Star Delta Motor Cable Size Calculator: Best Sizing Tool
Use the Star Delta Motor Cable Size Calculator below to quickly determine the correct cable sizes for both the line circuit and the motor winding circuit. Unlike a standard motor cable size calculator, this tool calculates two different cable sizes because a star-delta starter operates with different current values in different sections of the circuit.
Table of Contents
Table of Contents
Star Delta Motor Cable Size Calculator
Star Delta Motor Cable Size Calculator
Determine the correct incomer cable and motor terminal cable sizes for star-delta started induction motors, including current derating, ampacity selection and voltage drop verification.
Motor & Installation Data
Results
The incomer cable is sized on full line current. The six motor terminal cables are sized on winding (phase) current, which is lower than line current, since star-delta wiring splits each phase winding into two ends brought out separately to the starter.
Ampacity Reference Used
Base current ratings applied in this calculator (three loaded copper conductors, PVC insulation, 30 C ambient, Method B1), before material, insulation and installation adjustments:
| mm2 | Amps | mm2 | Amps |
|---|---|---|---|
| 1.5 | 17.5 | 50 | 144 |
| 2.5 | 24 | 70 | 184 |
| 4 | 32 | 95 | 223 |
| 6 | 41 | 120 | 259 |
| 10 | 57 | 150 | 299 |
| 16 | 76 | 185 | 341 |
| 25 | 96 | 240 | 403 |
| 35 | 119 | 300 | 464 |
Technical Details
A star-delta starter brings out both ends of each of the three motor windings to the starter panel, giving six conductors instead of three. During starting, the contactors connect these winding ends in star configuration; once the motor accelerates, a second contactor reconnects them in delta for normal running. Because these six conductors always carry winding current rather than the full line current, they can be sized smaller than a direct-on-line motor cable of the same rating, provided the reduction is verified against the actual winding current rather than assumed.
The incomer cable, from the protective device to the starter panel, is not part of this six-cable arrangement. It carries the full line current at all times, including during the brief starting transient, and must always be sized on that basis.
Cable ampacity from the reference table is adjusted for four site conditions before a size is selected: ambient temperature, the number of circuits grouped in the same tray or conduit, the installation method, and the conductor and insulation combination chosen. Each of these factors reduces or increases the safe current-carrying capacity relative to the base 30 C, single-circuit, conduit-installed reference condition. The combined derating factor is shown in the results panel so the margin between the selected cable and the calculated demand can be reviewed.
Aluminium conductors are rated lower than copper of the same cross-section, and XLPE-insulated cables are rated higher than PVC due to their higher permitted operating temperature. Both effects are built into the size selection.
Ampacity and voltage drop reference values used in this tool are drawn from tabulated data consistent with IEC 60364-5-52 (current-carrying capacity of cables) and IEC 60909 principles for motor circuit current behaviour. Where local practice follows NEC, BS 7671, or a national wiring regulation, the referee values in those documents should be used to confirm the final cable size, since permitted grouping, temperature, and installation factors vary between standards.
How to Use This Calculator
- Enter the motor rating in kW or HP, and select the supply voltage, or choose Custom to type a non-standard value.
- Enter the motor’s full load efficiency and power factor from its nameplate or datasheet. Typical defaults are provided if these are not yet known.
- Enter the one-way cable route length from the starter panel to the motor.
- Select the conductor material, insulation type, and installation method that match the actual site conditions.
- Set the ambient temperature and the number of circuits grouped together in the same containment.
- Choose an acceptable voltage drop limit, then press Calculate Cable Sizes.
- Review the incomer cable size, the motor terminal cable size, and both voltage drop results before finalising a selection.
Why Star-Delta Motors Need Different Cable Sizing
A standard Direct-On-Line (DOL) starter uses one set of cables that carries the full motor line current. Cable sizing is therefore straightforward because every conductor is designed for the same current.
A star-delta starter works differently. It contains two separate cable runs.
The first cable run connects the power source to the starter and carries the full line current.
The second cable run connects the starter to the motor windings. These cables carry only the phase current, which equals approximately 58% of the line current.
Use our online tool motor cable size calculator (for DOL/non-star-delta sizing)
This difference is the main reason a generic motor cable size calculator cannot accurately size star-delta installations.
| Cable Run | Current Carried | Typical Size |
|---|---|---|
| Supply to Starter | Full Line Current (IL) | Larger |
| Starter to Motor Windings | Phase Current (Iph = IL ÷ √3) | Usually Smaller |
Because the winding cables carry less current, they often require a smaller conductor while still complying with installation standards.
The Star Delta Motor Cable Size Calculator automatically performs this calculation, eliminating manual errors during cable selection.
Line Cable vs. Winding Cable: What’s the Difference?
Understanding the cable routing makes star delta current calculation much easier.
The electrical path follows this sequence:
Power Supply
↓
Line Cables
↓
Star-Delta Starter (Main, Star & Delta Contactors)
↓
Winding Cables
↓
Motor Terminals (U1, V1, W1, U2, V2, W2)
The line cables deliver power from the distribution panel to the starter.
The winding cables connect the contactors to each motor winding.
Since the winding conductors feed individual windings instead of the complete motor load, they carry phase current rather than line current.
