How Do I Size a Step-Up Transformer for Three VFDs?

One correctly selected step-up transformer can feed all three drives; a separate transformer for each VFD is not inherently required. Size the shared transformer from the sum of the drives’ rated single-phase input currents, then apply the documented 1.5 sizing allowance. For the installation described, the working result is approximately 4 kVA, which represents about 33 A from an ideal 120 V source before transformer losses.

Load Quantity and Reading Point

The number that matters is each VFD’s rated input line current. Read it from the VFD nameplate, input-rating table, or electrical specifications in the manual. Do not substitute motor full-load current or the VFD’s output-current rating: those values describe the motor side of the drive, not the load imposed on the transformer.

Record each drive separately even when all three are rated 1 HP. Input current can differ with drive design, input voltage, and rating method. Also confirm that the drives accept the transformer’s actual secondary voltage; the equipment is described as 220 VAC while the sizing calculation uses 240 V.

Source-Circuit Symptoms

A standard 120 V outlet may show normal voltage with no load yet be unable to deliver the calculated current when the drives run. The useful symptoms are voltage sag, breaker operation, drive undervoltage events, unstable acceleration, or excessive heating at the transformer, receptacle, terminals, or conductors. Read the VFD diagnostic history and measure both primary and secondary voltage while the machines accelerate and while all commanded loads are operating.

The approximately 33.3 A result is ideal current for a 4 kVA load at 120 V. Actual input current is higher because a real transformer has losses. A general-purpose receptacle circuit therefore cannot be accepted solely because its unloaded voltage measures near 120 V; verify the feeder’s breaker rating, conductor ampacity, terminal ratings, and available service capacity.

The installation estimate associated about 33 A with a 40 A breaker and #8 AWG conductors. Those values require project-specific verification by a qualified electrician because breaker sizing and conductor ampacity depend on the transformer data, wiring method, conductor insulation, terminal temperature ratings, and applicable electrical rules.

Transformer and VFD Current Mechanism

This is heat, not logic. A step-up transformer raises voltage but does not create power. Ignoring losses, moving 4 kVA from 120 V to 240 V changes current from about 33.3 A on the primary to about 16.7 A on the secondary. The 120 V supply must still deliver the full apparent power.

A VFD input rectifier draws current in pulses while charging its internal DC bus. That current waveform can produce more transformer and conductor heating than an equally sized linear load. Mechanical horsepower alone cannot describe this electrical burden; transformer selection starts with VFD input VA and must also account for the transformer manufacturer’s suitability or derating guidance for nonlinear drive loads.

Timing affects the maximum demand. If all three drives precharge or accelerate together, the transformer and feeder experience their highest combined loading at the same time. Staggered commands can reduce coincident demand, but they do not reduce the continuous transformer rating needed when all three drives can run together at their rated input loads.

Sizing and Connection Procedure

1. Verify the source. Measure the supply and identify the actual feeder capacity. Confirm whether the stated 110 V source is nominal 120 V and whether the circuit is dedicated to this equipment.
2. Verify each VFD input rating. Locate the single-phase input-voltage range and rated input line current in each manual or on each nameplate. Confirm that a 240 V secondary falls inside the permitted range for equipment described as 220 VAC.
3. Calculate each load. For drive i, calculate VAi = 240 V × Ii, where Ii is that drive’s rated single-phase input current.
4. Sum simultaneous loads. Calculate VAtotal = VA1 + VA2 + VA3. If operating modes prevent simultaneous use, document and enforce that interlock before using a reduced demand value.
5. Apply the working allowance. Calculate VAtransformer ≥ 1.5 × VAtotal. The documented result is close to 4,000 VA; select an available transformer rating that meets or exceeds the calculated requirement and is approved by its manufacturer for VFD loading.
6. Calculate primary current. Use Iprimary = VAtransformer ÷ 120 V. A 4 kVA transformer produces an ideal result of 33.3 A; incorporate transformer losses and nameplate primary current when selecting the feeder.
7. Distribute the secondary correctly. Feed the transformer from the 120 V primary circuit, then divide the stepped-up secondary into separately protected 240 V branches for the three VFD inputs. Each drive still needs protection and isolation appropriate to its input rating.

Loaded-System Verification

Complete verification with the machines under the operating condition that creates the greatest simultaneous demand. Use instruments suitable for nonsinusoidal VFD input current, and compare measurements with equipment nameplates rather than relying only on calculated ideal values.

1. Record primary voltage and current with the transformer unloaded.
2. Start one drive at a time and record primary voltage, secondary voltage, transformer current, and that drive’s input current.
3. Run all permitted drives together and repeat the measurements during acceleration and steady operation.
4. Check each VFD’s diagnostic history for input undervoltage, input loss, or other supply-related events.
5. Inspect terminations and monitor transformer, conductor, breaker, and enclosure temperature. Compare temperature and voltage results with the limits published on the installed equipment.

A passing test holds the secondary within every VFD’s specified input range, keeps measured current within the installed equipment ratings, completes acceleration without supply-related events, and shows no abnormal heating or connection discoloration.

Recurring Sizing and Wiring Pitfalls

• Using watts instead of volt-amperes: transformer loading is rated in VA or kVA. The stated 1 HP describes motor mechanical output and cannot size the transformer directly.
• Reading the wrong current: use VFD rated input line current, not motor current or VFD output current.
• Ignoring voltage terminology: 110 V, 120 V, 220 V, and 240 V labels cannot replace measurement and nameplate-range checks.
• Treating the 1.5 factor as universal: it is the working allowance used for this calculation. Apply any different loading or derating requirement published by the selected transformer manufacturer.
• Reusing the ideal 33.3 A everywhere: it excludes transformer losses and does not independently establish breaker or conductor sizes.
• Splitting the wrong voltage: after stepping up, the VFD branches originate on the 240 V secondary, not as three 120 V branches.
• Overlooking the service: a dedicated feeder can still exceed the building service’s available capacity when other loads are operating.

Frequently Asked Questions