The HT to LT Current Calculator is an online electrical calculator designed to calculate primary (High Tension-HT) and secondary (Low Tension-LT) currents for electrical power transformers.
Calculator
HT to LT Current Calculator
High Tension to Low Tension Transformer Current Converter
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It supports both 3 phase and single phase (1 phase) distribution systems commonly utilized in industrial and commercial electrical installations.
This calculator is essential for
- Electrical engineers in designing the transformer installations.
- Technicians/Electricians verifying the transformer ratings and protection settings.
- Energy auditors used to assess load distribution.
- Students learning power systems engineering.
The calculator assumes an ideal transformer model with efficiency applied only to LT side output.
3-phase detection is based on LT voltage threshold (>= 380 V). Adjust LT voltage accordingly for the system.
For HT calculations, efficiency correction is not applied on the primary side – losses are represented on the secondary.
Transformer magnetizing current and no load losses are not considered in this online calculator.
Results are rounded to 4 decimal places for ratios & 2 decimal places for currents.
Input Parameters
The calculator used to accepts the following inputs
| Parameter | Typical Values | Description |
| HT Voltage (V) | 11000 / 33000 V | Primary winding voltage of the transformer |
| Power (kVA) | 25 / 100 / 500 kVA | Transformer nameplate apparent power rating |
| Power Factor | 0.8 to 1.0 (as a default 0.9) | Ratio of real power to apparent power |
| Efficiency (%) | 96 to 99% (which is typical) | Ratio of output power to input power |
| LT Voltage (V) | 230 / 415 / 440 V | Secondary winding output voltage |
Transformer Types
The calculator provides the accurate efficiency values based on the transformer construction type.
| Type | Efficiency | Best Use Case |
| Oil Immersed | 98% | Industrial substations and outdoor distribution |
| Dry Type | 96% | Indoor installations and fire sensitive environments |
| Amorphous Core | 99% | Energy efficient applications and green buildings |
Users shall also enter a custom efficiency value for non standard transformer designs.
Calculation Formulas
System Type Detection
The calculator used to automatically determine the supply system type based on LT voltage.
- 3 Phase system: LT voltage >= 380 V (e.g., 440 V, 415 V and 380 V)
- Single-Phase system: LT voltage < 380 V (e.g., 230 V, 110 V)
HT Current (Primary Side)
For 3-Phase systems
IHT = (S x 1000) / (VHT x √3 x PF)
For Single-Phase systems
IHT = (S x 1000) / (VHT x PF)
LT Current (Secondary Side)
For 3-Phase systems (with efficiency correction)
ILT = (S x 1000 x Efficiency) / (VLT x √3 x PF)
For Single-Phase systems (with efficiency correction):
ILT = (S x 1000 x Efficiency) / (VLT x PF)
Transformation Ratios
Voltage Transformation Ratio
Voltage Ratio = VHT / VLT
Current Transformation Ratio:
Current Ratio = IHT / ILT
Where
S – Apparent power in kVA (transformer rating)
VHT – High Tension (primary) voltage in Volts
VLT – Low Tension (secondary) voltage in Volts
PF – Power Factor (dimensionless, 0 to 1)
√3 – Square root of 3 = 1.732 (for 3 phase systems)
Efficiency – Transformer efficiency as a decimal (e.g., 0.96 for 96%)
Solved Example
Below is a worked example using the calculator for a typical industrial transformer installation.
| Input Parameter | Value |
| HT Voltage | 11000 V (11 kV) |
| Power Rating | 100 kVA |
| Power Factor | 0.9 |
| Efficiency | 96% |
| LT Voltage | 440 V (3-Phase) |
| HT Current (Primary) | 5.25 A |
| LT Current (Secondary) | 127.02 A |
Step-by-Step Calculation
HT Current (3-Phase)
IHT = (100 x 1000) / (11000 x 1.732 x 0.9) = 100000 / 17146.8 = 5.25 A
LT Current (3-Phase with 96% efficiency)
ILT = (100 x 1000 x 0.96) / (440 x 1.732 x 0.9) = 96000 / 685.9 = 127.02 A
Voltage Ratio = 11,000 / 440 = 25 : 1
Current Ratio = 5.25 / 127.02 = 0.0413 (approximately 1:24)
Supported LT Voltage Options
230 V – Single Phase (standard residential/commercial supply)
440 V – 3-Phase (standard Indian industrial supply)
415 V – 3-Phase (BS standard industrial supply)
380 V – 3-Phase (IEC standard industrial supply)
110 V – Single Phase (North American / aviation standard)
127 V – Single Phase (some European systems)
Custom – User-defined voltage for special applications