Voltage drop equation
You can estimate cable voltage drop from load current, route length, conductor material, and conductor cross-section.
Vd = 2 × I × ρ × L / SVd = √3 × I × ρ × L / SVd% = Vd / Vn × 100where:
- Vd
- Calculated voltage dropV
- Vd%
- Voltage drop as a percentage of nominal voltage%
- I
- Load currentA
- ρ
- Conductor resistivity referenceohm mm2/m
- L
- One-way cable lengthm
- S
- Conductor cross-sectional areamm2
- Vn
- Nominal system voltageV
The result excludes reactance, harmonics, operating temperature, and installation correction factors, so long feeder runs need a standards-based cable check.
Assumptions
- Approximate conductor resistivity at normal temperature
- Reactance and installation temperature effects are not included
- One-way length is used
Important Warnings
- Use project voltage-drop limits and local electrical code for final design.
- Long runs, high temperature, and grouped cables need additional derating.
FAQ
Why does the tool ask for one-way length?
The formula adds the return path internally for DC and single-phase circuits. Enter the physical one-way cable run.
Does this replace a full cable sizing calculation?
No. It checks voltage drop only. You still need ampacity, protection coordination, installation method, and fault-current checks.