The table below highlights the difference.
| Feature | Line Cable | Winding Cable |
|---|---|---|
| Connected Between | Supply and Starter | Starter and Motor |
| Current | Line Current | Phase Current |
| Cable Size | Larger | Smaller in many cases |
| Protection Basis | Full Load Current | Winding Current |
This distinction is one of the biggest advantages of using a dedicated star delta motor cable size calculator instead of a general 3 phase motor cable size calculator.
How to Use This Calculator
Using the calculator requires only a few inputs.
- Enter the motor rating in kW or HP.
- Select the supply voltage.
- Confirm that the motor is a 6-terminal star-delta motor.
- Run the calculation.
- Read the calculated line current.
- Read the calculated phase current.
- View the recommended cable sizes for both cable runs.
The calculator displays two cable size recommendations because both circuits carry different currents.
For larger industrial motors such as a 55 kw motor cable size application, this distinction can significantly reduce unnecessary cable costs while maintaining safe operation.
Always verify the final cable selection against:
- Installation method
- Ambient temperature
- Grouping factors
- Cable insulation type
- Voltage drop limits
- Local electrical standards
Star-Delta Cable Sizing Formula
The calculator is based on standard three-phase motor equations.
Line Current
IL = P / (√3 × V × PF × η)
Use our online tool kW to Cable Size Calculator
Where:
- IL = Line Current (A)
- P = Motor Power (Watts)
- V = Line Voltage
- PF = Power Factor
- η = Motor Efficiency
Winding (Phase) Current
Iph = IL ÷ √3
Since:
√3 ≈ 1.732
Therefore,
Iph ≈ 0.577 × IL
This means the winding current is approximately 58% of the line current.
The table below summarizes the relationship.
| Parameter | Formula |
|---|---|
| Line Current | IL = P / (√3 × V × PF × η) |
| Phase Current | Iph = IL ÷ √3 |
| Current Ratio | Iph ≈ 0.577 × IL |
These equations form the basis of every accurate star delta motor current calculation.
Know more about star delta starter starting current reduction factor
Worked Example
Consider a 37 kW motor operating on a 400 V three-phase supply.
Assume:
- Motor Power = 37 kW
- Voltage = 400 V
- Power Factor = 0.85
- Efficiency = 92%
Step 1: Calculate Line Current
IL = 37,000 ÷ (1.732 × 400 × 0.85 × 0.92)
IL ≈ 68 A
Step 2: Calculate Phase Current
Iph = 68 ÷ 1.732
Iph ≈ 39 A
Step 3: Select Cable Sizes
| Circuit | Current | Typical Cable Size* |
|---|---|---|
| Line Cable | 68 A | 25 mm² Copper |
| Winding Cable | 39 A | 10 mm² or 16 mm² Copper |
*Actual cable size depends on installation method, ambient temperature, grouping factor, voltage drop, insulation type, and applicable electrical standards.
Know more about difference between dol and star delta starter
This example clearly demonstrates why both cable sizes are different. If the line current were incorrectly applied to the winding cables, larger conductors would be installed unnecessarily, increasing project cost.
Common Mistakes When Sizing Star-Delta Motor Cables
Even experienced installers occasionally make mistakes when working with star-delta starters.
The most common errors include:
| Mistake | Result |
|---|---|
| Using line current for both cable runs | Oversized winding cables and higher cost |
| Using phase current for the supply cable | Undersized line cable and overheating |
| Ignoring installation correction factors | Incorrect current capacity |
| Selecting cable based only on motor power | Poor protection coordination |
| Ignoring voltage drop | Reduced motor performance |
| Using DOL calculations for star-delta motors | Incorrect cable selection |
Another common issue occurs during replacement projects where only the motor rating is considered without verifying the starter configuration.
Always confirm that the motor has six accessible terminals before performing any star delta current calculation.
Find more about nema starter sizes based on motor horsepower and voltage
Star-Delta vs. DOL Cable Sizing
Although both systems power three-phase motors, their cable sizing requirements are different.
| Feature | Star-Delta Starter | DOL Starter |
|---|---|---|
| Number of Cable Runs | Two | One |
| Current Levels | Line & Phase | Line Only |
| Cable Sizes | Usually Two Different Sizes | One Size |
| Starter Complexity | Higher | Lower |
| Starting Current | Reduced | Higher |
A DOL installation requires only one cable size because every conductor carries the full motor current.
A star-delta installation requires separate calculations for:
- Incoming line cable
- Outgoing winding cable
If your motor uses a Direct-On-Line starter, use a dedicated motor cable size calculator instead. This page is specifically intended for star-delta starter applications where two current values must be evaluated.
Know more about Star Delta Starter Diagram with Timer
Frequently Asked Questions
Do star-delta motors need two different cable sizes?
Yes. The incoming line cable carries the full line current, while the cables between the starter and the motor windings carry only the phase current. As a result, the winding cable can often be smaller.
What is the current ratio between line and winding cables in a star-delta starter?
The winding current equals the line current divided by √3. This is approximately 57.7% or about 58% of the line current.
Can I use the same cable size for both line and winding connections?
You can, but it is often unnecessary and increases material cost. A proper Star Delta Motor Cable Size Calculator calculates both cable sizes independently for a more economical and technically correct design.
How do I size cables for a star-delta starter conversion?
First calculate the motor line current using the motor rating, voltage, efficiency, and power factor. Then calculate the winding current by dividing the line current by √3. Select the line cable using the line current and the winding cable using the calculated phase current, while also considering voltage drop, installation method, correction factors, and applicable electrical standards.
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