Calculator
Formula Used
The main resistance formula uses Ohm’s law:
R = Vdrop ÷ I
For a two conductor circuit, per wire resistance is:
Rwire = Vdrop ÷ 2I
For a three phase circuit, per conductor resistance is:
Rline = Vdrop ÷ √3I
Voltage drop percentage is:
Drop % = Vdrop ÷ Vsupply × 100
Power loss is:
P = Vdrop × I
Estimated conductor resistance is:
R = ρL ÷ A
How to Use This Calculator
- Enter the supply voltage and load voltage.
- Enter known voltage drop if it is already measured.
- Enter current in amperes.
- Select the circuit type.
- Add cable length, area, material, and temperature if needed.
- Enter your allowed voltage drop percentage.
- Press Calculate to view resistance and power loss.
- Use CSV or PDF buttons to save the result.
Example Data Table
| Supply Voltage | Voltage Drop | Current | Resistance | Power Loss | Drop Percent |
|---|---|---|---|---|---|
| 120 V | 2.4 V | 10 A | 0.24 Ω | 24 W | 2% |
| 230 V | 6 V | 15 A | 0.40 Ω | 90 W | 2.61% |
| 480 V | 14.4 V | 30 A | 0.48 Ω | 432 W | 3% |
About Resistance From Voltage Drop
Why Voltage Drop Matters
Voltage drop is the lost voltage between a source and a load. It appears when current flows through wire, terminals, connectors, fuses, switches, or bus bars. Every conductive path has resistance. That resistance turns part of the electrical energy into heat. A small drop may be normal. A high drop can reduce performance. Motors may run hot. Lights may dim. Electronics may reset. Batteries may seem weak. Long cable runs make the problem larger.
How Resistance Is Found
This calculator applies Ohm’s law to the measured loss. Divide voltage drop by current. The answer is the equivalent resistance in the current path. You can also estimate each conductor resistance. Single phase and two wire DC circuits usually use the outgoing and return conductor. Three phase circuits use a square root of three factor. These values help compare measured loss with expected cable loss.
Using Advanced Inputs
The extra fields help with deeper checks. Cable length and area estimate theoretical resistance. Material changes resistivity. Copper has lower resistance than aluminum. Temperature also matters. Hot conductors have higher resistance. The tool adjusts resistivity by the entered temperature coefficient. This gives a practical estimate for loaded conductors. It is useful when checking feeders, panels, solar wiring, vehicles, battery banks, and control circuits.
Reading the Result
Start with equivalent resistance. Then check power loss. Power loss shows heat created by the drop. Next review the voltage drop percentage. Many designs use a selected percent limit. The calculator compares your result with that limit. If the value is high, reduce current, shorten the run, improve terminals, or use a larger conductor. Always follow local electrical codes. Use proper instruments. De-energize circuits when required. For critical systems, ask a qualified electrician or engineer before changing equipment.
FAQs
1. What is resistance from voltage drop?
It is the equivalent resistance causing voltage loss in a circuit. It is found by dividing measured voltage drop by current.
2. Which formula does this calculator use?
It mainly uses R = voltage drop divided by current. It also calculates power loss, percent drop, and conductor estimates.
3. Can I leave voltage drop blank?
You can enter zero for known voltage drop. The tool will use supply voltage minus load voltage when both values are entered.
4. What does power loss mean?
Power loss is energy converted into heat by resistance. It equals voltage drop multiplied by current.
5. Why does circuit type matter?
Circuit type changes how conductor resistance relates to measured voltage drop. Three phase, single phase, and one way paths differ.
6. What conductor area unit should I use?
Use square millimeters for conductor area. The resistivity values in this calculator are based on Ω·mm²/m.
7. Why include temperature?
Conductor resistance rises with temperature. The temperature field helps estimate resistance under warmer operating conditions.
8. Is this suitable for final electrical design?
It is useful for planning and checking. Final designs should follow electrical codes, manufacturer data, and qualified professional review.