DC Voltage Drop Calculator

A DC Voltage Drop Calculator determines the voltage lost through the complete outgoing-and-return conductor path of a two-conductor DC circuit. The primary result is Voltage drop, stated in volts, followed by the corresponding Voltage drop percentage relative to the entered System voltage.

The calculation is used during DC branch-circuit and feeder design to evaluate whether conductor resistance and circuit length will leave adequate voltage at the load. It is particularly useful where loads are sensitive to supply voltage, including DC controls, battery systems, power supplies, low-voltage distribution, communications equipment, and DC motor applications.

For a selected conductor, the result helps evaluate whether a larger AWG or kcmil conductor may be needed to reduce resistance. It does not select a conductor or establish ampacity.

Circuit Path and Inputs

DC voltage-drop arithmetic includes both conductors in the circuit path:

• The ungrounded or positive conductor carries current from the source to the load.
• The return conductor carries the same current back to the source.
• One-way length is doubled through the DC circuit multiplier of 2 x.

The calculator uses the following fields.

Input	Unit	Electrical use
Load current	A	DC current expected to flow through both conductors
One-way length	ft	Distance from the source to the load, not the total loop length
System voltage	V	Nominal DC voltage used to calculate percentage voltage drop
Conductor resistance	ohm/1000 ft	Verified resistance of the installed conductor at the applicable temperature basis
Entered drop limit	%	Comparison value used to calculate the entered limit in volts and entered margin

Conductor resistance is the controlling conductor property in this mode. Resistance must correspond to the actual conductor material, AWG or kcmil size, strand construction where applicable, and temperature basis used for the design review. A resistance value at one temperature should not be treated as equivalent to a value at another temperature.

The calculator does not derive resistance from ampacity. A conductor can have sufficient ampacity yet still produce unacceptable voltage drop over a long run; conversely, low voltage drop does not prove acceptable ampacity, terminal rating, insulation temperature rating, or overcurrent protection.

DC Voltage-Drop Formula

The calculator applies the complete two-conductor loop length:

\(\displaystyle \text{voltage drop} = \frac{2 \times \text{current} \times \text{one-way length} \times \text{resistance per 1000 ft}}{1000}\)

The percentage result is:

\(\displaystyle \text{voltage drop percent} = \frac{\text{voltage drop}}{\text{system voltage}} \times 100\)

The entered comparison limit is converted from percent to volts:

\(\displaystyle \text{entered limit in volts} = \text{system voltage} \times \frac{\text{entered drop limit}}{100}\)

The calculator then reports:

\(\displaystyle \text{entered margin} = \text{entered drop limit percent} - \text{voltage drop percent}\)

A positive Entered margin indicates that the calculated voltage drop is below the entered percentage. A negative margin indicates that calculated voltage drop exceeds the entered comparison limit.

Calculation Example

For the displayed values:

Field	Value
Load current	20 A
One-way length	100 ft
System voltage	120 V
Conductor resistance	1.588 ohm/1000 ft
Entered drop limit	3%

First, calculate the conductor-loop voltage drop:

\(\displaystyle \text{Voltage drop} = \frac{2 \times 20 \times 100 \times 1.588}{1000} = 6.352\text{ V}\)

Next, convert that drop into a percentage of the 120 V system voltage:

\(\displaystyle \text{Voltage drop percent} = \frac{6.352}{120} \times 100 = 5.2933\%\)

The calculator converts the 3% entered limit to volts:

\(\displaystyle \text{Entered limit in volts} = 120 \times 0.03 = 3.6\text{ V}\)

Finally:

\(\displaystyle \text{Entered margin} = 3\% - 5.2933\% = -2.2933\text{ percentage points}\)

The calculated Voltage drop is 6.352 V, or 5.2933% of the System voltage. The entered 3% comparison limit equals 3.6 V, so the circuit exceeds that entered limit by 2.2933 percentage points.

Applying the Result

When the calculated voltage drop is excessive, the electrical design response is generally to reduce loop resistance or reduce circuit current. Common design changes include:

• Increase conductor size from a smaller AWG conductor to a larger AWG or kcmil conductor.
• Shorten the One-way length by relocating a source, distribution point, battery bank, power supply, or load.
• Split the load across separate circuits where the system design permits.
• Recheck the actual Load current, including continuous operating conditions and expected simultaneous load.
• Verify whether the Conductor resistance value reflects copper or aluminum construction, conductor temperature, and the actual installed conductor.

For a raceway installation, conductor enlargement can affect raceway fill, pull tension, bending space, box fill, termination compatibility, lug range, equipment rating, and available installation space. Those decisions are separate from the DC voltage-drop equation.

Field Verification

The calculation assumes the entered Load current flows through a two-conductor DC circuit with the stated One-way length and conductor resistance. It does not account for connection resistance, splices, lugs, corrosion, damaged conductors, parallel paths, source-voltage sag, nonlinear equipment behavior, fault current, or transient load conditions.

Voltage drop also does not establish compliance with ampacity or protection requirements. Verify conductor ampacity using the applicable conductor properties, insulation temperature rating, terminal rating, ambient-temperature correction factor, adjustment factor for current-carrying conductors, and overcurrent protective-device requirements. Confirm installation details such as raceway fill, equipment listings, termination ratings, and local AHJ requirements separately.

The Entered drop limit is a project comparison value. The calculator reports whether the entered percentage is met mathematically; it does not make a code determination, approve an installation, or select a conductor.