Buck boost transformer calculator
Single phase. The unit only carries the voltage it adds or removes, so the kVA is that voltage times the load current divided by 1,000. Supply current is the load current times the voltage ratio, ideal with no losses. Standard sizes shown are the common ratings from 0.05 to 10 kVA, and a maker catalog sets what is actually stocked. Pick a unit whose connection gives your exact voltage pair.
How the sizing works
A buck boost transformer is a small isolation style transformer connected as an autotransformer. The secondary winding sits in series with the supply, so it adds its voltage to raise the output (boost) or subtracts it to lower the output (buck). The winding only has to carry the load current at that small voltage.
- Single phase kVA equals load amps times the boost or buck volts, divided by 1,000.
- Three phase wye with three units, total kVA equals 1.732 times the line voltage change times the load amps, divided by 1,000.
- Supply current equals load current times the load voltage divided by the supply voltage.
- Pick the next standard size up from the result, and never run a unit past its nameplate current.
Worked example. A 30 amp load needs 240 volts and the panel gives 208 volts. The boost is 32 volts, so 30 x 32 / 1,000 = 0.96 kVA and a 1 kVA unit covers it. The supply current is 30 x 240 / 208 = 34.6 amps, so size the wire and breaker feeding the unit for the larger number. Because the output is the supply times the winding ratio, a 240 volt winding fed at 208 volts with a 32 volt boost winding gives 208 + 32 x 208 / 240 = 235.7 volts, a little under 240.
Buck boost sizing chart, 208 to 240 volts single phase
| Load amps at 240 V | Required kVA | Standard size | Supply amps at 208 V |
|---|---|---|---|
| 5 A | 0.16 kVA | 0.25 kVA | 5.77 A |
| 10 A | 0.32 kVA | 0.5 kVA | 11.5 A |
| 15 A | 0.48 kVA | 0.5 kVA | 17.3 A |
| 20 A | 0.64 kVA | 0.75 kVA | 23.1 A |
| 25 A | 0.8 kVA | 1 kVA | 28.8 A |
| 30 A | 0.96 kVA | 1 kVA | 34.6 A |
| 40 A | 1.28 kVA | 1.5 kVA | 46.2 A |
| 50 A | 1.6 kVA | 2 kVA | 57.7 A |
| 60 A | 1.92 kVA | 2 kVA | 69.2 A |
| 75 A | 2.4 kVA | 3 kVA | 86.5 A |
| 100 A | 3.2 kVA | 5 kVA | 115 A |
| 125 A | 4 kVA | 5 kVA | 144 A |
| 150 A | 4.8 kVA | 5 kVA | 173 A |
| 200 A | 6.4 kVA | 7.5 kVA | 231 A |
Free to cite with a link to this page. Required kVA is load amps times 32 volts divided by 1,000. Supply amps are load amps times 240 divided by 208. Standard sizes are the common ratings from 0.05 to 10 kVA and each maker lists its own range.
Required kVA for other voltage pairs
The same formula covers every common pair. Each cell is load amps times the volts added or removed, divided by 1,000, rounded up to the next standard size in brackets.
| Load amps | 208 to 240 V 32 V change | 208 to 230 V 22 V change | 240 to 208 V 32 V change | 240 to 277 V 37 V change | 480 to 440 V 40 V change |
|---|---|---|---|---|---|
| 10 A | 0.32 kVA (0.5) | 0.22 kVA (0.25) | 0.32 kVA (0.5) | 0.37 kVA (0.5) | 0.4 kVA (0.5) |
| 20 A | 0.64 kVA (0.75) | 0.44 kVA (0.5) | 0.64 kVA (0.75) | 0.74 kVA (0.75) | 0.8 kVA (1) |
| 30 A | 0.96 kVA (1) | 0.66 kVA (0.75) | 0.96 kVA (1) | 1.11 kVA (1.5) | 1.2 kVA (1.5) |
| 50 A | 1.6 kVA (2) | 1.1 kVA (1.5) | 1.6 kVA (2) | 1.85 kVA (2) | 2 kVA (2) |
| 75 A | 2.4 kVA (3) | 1.65 kVA (2) | 2.4 kVA (3) | 2.77 kVA (3) | 3 kVA (3) |
| 100 A | 3.2 kVA (5) | 2.2 kVA (3) | 3.2 kVA (5) | 3.7 kVA (5) | 4 kVA (5) |
| 150 A | 4.8 kVA (5) | 3.3 kVA (5) | 4.8 kVA (5) | 5.55 kVA (7.5) | 6 kVA (7.5) |
Load amps are at the output voltage. A large change, such as 480 to 440 volts at 150 amps, needs a bigger unit than most catalogs list, so check the range before ordering. For a 480 volt service that needs 240 volts or 208 volts, a step down transformer is the right product, see the 480 to 240 volt transformer page.
