Buck Boost Transformer Wiring Diagram & Sizing | 208 to 240V 

A buck boost transformer can raise 208V to around 236V, or reduce 240V closer to 208V when wired as an autotransformer. It is used for small voltage correction. It is not a replacement for a full-size isolation transformer. A common case is the 240V machine supplied from a 208V panel. A 16/32V unit can raise 208V to about 236V in the proper boost connection. The actual output depends on the transformer connection and the voltage at the input. 

Before installation check the equipment nameplate first. If the machine is rated for 208-230V or 208/230V, then 208V may already be within the manufacturers rated range. In such scenario, adding a buck boost only to bring the voltage closer to 230V does not make much sense.

Do You Need a Buck Boost Transformer? Check the Nameplate

You need a buck boost when the voltage at the equipment is outside the equipment’s allowed voltage range. A 240V-only machine on a 208V supply is a common example. But if the nameplate says 208-230V or 208/230V, the 208V supply may already be acceptable. Do not add a transformer just to move the voltage closer to 230V.

Check the nameplate first. Then measure the voltage at the equipment terminals. I also like to check it while the machine is running because the voltage can change when a motor or compressor starts.

Motor voltage tolerance does not mean every 230V motor should be run at 208V. Check the motor nameplate and manufacturer’s data first.

For a 230V motor, 208V is about 9.6% below 230V. That may fall within the commonly referenced NEMA voltage-tolerance range, but low voltage can still affect motor performance. Starting torque decreases as voltage falls, and the motor can run hotter if the operating conditions are not suitable.

A compressor with a hard start can show this pretty quickly. So I would not make the transformer decision from the 10% number alone.

My check is simple. If the voltage you measure is inside the nameplate range, I don’t add a buck boost. If it is outside the range, then check the transformer size and connection.

What Is a Buck Boost Transformer?

A buck boost transformer is a small dry type transformer, air cooled and with no oil. You wire it in the field as autotransformer when you need to raise or lower the voltage by a small amount. Once wired this way, it does not provide electrical isolation.

A common buck boost transformer has dual primary and dual secondary windings. For example, the nameplate may show 120 x 240V on the primary and 16 x 32V on the secondary. The windings are connected in different ways depending on the supply and the required boost or buck voltage. 

When you make the autotransformer connection, part of the winding is shared between the input and output. So it is the same coils, just connected differently.

Don’t confuse this with a distribution transformer. A 480V to 208V transformer is used to change the voltage for a larger electrical system. A buck boost transformer is mainly there to correct a smaller voltage difference at the equipment.

How a Buck Boost Transformer Works

A buck boost transformer adds or subtracts a small amount of voltage from the supply. The secondary winding provides this correction voltage. Connect it so the secondary voltage adds to the supply and you get a boost. Reverse the connection and it bucks the voltage. Since only the correction voltage is being transformed, a relatively small transformer can handle a much larger load.

There are two terms that get confused here. Additive or subtractive refers to the polarity marking of the transformer terminals. Buck and boost refer to how those terminals are connected in the circuit. Many small units are marked additive, but check the nameplate before making the connection.

The output voltage depends on how the windings are connected. For the common buck-boost calculation, the correction voltage is added for a boost connection and subtracted for a buck connection.

For a quick estimate, think of the buck boost transformer as adding or subtracting the connected correction voltage from the supply:

Boost: Vout ≈ Vin + Vcorrection

Buck: Vout ≈ Vin – Vcorrection

The correction voltage depends on the winding section used and the actual transformer connection. Use the manufacturer’s voltage table for the final selection rather than treating the calculation as a universal transformer formula.

For example, a 16/32V unit can provide a 32V correction when connected for that application. With a 208V supply, the simple voltage addition gives about 240V in an ideal calculation. The actual equipment voltage can be different because the transformer connection, source voltage and transformer regulation all affect the result.

This is why the manufacturer’s buck-boost selection table matters. Do not choose a transformer from the correction voltage alone.

These are calculated values, not guaranteed equipment-terminal voltages. Transformer regulation, source voltage and the actual connection can change the final reading.

