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A buck-boost transformer is a type of electrical device that reduces (bucks) or increases (boosts) line voltage in industrial, commercial & residential applications. 

These compact single-phase isolation transformers are one of the most cost-effective alternatives for voltage correction in the 5% to 20% range. 

The most typical buck-boost transformer application is to boost 208 V to 230 V, allowing 230 V rated equipment to run efficiently off a 208 V supply line.

What is a Buck-Boost Transformer?

A buck-boost autotransformer is an electrical transformer that reduces or boosts the incoming supply voltage via a series connection. 

Buck-boost transformers unlike normal isolation transformers have a distinct winding structure that allows the secondary voltage to be enhanced (boosted) (or) dropped from (bucking) the primary voltage. 

This unique design makes them a great option for voltage correction & line voltage modification.

Buck-Boost Transformer Size: Technical Specifications

Single-Phase vs Three-Phase Configurations

To use buck-boost transformer sizing first evaluate whether your application requires a single-phase (or) three-phase choice:

Single-Phase Applications

One transformer manages single-phase voltage boosting (or) bucking.

Standard primary voltages: 120V, 240V and 480V.

Secondary voltages: 12V, 16V, 24V, 32V and 48V

Power ratings: 50 VA to 10 kVA.

Three-Phase Applications

Three-phase open delta: Requires two transformers for three-wire systems.

Three-phase WYE Configuration: Uses three transformers for four-wire systems with neutral available.

Ideal for balanced three-phase loads.

Requires appropriate transformer banking techniques.

Selection Criteria for Buck-Boost Transformers

Input Voltage (Line Voltage)

The line voltage is the current supply voltage measured by your electrical service. 

Typical input voltage possibilities include:

120 V, Single-Phase (Residential),

208 V, Three-Phase (Commercial),

240 V, Single-Phase (Residential/Commercial) &

480 V, Three-Phase (Industrial)

Output Voltage (Load Voltage)

The output voltage (or) load voltage must correspond to the equipment nameplate specification. 

The voltage requirement is specified on the nameplate of your device by the manufacturer. 

Common output voltage requirements:

240 V for single phase devices,

230 V for HVAC systems,

120 V for control circuits &

480 V for industrial machinery.

Load Amperage Calculation

Load amps (or) full load current are often listed on the equipment’s nameplate. 

Manufacturers give current amperage requirements for NEMA standard motors. 

The load current is important for determining the appropriate transformer size & KVA rating.

How to use Buck-Boost Transformer Calculator?

Step 1: Select Phase Configuration

Choose the system type from the following options:

Single Phase: Single phase is standard for residential and light commercial.

Three Phase Open Delta: Two transformers for three-wire industrial systems.

Three Phase WYE: Three transformers for four-wire systems with neutral.

Step 2: Enter Input Voltage

Enter the current supply voltage (or) line voltage. 

This is the voltage available through the electrical service panel. 

If necessary utilize a multimeter to verify the real voltage measurements.

Step 3: Set Output Voltage

Enter the appropriate output voltage from the equipment’s nameplate. 

The voltage differential between the input and output determines how much boost or buck is required.

Step 4: Input Load Amperage

Enter the load current in amperes as specified in the device being used. 

This value is essential for proper transformer sizing & KVA calculation.

Step 5: Adjust Tolerance Setting

The calculator sets to a 5% tolerance but it can be adjusted. 

Voltage tolerance refers to permissible variance in output voltage. 

It is typical to keep voltage within ±5% of nominal requirements.

Buck-Boost Transformer Calculator Formula

Voltage Difference Formula

Voltage Difference = (Output Voltage – Input Voltage)

Boost/Buck Percentage Formula

Boost/Buck % = (Voltage Difference / Input Voltage) x 100

Load KVA Calculation

Load KVA is the entire power of your equipment.

Single-Phase:

Load KVA = (Input Voltage x Load Amps) / 1000

Three-Phase:

Load KVA = (Input Voltage x Load Amps x 1.732) / 1000

This is the reason a small transformer can serve a large load with minimum changes in voltage.

Transformer KVA Requirement

The transformer’s needed KVA rating is simply based on the voltage change not the overall load:

Single-Phase:

Transformer KVA = (Voltage Difference x Load Amps) / 1000

Three-Phase:

Transformer KVA = (Voltage Difference x Load Amps x 1.732) / 1000

This is the reason a small transformer can serve a large load with minimum changes in voltage.

Voltage Range with Tolerance

Calculate the minimum and maximum permissible voltages.

Minimum Voltage = Output Voltage x (1 – Tolerance %)

Maximum Voltage = Output Voltage x (1 + Tolerance %)

Solved Example

A single-phase boost transformer is required to increase the supply voltage from 208 V to 240 V for a given load. The connected load draws 50 A, and the allowable voltage tolerance is ±5%. What should be the boost transformer voltage rating (boost voltage) and the required kVA rating to safely supply this load within the specified tolerance?

Given

Input Voltage: 208 V

Output Voltage: 240 V

Load Amps: 50 A

Tolerance: 5%

Calculation

Voltage Difference: 240 – 208 = 32 V

Boost %: (32 / 208) x 100 = 15.4 %

Load KVA: (208 x 50) / 1000 = 10.4 kVA

Transformer KVA: (32 x 50) / 1000 = 1.6 kVA

Min Voltage: 240 x 0.95 = 228 V

Max Voltage: 240 x 1.05 = 252 V

Selection (Answer)

Use 2 kVA transformer (next standard size above 1.6 kVA)