SpecCalc Hub
Maximum Cable Length Calculator: Voltage-Drop Limit
Estimate one-way cable length for a chosen voltage-drop limit. This is a voltage-drop calculation, not an ampacity or protection approval.
Preliminary estimate only. Not a code-compliance document, not installation approval, not final engineering design. Verify with qualified professional and local codes.
Enter the values
Client-side estimate. SpecCalc Hub does not store entered values.
Formula: L = (V x drop% x A) / (factor x I x rho)
Reference formula
This simplified formula block is educational and depends on the page status and limitations.
L = (V x drop% x A) / (factor x I x rho)
Assumptions
- Inputs are user-provided.
- The result is an informational estimate.
Limitations
- Professional verification is required before real-world use.
What this page helps with
Estimate one-way cable length for a chosen voltage-drop limit. This is a voltage-drop calculation, not an ampacity or protection approval.
Find the maximum one-way run allowed by your chosen voltage-drop percentage for an already selected conductor and load.
Formula or method
Use Lmax = ΔVallowed × A / (k × I × ρ). k is 2 for a two-wire circuit and √3 for the simplified balanced three-phase model. Convert a percent limit into volts first.
The working method on this page uses inputs such as System type, Voltage, Current, and Conductor material and reports outputs such as Maximum one-way length, Voltage drop limit, and Circuit factor. The formula, assumptions and limitations stay visible so the result can be reviewed instead of simply trusted.
This makes the page useful for comparison and documentation, but it does not automatically include every manufacturer coefficient, installation condition, environmental factor or local-code requirement that may matter in a real project.
How to use
- Enter the page inputs, starting with System type, and confirm that the units match the real scenario you are checking.
- Read the main output Maximum one-way length together with the other reported values instead of focusing on only one number.
- Before a real decision, review the assumptions, limitations, source links and related tools on the same page.
Practical examples
A common question, worked through
For 230 V, a chosen 3% drop, 16 A, 2.5 mm² copper and ρ = 0.017241 Ω·mm²/m: ΔV = 6.9 V and one-way length is about 31.266 m. The 3% limit is an example, not a universal rule.
Worked example with the form defaults
Example inputs: System type: Single-phase AC; Voltage: 230 V; Current: 16 A; Conductor material: Copper; Cross-section: 2.5 mm²; Max voltage drop: 3 %. L = (V x drop% x A) / (factor x I x rho). Calculated outputs: Maximum one-way length: 31.266 m; Voltage drop limit: 6.9 V; Circuit factor: 2.
Common mistakes
- Using the page without checking units, document revision or the real operating context.
- Treating a preliminary result or reference table as a final engineering decision.
- Ignoring the source links, limitations and related tools needed for the next validation step.
Limitations
- Do not double the input length again. Temperature, installation, reactance, protective-device operation and local limits require separate checks.
- The percentage is a user-selected design target, not a statement of local code. Temperature, starting current and protection may require a shorter run or a larger conductor.
- Professional verification is required before real-world use.
- The page does not replace official manufacturer documentation, local rules or site measurements.
- Separate professional verification is required for safety, protection, cable or installation decisions.
FAQ
Can I enter VA instead of amperes?
No. For single-phase apparent power, I = S / V; for balanced three-phase apparent power, I = S / (√3 × Vline). A value in watts also requires power-factor context.
Is the returned length one way or out and back?
It is the source-to-load, one-way route length. The DC and single-phase model already accounts for the outward and return conductors; do not double the route length again.
Does meeting the voltage-drop limit make the cable safe?
No. Current capacity, installation method, temperature, protection and fault-disconnection conditions require separate checks. The selected percentage is an input, not a universal installation rule.
Related tools
Explanation updated: 2026-09-16
Graphics
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.