For example, a 16/32V unit does not guarantee a 240V output from a 208V supply. The actual output depends on the manufacturer’s connection and transformer characteristics. Check the manufacturer’s voltage table before using the transformer for a specific load.  If the incoming voltage falls to 200V, the boosted voltage falls too. A buck boost transformer does not regulate the supply like a voltage regulator.

There is another point here. The transformer does not hold the output at one fixed voltage. If the supply is 208V, the 16/32 setup adds its correction voltage. If that supply drops to 200V, the output drops too, to around 227V. So if the plant voltage changes during the day, take a few readings and see what is really happening before selecting the transformer.

208V to 240V Buck Boost Transformer Sizing 

Buck boost transformer sizing starts with the correction voltage, not the full load voltage. A simple correction-winding check is:

kVA = Load amps × correction volts ÷ 1,000

For a 50A load using a 32V correction:

50 × 32 ÷ 1,000 = 1.6 kVA

That gives a calculated correction-winding requirement of 1.6 kVA. Then check the manufacturer’s available sizes and application table. If the next suitable size is 2 kVA, that is the transformer size to consider.

1. Determine the required correction voltage from the equipment nameplate and measured supply voltage. Then select the secondary voltage from the manufacturer’s application table. 
2. Read the full-load amps from the equipment nameplate. We use 50A.
3. Work out kVA (kilovolt-amps, the size rating). 50A × 32V ÷ 1,000 = 1.6 kVA.
4. Select the next suitable standard transformer size listed by the manufacturer. For this example, a 2 kVA unit would be the next size if the manufacturer’s table permits it. 
5. Check the secondary current. 2 kVA ÷ 32V is 62.5A, which covers 50A.

The most common mistake is sizing from the load kVA instead of the correction kVA. A 50A load at 240V is 12 kVA, but the transformer only needs 1.6. You pay for 32 volts of correction, not 240 volts of power. On a 208V supply the real correction is about 28V, so you get a little margin.

Now look at the input side. The source has to supply the load power, so the 208V input current will be higher than the 50A load current at 236V.

For this example:

50 × 236 ÷ 208 = about 56.7A

So the source side current is about 57A before considering transformer losses. Do not size the input conductors from the 50A output current alone. Check the actual transformer rating, connection and applicable NEC requirements.

If the load meets the NEC definition of a continuous load, apply the required 125% continuous-load calculation. Do not use a three hour rule without checking the applicable NEC edition and application and conductors and breakers get sized at 125 percent. I also add about 25 percent spare transformer size on continuous loads. That is my habit, not an NEC rule.

How Much Voltage Does a Buck Boost Transformer Add?

The voltage added by a buck boost transformer depends on the winding section selected and the way the transformer is connected. A 16/32V transformer can provide different correction voltages depending on the connection.

For example, a 32V correction can be used when the manufacturer’s application table calls for that connection. With a 208V supply, the simple calculation is:

208V + 32V = 240V

The actual output at the equipment can be different because of transformer regulation, source voltage and the connection used.

This is why I check the manufacturer’s voltage table before selecting the transformer. The nameplate voltage of the equipment also matters. If the equipment is already rated for 208V, there may be no reason to boost it to 240V.

208V to 240V Buck Boost Transformer Wiring Diagram 

For a 208V to approximately 236V boost, the secondary winding is placed in series with the supply so its voltage adds to the source voltage. The exact primary and secondary connections depend on the transformer model.

If the secondary is connected with the wrong polarity, its voltage subtracts instead of adding. The output can then be much lower than expected. Check the manufacturer’s wiring diagram before making the connection.

This diagram is an example of a 120 x 240V primary and 16/32V secondary connection. Terminal numbers and jumper locations vary by manufacturer and model. The wiring diagram on the actual transformer always takes priority. Hubbell’s Acme installation manual lists the maker diagrams by catalog number.

Only a qualified electrician should do this. Follow local code, lock out and tag out the supply, and let your inspector have the final word.

1. Lock out the supply and test it dead with a meter.
2. Primary. Jumper H2 to H3, making one 240V winding between H1 and H4.
3. Secondary. Jumper X2 to X3, putting the two 16V windings in series, 32V between X1 and X4.
4. Supply. Connect L1 to H1 and to X1. Connect L2 to H4.
5. Load. Connect the boosted output to the load exactly as shown on the manufacturer’s wiring diagram. In this example, the output is approximately 236V. 
6. Torque all lugs to the maker’s values.
7. Energize the transformer with the load disconnected and measure the output voltage. For this example, you should see approximately 236V. If the reading is not what you expect, de-energize the circuit and recheck the connections before connecting the load.

