DC Voltage Drop Calculator
Enter the load current and either the total round-trip resistance of the run or the resistance per meter plus the one-way length. The calculator returns the voltage dropped in the conductors, and if you add a source voltage it also gives the percent drop and the voltage left at the load.
How to use the voltage drop calculator
- Enter the load current in amperes.
- Enter the total round-trip resistance, or the resistance per meter and the one-way run length.
- Optionally enter the source voltage to see percent drop and end voltage.
- Read the voltage drop in volts.
Examples
Known total resistance
I = 10 A, total R = 0.5 ohm
drop = 5 V
Per-meter resistance over 25 m at 120 V
I = 10 A, 0.01 ohm/m, length 25 m, source 120 V
drop = 5 V, 4.17%, end = 115 V
Frequently asked questions
How is DC voltage drop calculated?
Voltage drop is current times the total conductor resistance, from Ohm's law V = I R. For 10 A through 0.5 ohm the drop is 5 V.
Why is the length doubled?
Current flows out to the load and back, so the round-trip resistance uses twice the one-way length: total = resistance per meter times 2 times length.
What is an acceptable voltage drop?
Many guidelines keep total drop under about 3 percent for branch circuits and 5 percent overall, but always follow the code that applies to your installation.
How do I get the resistance per meter?
Use the conductor resistance from a wire table for the gauge and material, given in ohms per meter or per 1000 feet, then convert to ohms per meter.
Does this work for AC circuits?
It covers resistive DC drop. For AC you may also need conductor reactance and power factor, which this simple model does not include.
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