
Three-phase sizes are 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1,000, 1,500, 2,000, 2,500, 3,000, 3,750 and 5,000 kVA; single-phase 5 through 500 kVA. Round up, never down, and aim to land at 60 to 80% loading.
Check the motors and the fault current
The largest motor must start without dipping the voltage too far (below), and the chosen transformer’s impedance sets the fault current the switchgear must interrupt (the impedance guide). A size up may be the answer to the first; a higher impedance to the second.
kVA needed = kW ÷ power factor × (1 + growth)
Sizing chart: kVA by amps at 208 V and 480 V
Full-load amps for each standard three-phase size, and the current it serves at 80% loading: read across from the amps you need to the size that covers it.
| kVA | Full load at 480 V | 80% at 480 V | Full load at 208 V | 80% at 208 V |
|---|---|---|---|---|
| 45 | 54.1 A | 43.3 A | 125 A | 99.9 A |
| 75 | 90.2 A | 72.2 A | 208 A | 167 A |
| 112.5 | 135 A | 108 A | 312 A | 250 A |
| 150 | 180 A | 144 A | 416 A | 333 A |
| 225 | 271 A | 217 A | 625 A | 500 A |
| 300 | 361 A | 289 A | 833 A | 666 A |
| 500 | 601 A | 481 A | 1,388 A | 1,110 A |
| 750 | 902 A | 722 A | 2,082 A | 1,665 A |
| 1,000 | 1,203 A | 962 A | 2,776 A | 2,221 A |
| 1,500 | 1,804 A | 1,443 A | 4,164 A | 3,331 A |
| 2,000 | 2,406 A | 1,925 A | 5,551 A | 4,441 A |
| 2,500 | 3,007 A | 2,406 A | 6,939 A | 5,551 A |
Amps = kVA × 1,000 ÷ (1.732 × V). The kVA to amps guide has every size at every voltage.
The motor-starting check
A motor started across the line draws about six times its running current for a second or two. That inrush flows through the transformer’s impedance and pulls the bus voltage down for every other load. The dip, as a fraction of the voltage, is roughly:
Voltage dip ≈ %Z × (motor starting kVA ÷ transformer kVA)
A 100 HP motor has a locked-rotor draw of about 600 kVA (code letter G, 5.6 to 6.3 kVA per HP). On a 500 kVA transformer at 5.75% impedance the dip is 5.75% × 600 ÷ 500 = 6.9%, which most equipment rides through. The same motor on a 150 kVA transformer would see a 23% dip: contactors drop out, drives trip. The rule of thumb that follows is that a transformer should be at least a third of the starting kVA of the largest motor it starts across the line, and more where the supply behind it is weak. A soft starter or a drive cuts the starting draw to two or three times running current, or less, and takes the problem away.
| Motor | Starting kVA (code G) | Transformer for a 10% dip at 5.75% Z |
|---|---|---|
| 25 HP | 150 | 112.5 kVA or more |
| 50 HP | 300 | 225 kVA |
| 100 HP | 600 | 500 kVA |
| 200 HP | 1,200 | 750 kVA |
| 300 HP | 1,800 | 1,000 kVA |
| 500 HP | 3,000 | 2,000 kVA |
Starting kVA from the motor’s code letter on its nameplate; code G is typical for 10 to 200 HP. The dip also depends on the utility source behind the transformer, so treat the last column as a floor.
How hard to load a transformer
The nameplate kVA is a continuous rating at the rated temperature rise in a 30 °C average ambient (IEEE C57.12.00), so a transformer can carry 100% of it all day. Three things argue for sizing to 60 to 80% at peak anyway: growth, which otherwise forces a replacement; hot days, since every 8 to 10 °C above the design ambient takes years off the insulation at full load; and motor starting, which needs headroom. IEEE C57.91 (the loading guide) allows short overloads on liquid-filled units, 110 to 150% for an hour or two after a lighter period, at a cost in insulation life that the guide quantifies.
- ONAN / ONAF (liquid-filled)
- Self-cooled, and with fans. A 1,500/2,000 kVA unit carries 1,500 kVA without fans and 2,000 with them: the fan rating is a cheap way to cover a peak.
- AA / FA (dry-type)
- The same idea; fans add about a third.
- Temperature rise (dry-type)
- A 115 °C-rise unit carries about 15% over nameplate continuously and an 80 °C-rise unit about 30%, because their insulation is rated for 150 °C rise. The dry-type guide covers it.