If the output is lower than expected, do not simply swap the secondary wires while troubleshooting live. Lock out the circuit, verify it is dead, and compare the connection with the manufacturer’s diagram. A polarity error can make the secondary voltage subtract instead of add.

For three-phase buck boost applications, two or three single-phase transformers can be used depending on the source, load and required connection. Open-delta arrangements are used for some three-wire systems. Wye arrangements are used for applications where the manufacturer’s connection requires the source neutral.

Do not select the number of transformers just because the system has or does not have a neutral. Check the manufacturer’s three-phase application table and wiring diagram for the exact voltage combination.

With open delta, two transformers form the required three-phase connection without creating a neutral from the delta point. Do not treat an open-delta connection point as a neutral.

For a wye bank, the three transformer windings meet at a common point and the source neutral connection depends on the manufacturer’s specified arrangement. The line-to-line voltage can be correct even when the line-to-ground readings are not what you expect, so check the complete connection before energizing the load.

For a quick correction-kVA check, start with the load current and the correction voltage:

kVA = Load amps × correction volts ÷ 1,000

For example, 30A with a 32V correction gives:

30 × 32 ÷ 1,000 = 0.96 kVA

That is a starting calculation only. For a three-phase bank, use the manufacturer’s sizing table to determine the number and size of transformers required. The phase connection changes the calculation from a simple single-phase example.

 I would still check the manufacturer’s three-phase chart before ordering the transformers. The connection and phase angle make this different from sizing one single-phase transformer by itself.

Before putting the motor on, check all three line-to-line voltages. Don’t just check one phase and assume the others are the same. Voltage unbalance is a problem for motors. Even around 1% unbalance can require motor derating, and the motor can heat up pretty fast when the phases are not balanced.

Other Buck Boost Voltage Combinations

Some common buck boost applications are 208 to 240V, 208 to 230V, 240 to 208V, 120 to 132V, and 240 to 264V.

The transformer is only making up the voltage difference. So if you have 208V and need around 240V, the buck boost unit supplies the extra voltage. These transformers are normally used for smaller voltage corrections, not large changes. Buck boost transformers are intended for relatively small voltage corrections. If the required voltage change becomes large, check the manufacturer’s allowed voltage combinations and compare the application with an isolation transformer or another transformer arrangement.

Do not choose a buck boost just because it is smaller. If the job needs electrical isolation, a new neutral or a substantially different voltage ratio, another transformer arrangement may be more suitable.

The same transformer can also buck 240V down toward 208V. With the subtracting connection, you get around 212V in this example, which is only about 2% above 208V. That is within the normal range for many 208V loads.

Don’t select the transformer from the nominal supply voltage alone. Check what voltage you actually have at the equipment. Then use the manufacturer’s selection chart and wiring diagram for that particular unit.

Protection, Wire Size and Grounding: What the NEC Says

For a buck boost transformer, start with the autotransformer overcurrent requirements in the NEC edition adopted for the project. The section number can change between NEC editions, so do not rely on one section number without checking the edition.

The applicable requirements include overcurrent protection for the ungrounded input conductors. The allowed protection can be based on 125% of the rated full-load input current, with a next-standard-size allowance when the applicable conditions are met.

A buck boost transformer can serve a much larger load when connected as an autotransformer because only the voltage difference is being transformed. The nameplate kVA still has to be applied correctly for the actual connection.

Do not take a breaker size from a generic example and apply it to another transformer. Calculate the input current from the actual transformer connection, then check the manufacturer’s data and the applicable NEC requirements before selecting the overcurrent device.

The wire size is based on the current in that part of the circuit. On the output side, use the load current. The input current depends on the voltage ratio, so don’t use the output current for the input wire size. For a continuous load, apply the 125% requirement and then check the conductor ampacity in NEC Table 310.16. Also check the temperature rating of the terminals before finalizing the wire size.

