Voltage Drop & Circuit Screening Planner
Estimate resistive voltage loss from current, one-way length, conductor area, resistivity and circuit arrangement.
Use the Voltage Drop Calculator
Estimate resistive voltage loss from current, one-way length, conductor area, resistivity and circuit arrangement.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary engineering calculation only. It does not replace applicable codes, manufacturer data, licensed electrical or mechanical design, protection studies, derating, fault checks, site conditions or professional verification.
How to use it
Replace the worked-example values with the actual project inputs. Review the intermediate quantities and assumptions before using the result elsewhere.
How the Voltage Drop Calculator works
The Voltage Drop Calculator estimates resistive voltage drop along a conductor using current, one-way length, conductor area, resistivity and circuit arrangement.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Voltage drop formula used by this screening calculator
ToolLott applies a simplified resistive conductor model. Single-phase/DC uses a factor of 2 for the outgoing and return path; balanced three-phase uses the square-root-of-3 factor.
Delta V = 2 x rho x L x I / AThe one-way length L is doubled by the single-phase/DC circuit factor.
Delta V = sqrt(3) x rho x L x I / AThis simplified mode ignores conductor reactance and power factor.
Drop % = Delta V / V_system x 100The percentage is referenced to the entered system voltage.
| Symbol / input | Meaning | Unit |
|---|---|---|
rho | Conductor resistivity | ohm mm^2/m |
L | One-way cable length | m |
I | Load current | A |
A | Conductor cross-sectional area | mm^2 |
V_system | Entered system voltage | V |
Step-by-step method
- Select single-phase/DC or balanced three-phase.
- Calculate resistive conductor drop from resistivity, current, length and cross-sectional area.
- Divide the voltage drop by system voltage to express it as a percentage.
- Subtract the estimated drop from system voltage to show a screening receiving voltage.
Worked example
A 230 V single-phase circuit carries 20 A over a 35 m one-way run using a 6 mm^2 conductor and a resistivity input of 0.0175 ohm mm^2/m.
Example inputs
- System voltage = 230 V
- Current = 20 A
- One-way length = 35 m
- Area = 6 mm^2
- Resistivity = 0.0175 ohm mm^2/m
- Circuit = single-phase/DC
Calculation / processing
Delta V = 2 x 0.0175 x 35 x 20 / 6Delta V = 4.083 VDrop % = 4.083 / 230 x 100 = 1.775%Estimated receiving voltage = 230 - 4.083 = 225.917 V
The result is a resistive screening estimate. It helps compare candidate conductor areas, but it is not a complete cable-sizing or installation-compliance calculation.
Assumptions
- The entered resistivity represents the conductor condition the user wants to screen.
- The three-phase mode assumes a balanced circuit.
- The model treats the circuit as resistive and does not calculate reactance or power-factor effects.
Limitations
- Cable installation method, conductor temperature, grouping/derating, protection, fault level and applicable wiring rules are outside this calculation.
- Real AC voltage drop can require resistance and reactance values appropriate to conductor construction and operating conditions.
- Use the applicable electrical standard and qualified design review for final conductor selection.
Common questions
Why is one-way cable length entered?
The circuit factor handles the return path for the single-phase/DC screening formula, so the user enters the physical one-way run length.
Why can the real voltage drop differ from this result?
AC conductor resistance changes with temperature and real circuits can include reactance, power factor, harmonics and installation effects.
Can this tool approve a cable size?
No. It is a transparent voltage-drop screening calculation, not a standards-compliance or cable-protection design tool.
Methodology sources
Related ToolLott tools
A realistic way Nina could use this tool
Nina is a project engineer.
Nina has a 230 V circuit carrying 20 A over a 35 m one-way copper run and needs to test candidate conductor sizes against a voltage-drop limit.
The scenario is tested with voltage = 230; current = 20; length = 35; area = 6; resistivity = 0.0175; phase = “single”. Those entries lead to Voltage drop: 4.083 V (1.775%). Because the inputs and result are both shown, Nina can compare the example with real project/source data before reuse.
The worked result is Voltage drop: 4.083 V (1.775%); in this situation that lets Nina test candidate conductor sizes against a voltage-drop limit. If the underlying values change, the example makes clear which inputs need to be updated rather than hiding the assumption. The page also keeps this limitation explicit: Preliminary screening or quantity calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this calculator is for
Use it to estimate resistive voltage loss from current, one-way length, conductor area, resistivity and circuit arrangement.
It sits within ToolLott’s Electrical collection, where you can also estimate battery banks, cable sizes, generator sizes, motor current, Ohm’s law values, solar systems, transformers and voltage drop.