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Voltage Drop Calculator: Check Compliance for DC, Single-Phase and Three-Phase Circuits

A cable can be rated to carry your current just fine and still be the wrong choice – if the run is long, the voltage at the far end drops enough to affect motors, dim lights or trip sensitive equipment. This voltage drop calculator works for DC, single-phase and three-phase circuits, using temperature-corrected resistance for accuracy. It also has a reverse mode – instead of just checking one cable size, it tells you the minimum size needed to stay compliant and shows a comparison table of nearby sizes so you can see the trade-off.

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How to Use

1. Choose your mode – Check This Cable to test a specific size or Find Minimum Size for reverse mode.
2. Select the system type (Three-phase AC, Single-phase AC, or DC) and enter the voltage.
3. Enter your load as current (A) or switch to power (kW) mode and set the power factor for AC systems.
4. In “Check This Cable” mode, select the cable size; in reverse mode, skip this – the tool finds it.
5. Choose conductor (Copper/Aluminium), temp rating (70°C PVC or 90°C XLPE) and enter the length (one-way – return path is added automatically).
6. Select your drop limit – 5% power, 3% lighting (as per IS 732) or set a custom limit.
7. Click Calculate to see the result, compliance status and a comparison table of nearby cable sizes.

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Key Features

• Works for DC, single-phase AC and three-phase AC in one tool
• Temperature-corrected resistance – accounts for actual conductor operating temperature, not a fixed value
• Splits voltage drop into resistive (Vr) and reactive (Vx) components
• Reverse mode – tells you the minimum cable size instead of checking one at a time
• Side-by-side comparison table of nearby cable sizes
• Automatic return-path handling – 2× length for DC/single-phase, √3 factor for three-phase
• IS 732-based compliance limits (5% power, 3% lighting) with custom limit option
• Export as PDF or copy the result

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Formula / Logic Used

Temperature-Corrected Resistance$$R = \frac{R_{20}}{Size} \times \left[1 + \alpha \times (T_{rating} – 20)\right]$$

Where $R_{20}$​ is the conductor resistance constant at 20°C (Copper/Aluminium specific), $\alpha$ is the temperature coefficient of resistance and $T_{rating}$​ is the cable’s rated operating temperature (70°C PVC or 90°C XLPE).

Voltage Drop$$V_{drop} = k \times I \times L \times (R\cos\phi + X\sin\phi) / 1000$$

Where $k$ = 2 for DC/single-phase (return conductor) or √3 for three-phase, $I$ is load current, $L$ is one-way length, $R$ is temperature-corrected resistance and $X$ is reactance (~0.08 Ω/km for AC, 0 for DC).$$Percentage\ Drop = \frac{V_{drop}}{System\ Voltage} \times 100$$

Reverse Mode Logic

In “Find Minimum Size” mode, the tool tests each standard cable size from the smallest upward and picks the first one whose percentage drop falls within your compliance limit.

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Who Should Use This Tool

Electrical site engineers checking long cable runs – motor feeders, street lighting, solar DC wiring – before installation. Also useful for diploma and B.Tech Electrical students learning IS 732 limits and the resistive/reactive drop split.

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Frequently Asked Questions (FAQs)

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