Three phase full load current, kVA times 1000 divided by volts times the square root of 3. This is the nameplate figure. Size conductors and protection to the code that applies to your job, not to this number alone.
Full load current for every standard rating
Secondary current at the common utilisation voltages, and primary current at the two distribution voltages we quote most. All figures are nameplate full load current.
| kVA | 480Y/277 | 208Y/120 | 240 | 600Y/347 | 12470 GY/7200 primary | 13200 GY/7620 primary |
|---|---|---|---|---|---|---|
| 45 | 54.1 A | 125 A | 108 A | 43.3 A | 2.1 A | 2.0 A |
| 75 | 90.2 A | 208 A | 180 A | 72.2 A | 3.5 A | 3.3 A |
| 112.5 | 135 A | 312 A | 271 A | 108 A | 5.2 A | 4.9 A |
| 150 | 180 A | 416 A | 361 A | 144 A | 6.9 A | 6.6 A |
| 225 | 271 A | 625 A | 541 A | 217 A | 10.4 A | 9.8 A |
| 300 | 361 A | 833 A | 722 A | 289 A | 13.9 A | 13.1 A |
| 500 | 601 A | 1,388 A | 1,203 A | 481 A | 23.1 A | 21.9 A |
| 750 | 902 A | 2,082 A | 1,804 A | 722 A | 34.7 A | 32.8 A |
| 1000 | 1,203 A | 2,776 A | 2,406 A | 962 A | 46.3 A | 43.7 A |
| 1500 | 1,804 A | 4,164 A | 3,608 A | 1,443 A | 69.4 A | 65.6 A |
| 2000 | 2,406 A | 5,551 A | 4,811 A | 1,925 A | 92.6 A | 87.5 A |
| 2500 | 3,007 A | 6,939 A | 6,014 A | 2,406 A | 116 A | 109 A |
Free to cite with a link to this page. Figures are computed from the standard three phase formula, not from any one manufacturer's catalogue, so they hold for any unit at that rating.
Smaller and less common ratings
The same formula for the dry type and odd sizes people look up most, three phase at the two common building voltages and single phase at 240 volts.
| kVA | 480Y/277, 3 phase | 208Y/120, 3 phase | 240 V, single phase |
|---|---|---|---|
| 3 | 3.6 A | 8.3 A | 12.5 A |
| 5 | 6.0 A | 13.9 A | 20.8 A |
| 10 | 12.0 A | 27.8 A | 41.7 A |
| 15 | 18.0 A | 41.6 A | 62.5 A |
| 20 | 24.1 A | 55.5 A | 83.3 A |
| 25 | 30.1 A | 69.4 A | 104 A |
| 30 | 36.1 A | 83.3 A | 125 A |
| 37.5 | 45.1 A | 104 A | 156 A |
| 40 | 48.1 A | 111 A | 167 A |
| 50 | 60.1 A | 139 A | 208 A |
| 60 | 72.2 A | 167 A | 250 A |
| 100 | 120 A | 278 A | 417 A |
| 125 | 150 A | 347 A | 521 A |
| 200 | 241 A | 555 A | 833 A |
| 250 | 301 A | 694 A | 1,042 A |
| 630 | 758 A | 1,749 A | 2,625 A |
Current for a specific voltage change
Every formula behind these tables, with worked examples, is on the three phase transformer formulas page.
Ratings we stock and quote
Common questions
How do you convert kVA to amps on a three phase transformer?
Multiply the kVA rating by 1000, then divide by the voltage multiplied by the square root of three, about 1.732. A 300 kVA transformer at 480Y/277 gives 300 times 1000 divided by 480 times 1.732, which is 361 amps.
What is the full load current of a 300 kVA transformer at 480 volts?
About 361 amps on the secondary at 480Y/277. The matching primary current at 12470 GY/7200 is about 14 amps, because current falls as voltage rises for the same kVA.
Why is the primary current so much lower than the secondary current?
Because kVA is the same on both sides. At 12470 volts the current needed to carry 300 kVA is roughly 26 times smaller than at 480 volts, which is why medium voltage cable into a pad mounted transformer is far smaller than the secondary conductors leaving it.
How many amps is a 75 kVA transformer good for?
About 90 amps per phase at 480Y/277 and about 208 amps at 208Y/120, three phase. That is the nameplate full load current, and a liquid filled unit carries it continuously. The secondary breaker and conductors are then sized from that figure under the electrical code.
How many amps is a 45 kVA transformer?
About 54 amps at 480Y/277 and 125 amps at 208Y/120, three phase. On a single phase 45 kVA unit at 240 volts it is about 188 amps.
How many amps is a 25 kVA transformer good for?
A 25 kVA single phase transformer at 120/240 volts delivers about 104 amps at 240 volts. Three phase at 208Y/120 it is about 69 amps, and at 480Y/277 about 30 amps.
Does this number include any safety or continuous load factor?
No. This is nameplate full load current only. Continuous load factors, conductor derating, ambient correction and overcurrent protection are separate calculations under the code that applies to your installation.
Need the unit rather than the number. Ask for a quote and we will come back with pricing and lead time from stock and from production. Sizing a feed for a server hall, see data center power equipment. Working with used or surplus gear, see buying used transformers and buying used switchgear. Feeding a motor load, the HP to kVA calculator gives the kVA to start from, and for a plant substation see substation transformers. More lookup tables sit in the charts library.