Delta to Star Resistance Conversion Calculator
Equivalent star resistors for a delta network of three resistors. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates ra, rb and rc from rab, rbc and rca. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
For a balanced delta, each star resistor is one third of the delta resistor.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: Textbook circuit identity (Ohm's law, power, series/parallel, AC circuit theory); no governing standard |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Rab | 30 Ω |
| Rbc | 60 Ω |
| Rca | 90 Ω |
- Σ = 180 Ω
- Ra = 15 Ω, Rb = 10 Ω, Rc = 30 Ω
Assumptions
- Node A joins Rab and Rca, node B joins Rab and Rbc, node C joins Rbc and Rca
For a balanced delta, each star resistor is one third of the delta resistor.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Rab | Ω | greater than 0 Ω | |
| Rbc | Ω | greater than 0 Ω | |
| Rca | Ω | greater than 0 Ω |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Ra (at node A) | Ω | Shown to 3 decimal places | |
| Rb (at node B) | Ω | Shown to 3 decimal places | |
| Rc (at node C) | Ω | Shown to 3 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with rab
| Rab (Ω) | Ra (Ω) | Rb (Ω) | Rc (Ω) |
|---|---|---|---|
| 3 | 1.765 | 1.176 | 35.294 |
| 7.5 | 4.286 | 2.857 | 34.286 |
| 15 | 8.182 | 5.455 | 32.727 |
| 30 | 15 | 10 | 30 |
| 60 | 25.714 | 17.143 | 25.714 |
| 120 | 40 | 26.667 | 20 |
| 300 | 60 | 40 | 12 |
Other inputs are the same as in the worked example above.
Standards and further reading
These are textbook relationships (Ohm's law, series and parallel networks, AC phasors, three-phase quantities). No standard sets the method; standards apply when the result feeds a design, for example cable or protection selection.
Frequently asked questions
What do I need to enter?
Enter rab, rbc and rca. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Node A joins Rab and Rca, node B joins Rab and Rbc, node C joins Rbc and Rca.
Which calculators should I use next?
Common next steps are Resistors in Series and Resistors in Parallel. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Delta-Star Resistor Conversion Formulas: Delta to star and star to delta resistor formulas: the equivalent star resistors of a delta network
- Star-Delta Conversion Formulas for Resistors: Star-delta (wye-delta) conversion formulas for resistors or impedances in both directions, equal-resistor shortcut
More Electrical Fundamentals calculatorsAll engineering calculatorsDelta to Star Resistance Conversion in Elecmetric