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Voltage Drop Calculator

Estimate actual conductor voltage drop, drop percentage, resistance, and power loss from current, one-way distance, wire size, conductor material, and system voltage.

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Wiring

Voltage drop calculator: estimate actual drop, percentage, and power loss for a chosen conductor

A voltage drop calculator estimates how much voltage is lost along a conductor run when you already know the wire size, current, one-way distance, conductor material, and system voltage. This version also compares the selected conductor with the minimum supported size needed to stay within your entered drop target.

What this voltage drop calculator solves

Use this page when you already have a conductor size in mind and want to see what it does in practice. The calculator returns the actual voltage drop in volts, the percentage drop relative to system voltage, the conductor resistance implied by the chosen size, and the power lost in the run.

It also calculates the circular-mil area required to stay within your chosen voltage-drop percentage and flags the smallest supported AWG or kcmil size that would meet that limit. That makes it a good partner to a wire-size estimator when you want to compare specific conductor choices instead of only asking for the minimum size.

Core voltage drop relationships

The calculator uses the circular-mil method for a two-conductor circuit. It doubles the one-way distance to reflect the full circuit path, applies a resistivity constant based on copper or aluminum, and then computes resistance, voltage drop, and power loss for the chosen conductor size.

Because the percentage limit is entered separately, the calculator can show both the actual behaviour of the selected conductor and whether that behaviour stays inside the design target you set.

Resistance = (2 x K x Distance) / Circular mils

K is the material resistivity constant used by the calculator, Distance is the one-way run in feet, and the factor of 2 accounts for the round-trip circuit length.

Voltage drop = Current x Resistance

Calculates the actual voltage lost along the run at the entered load current.

Power loss = Current x Voltage drop

Shows the power dissipated in the conductor as heat.

How to interpret the supported conductor table

The table shows the same current, distance, voltage, and material across a set of common AWG and kcmil sizes. That lets you compare how fast voltage drop falls as conductor area increases, without re-entering the scenario for every wire size.

When the selected conductor exceeds the target, the result panel points you to the minimum supported size that would satisfy the target. If the required circular-mil area is larger than the built-in table, the warning is a sign that the design should move into a more detailed engineering workflow.

What this result does not replace

Voltage drop is only one part of conductor selection. Final wiring design still has to account for ampacity, insulation temperature rating, ambient derating, bundling, conduit fill, terminations, and the electrical code that applies to the installation.

Treat this as a planning and comparison tool for run performance, not a complete code-compliance or engineering sign-off document.

Frequently asked questions

Why does the calculator ask for one-way distance instead of round-trip length?

The tool expects one-way distance and doubles it internally to model the full circuit path from source to load and back again.

 What is a common voltage drop target?

Many designers use low single-digit percentage targets for branch and feeder circuits, but the right number depends on equipment sensitivity, system voltage, and the code or design standard you are following.

 Why can the recommended size be larger than the selected conductor?

That happens when the conductor you chose produces more voltage drop than the target allows. The recommendation points to the smallest supported size in the table that keeps the design within the entered limit.

 Does this calculator perform an ampacity check?

No. It models voltage drop and conductor loss only. You still need a separate ampacity and code-compliance check before choosing a final wire size.

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