Limits to know before you buy
- No isolation. Primary and secondary share a conductor, so a fault on the supply side reaches the load.
- No new neutral. The unit does not make a grounded neutral at the new voltage.
- No regulation. The output follows the supply by a fixed ratio, so a sagging line stays low.
- Check the wiring diagram. The same unit gives different output voltages and different kVA ratings depending on the H and X jumpers, so use the nameplate table for your connection.
- Overcurrent protection. NEC 450.4 limits the protective device on an autotransformer to 125 percent of full load input current, read the edition in force locally.
If the equipment is rated for 208 to 230 volts, it may not need any transformer at all, so read its nameplate first. For loads that need isolation or a new neutral, size an isolation transformer at the full load kVA, and for current at any rating use the kVA to amps calculator and the transformer sizing calculator.
Common questions
What size buck boost transformer do I need for 208 to 240 volts
Multiply the load amps by 32 volts and divide by 1,000. A 30 amp load at 240 volts needs 30 x 32 / 1,000 = 0.96 kVA, so a 1 kVA unit works. The kVA is far smaller than the load kVA because the unit only carries the 32 volts it adds.
How do I size a buck boost transformer for 208 to 230 volts
The boost is 22 volts, so kVA equals load amps times 22 divided by 1,000. A 40 amp load needs 0.88 kVA, so a 1 kVA unit. Check the maker connection table for the exact output of the unit you pick.
Does a buck boost transformer regulate voltage
No. It adds or removes a fixed percentage, so the output moves with the supply. If the supply sags or swings, the output sags or swings by the same ratio. A voltage regulator is a different product.
Does a buck boost transformer isolate the load
No. Wired as an autotransformer the primary and secondary share a conductor, so there is no isolation and no new neutral. When a load needs isolation, a separately derived system or a new neutral, use an isolation transformer.
Can I use a buck boost transformer for 208 to 240 volts three phase
Yes, with three single phase units in wye or two in open delta, as the maker connection diagrams show. The calculator above sizes the three unit wye. Boosting 208Y/120 to a 240 volt line gives each unit about 18.5 volts of boost, so the winding choice matters, check the catalog table.
How much overcurrent protection does a buck boost transformer need
Code treats a unit connected as an autotransformer under NEC 450.4. The rule limits the protective device to 125 percent of the rated full load input current. Read the edition your jurisdiction enforces, and confirm the connection with the maker, because the nameplate rating changes with the wiring.
When should I use an isolation transformer instead
Use one when the load needs isolation, a grounded neutral at the new voltage, or when the current is high enough that an autotransformer connection would be a large unit. A full 208 to 240 isolation transformer is rated at the load kVA, not the boost kVA. See the isolation transformers page for sizes.
Need a larger transformer for a building or plant service, request a quote with the kVA and voltage.
Sources
- kVA, supply current and output voltage figures are computed on this page from the formulas above, with no losses.
- Standard voltage groups of 120x240 with 12/24 or 16/32 secondaries, and 240x480 with 24/48, and the 0.05 kVA lower end of the range, from the Dongan Electric buck boost catalog page. Ratings up to 7.5 kVA appear in Sola Hevi Duty tables. Each maker sets its own range.
- Boosting 208 volt lines to 230 or 240 volts is the most common use, per Dongan Electric.
- Autotransformer overcurrent limit of 125 percent of full load input current, NEC 450.4 as quoted in trade and Hammond Power Solutions material. Check the NEC edition your jurisdiction enforces.
- Elliott Electric publishes a multiplier table for approximate sizing by connection.