Grounding and grounded-conductor connections need to be checked separately from the voltage calculation. A buck boost autotransformer does not become a separately derived system just because it changes the voltage.

Do not add a neutral-to-ground bond at the transformer simply because it is a buck boost connection. Follow the applicable NEC requirements and the manufacturer’s wiring diagram for the specific system.

One point that gets missed is the system grounding. A buck boost transformer connected as an autotransformer is not a separately derived system. Do not add a second neutral-to-ground bond at the transformer just because the voltage is being changed. 

Worked Buck Boost Transformer Examples

These examples use typical nameplate values, not one specific job. Each follows the same steps. Pick the secondary, multiply by load amps, then round up.

On the condenser, 28A is the minimum circuit ampacity (MCA) from the nameplate. Input current at 208V is about 32A, so the feeder has to carry that.

A motor can draw several times its running current during starting. If the source voltage is already low, that starting current can pull the voltage down further. I would check the motor starting conditions and voltage drop before deciding to increase the transformer size.  The 12/24 secondary gives about 229V, inside the 230V nameplate. Size the overload protection from the motor nameplate.

The heater surprises people. Output falls with the square of voltage, so a 240V heater on 208V gives only about 75 percent of its rated heat. Even on 236V it gives about 97 percent. Heaters can be continuous loads, so I would apply the applicable continuous-load requirements and any design margin separately. The 25% spare capacity I used here is my design margin, not an NEC transformer-sizing rule. 

Buck Boost vs Isolation Transformer vs Voltage Regulator

A buck boost transformer fixes a steady voltage gap cheaply but gives no isolation. An isolation transformer separates input from output and can change voltage by any ratio, but costs more and weighs more. A voltage regulator holds output steady when input swings. Pick by the problem, not the price.

Some people say fit isolation every time because it is safer. The other side is size and cost. On a 28A job at 240V, isolation means about 6.7 kVA instead of 0.9 kVA. Use isolation when the job needs it, not out of habit.

Skip the buck boost when the supply voltage swings too much, when the job needs isolation, or when the required voltage change is outside the manufacturer’s buck-boost range.

Frequently Asked Questions

Can a buck boost transformer turn 208V into 240V?

It can raise 208V close to 240V, but the output is not always exactly 240V. For example, a 16/32V transformer using the 32V correction can produce about 236V from a 208V supply in the appropriate connection. Check the equipment nameplate and the manufacturer’s voltage table before selecting the transformer.

Does a buck boost transformer provide isolation?

No. Wired as an autotransformer, input and output share the same conductors. If you need separation, use a true isolation transformer.

How do I size a buck boost transformer for 208 to 240V?

Multiply the load current by the correction voltage and divide by 1,000 for a basic correction-kVA check. For a 50A load using a 32V correction:

50 × 32 ÷ 1,000 = 1.6 kVA

Then check the manufacturer’s available sizes and application table. A 2 kVA unit may be the next suitable size, but the final selection depends on the transformer connection and manufacturer data.

What is the difference between additive and subtractive polarity?

It is how the maker marked the terminals. On additive units H1 sits across from X1, and on subtractive units they sit on the same side. Boost or buck is how you connect the windings, so the marking tells you which way to wire for each.

What size breaker does a buck boost transformer need?

There is no single breaker size for every buck boost transformer. Start with the rated input current for the actual autotransformer connection, then apply the overcurrent requirements of the NEC edition used on the project. The manufacturer’s data and transformer connection also need to be checked before selecting the breaker.

Can I use a buck boost transformer outdoors?

Only if it is built for it. Look for a NEMA 3R enclosure or an outdoor listing. Standard open units belong indoors.

When should I choose an isolation transformer instead?

Choose isolation when you need electrical separation, a new neutral, noise protection, or a voltage change bigger than about 20 percent.

Before You Buy or Wire One

Measure the supply at the equipment, preferably while the machine is running. Write down the nameplate voltage and amps, then determine how much voltage correction is actually needed.

Calculate the correction kVA and compare it with the manufacturer’s selection table. After wiring, keep the load disconnected and measure the output voltage first. If the voltage is not what you expect, lock out the circuit and check the transformer connection before putting the equipment into service.