- Peak versus average
- A load that peaks for an hour a day can use the overload allowance; a data hall at constant load can’t. Size the second kind with more margin.
Single-phase and small loads
Single-phase transformers (pole-mounts, single-phase pad-mounts, small dry-types) are sized the same way with the single-phase formula, kVA = volts × amps ÷ 1,000. A 200 A, 240 V house service is 48 kVA on paper, but utilities size residential transformers on diversity: a 25 kVA pole-mount commonly serves four to six houses, because their peaks don’t coincide. For a single large single-phase load, a well pump or a commercial kitchen, size to the demand without diversity. Standard single-phase sizes are 5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 and 500 kVA.
A worked example
A machine shop is moving into a building with a 12.47 kV service. Its connected load is 420 kW: 300 HP of motors across eight machines, 60 kW of welding, 40 kW of lighting and HVAC, 20 kW of receptacles. The largest motor is a 75 HP compressor, started across the line.
- Demand: the shop never runs more than five machines at once, and the welding is intermittent. The demand load is estimated at 260 kW.
- kVA: motors and welders dominate, so 0.85 power factor: 260 ÷ 0.85 = 306 kVA.
- Growth: the owner plans two more machines. 306 × 1.25 = 382 kVA.
- Standard size: 500 kVA (the 300 kVA unit would run at 127%). Loaded at 76% at the projected peak, 61% today.
- Motor check: 75 HP starts at about 450 kVA. On 500 kVA at 5.75% Z the dip is 5.2%. Fine.
- Fault current: 601 A ÷ 0.0575 = 10,500 A at 480 V with an infinite source, so 22 kA switchgear is more than enough (the impedance guide).
The specification: 500 kVA, 12.47 kV delta to 480Y/277 V, pad-mount, loop or radial feed as the utility requires, 5.75% impedance. Protection follows from it (the fuse and breaker sizing guide), and Magic Search finds the units for sale that match it.
Questions people ask
How do you size a transformer?
Add up the load in kW, divide by the power factor to get kVA, multiply by the demand factor (the share of connected load that actually runs at once) and by an allowance for growth, then choose the next standard kVA above the result. Then check that the largest motor can start without an unacceptable voltage dip, and that the transformer will run at a sensible fraction of its rating, usually 60 to 80% at peak.
What size transformer do I need for 400 amps at 480 V?
A 400 A three-phase service at 480 V is 1.732 × 480 × 400 ÷ 1,000 = 333 kVA, so a 500 kVA transformer (601 A full load) if the 400 A is the demand, or a 300 kVA unit (361 A) if the 400 A is a conservative main-breaker rating and the real demand is under about 290 A. At 208 V, 400 A is 144 kVA, which is a 150 kVA transformer.
What size transformer do I need for 200 amps?
At 208Y/120 V three-phase, 200 A is 72 kVA, so a 75 kVA transformer. At 480 V three-phase it’s 166 kVA, a 225 kVA unit (or 150 kVA at 180 A if the load is well below 200 A). For a single-phase 240 V service, 200 A is 48 kVA, a 50 kVA transformer.
Should a transformer be sized at 80% of its rating?
As a target, yes. A transformer can carry its full nameplate kVA continuously at its rated temperature rise, so 80% isn’t a code limit the way it is for breakers; it’s a design margin that leaves room for growth, for a hot day, and for a motor to start. Liquid-filled units with fans and dry-types with a low temperature rise have more margin built in.
What is the demand factor for a transformer?
The fraction of the connected load that runs at the same time. The NEC’s Article 220 gives demand factors by type of load (lighting, receptacles, kitchen equipment, motors), and for an existing building the utility bill’s peak demand in kW is the best figure. Industrial loads with large motors often see 50 to 70% of connected load; offices 30 to 50%.
Can you oversize a transformer?
Yes, within reason. A transformer loaded at 20% wastes little energy (its no-load loss is fixed, perhaps 0.2% of rating, and its load loss falls with the square of the load), but it costs more, takes more room, and lets through more fault current, which can push the switchgear into a higher rating. One size above the calculated need is normal; two sizes is money spent on nothing.
Sources: NEC Article 220 (load calculations) and Article 450; IEEE C57.12.00 (ratings) and C57.91 (loading guide); IEEE C57.96 (loading dry-type transformers).