Buck Boost Transformer Sizing Calculator
Size your buck-boost transformer in 2 methods — by load power or load current — with built-in NEC 450-4
overcurrent protection sizing.
7 Common Presets
NEC 450-4 OCP Sizing
Cost Savings vs Isolation
Example: 28A load, 208V→230V: kVA
= 28 × 22 / 1000 = 0.616 kVA
The equivalent isolation transformer would need 6.44
kVA — the buck-boost autotransformer is 90% smaller for the same job.
Two Methods to Size a Buck Boost Transformer
Unlike standard distribution transformers sized purely by load kVA, a buck-boost transformer can be wired in
two fundamentally different ways — and each requires a different sizing formula:
Method 1: Load Power (Isolation Mode)
When the buck-boost is connected as a standard isolation transformer (e.g., 120V
primary, 24V secondary for low-voltage control). Full load current flows through the secondary. kVA
= I × V_secondary / 1000.
Method 2: Load Current (Autotransformer Mode)
When field-connected as an autotransformer (most common). Only the voltage
difference flows through the transformer — kVA = I × ΔV / 1000. This is dramatically
smaller than the load kVA.
I’ve seen installers use Method 1 for an autotransformer application and oversize the transformer by
8–10×. A job that needed a 1 kVA buck-boost ended up with a 10 kVA unit because someone calculated load
kVA instead of buck/boost kVA. Always use Method 2 (Load Current × ΔV) when connecting as an
autotransformer. The buck-boost transformer is shipped from factory as an isolation
transformer but is always field-connected as an autotransformer — that’s why manufacturers show the
nameplate kVA as the isolation rating.
Common Buck Boost Applications & kVA Sizing Reference
These are the most common voltage correction combinations seen in HVAC, lighting, and industrial
applications. All values for single-phase; multiply load current by actual load amps for your project.
| Application | Line V → Load V | ΔV | %Change | Auto kVA (at 28A) | Isolation kVA equiv. | Savings |
|---|---|---|---|---|---|---|
| A/C, Refrigeration | 208V → 230V | 22V | 10.6% | 0.616 kVA | 6.44 kVA | 90% |
| General Equipment | 208V → 240V | 32V | 15.4% | 0.896 kVA | 6.72 kVA | 87% |
| Motor Correction | 240V → 208V | 32V | 13.3% | 0.896 kVA | 5.82 kVA | 85% |
| Lighting Boost | 120V → 132V | 12V | 10.0% | 0.336 kVA | 3.70 kVA | 91% |
| Line End Correction | 240V → 264V | 24V | 10.0% | 0.672 kVA | 7.39 kVA | 91% |
| Fluorescent Lighting | 240V → 277V | 37V | 15.4% | 1.036 kVA | 7.76 kVA | 87% |
| >20% — NOT suitable | 120V → 208V | 88V | 73.3% | Use isolation transformer instead | ||
All autotransformer kVA = 28A × ΔV / 1000. Isolation kVA = 28A ×
V_load / 1000. Savings = (1 − ΔV/V_load) × 100%.
NEC Article 450-4 — Overcurrent Protection for Autotransformers
The National Electrical Code (NEC) 450-4(a) has specific
overcurrent protection requirements for autotransformers:
General Rule: OCP device = Input Current × 125%, rounded up to next standard size (NEC
240-6)
Exception (≥9A input): If 125% doesn’t match a standard size, use the next higher
standard size
Exception (<9A input): OCP ≤ 167% of input current is permitted
Important: Do NOT install the OCP device in the shunt winding (the common winding
between input and output terminals)
| Application | Input Voltage | Auto kVA | Input Current | ×125% | Standard OCP Size |
|---|---|---|---|---|---|
| 208V→230V, 28A load | 208V, 1Φ | 0.616 kVA | 2.96A | 3.70A | 15A (≤9A → 167% rule) |
| 208V→240V, 25A load | 208V, 1Φ | 0.800 kVA | 3.85A | 4.81A | 15A |
| 208V→240V, 100A load | 208V, 1Φ | 3.200 kVA | 15.38A | 19.23A | 20A |
| 480V→504V, 60A 3Φ | 480V, 3Φ | 1.440 kVA | 1.73A | 2.17A | 15A (min circuit) |
| 240V→264V, 50A load | 240V, 1Φ | 1.200 kVA | 5.00A | 6.25A | 15A |
When to Use (and When NOT to Use) a Buck Boost Transformer
Use Buck-Boost When:
• Voltage correction needed: 5–20% adjustment
• Common: 208V→230/240V for HVAC
• End-of-line
voltage sag correction
• Low-voltage lighting (12V, 16V, 24V, 48V circuits)
• Space and cost
are constraints
• Motor runs on slightly off-spec voltage
Do NOT Use When:
• Voltage change >20% — use isolation transformer
• Electrical isolation is required (medical,
IT)
• Fluctuating supply voltage (doesn’t stabilize)
• Phase conversion needed
• Delta
circuit without neutral
• Sensitive electronics needing harmonic isolation
Frequently Asked Questions
where ΔV = |V_out − V_in|. Example: 28A load, 208V→230V: kVA = 28 × 22/1000 = 0.616 kVA → use 0.75 kVA
nameplate. Isolation mode: kVA = Load Current × V_secondary / 1000. Use our calculator
above for instant results with NEC 450-4 OCP sizing.
transformers?+
standard size per NEC 240-6. Exception: For input currents ≥ 9A where 125% doesn’t land on a standard
size, use next standard size up. For input <9A, OCP up to 167% is permitted. The OCP must NOT be
installed in the shunt (common) winding.
isolation transformer?+
primary/secondary windings). When field-connected as an autotransformer, it loses isolation but gains
enormous kVA capacity. A 1 kVA buck-boost in autotransformer mode can supply the same voltage correction
as a 10 kVA isolation transformer — at typically 75%+ less cost.