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Voltage drop calculator

Calculate cable voltage drop, end voltage and power loss for one or many circuits.

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Voltage drop calculator
Paste a list of circuits and this tool calculates the voltage drop across the cable, the voltage remaining at the load, the percentage drop against your supply voltage, and the power lost as heat in the conductor. Electricians, panel designers and hobbyists use it to check that a run of cable to a workshop, a pump, an outbuilding or a long lighting circuit stays within a safe drop before wiring it.
Each line describes one circuit as name;length;cross-section;current — for example "Garage;60;1.5;16" is a 60 m run on 1.5 mm² cable carrying 16 A. Options control how the physics is applied: conductor material (copper or aluminum, which has noticeably higher resistance), DC, AC single-phase or AC three-phase (three-phase uses the √3 line-to-line factor instead of the round-trip factor), conductor operating temperature (resistance rises with heat), the supply voltage, and the maximum drop percentage you're designing to — commonly 3% for lighting or 5% overall. Length can be entered in meters or feet, and cross-section in mm² or AWG gauge; choose "Table" instead of "Text" to see every circuit's numbers lined up in columns.
Any circuit that exceeds the drop limit is marked and, where a larger standard cross-section would bring it back under the limit, the tool suggests the next size up. A line that doesn't parse as name;length;cross-section;current is flagged rather than discarded, so one typo in a list of twenty circuits doesn't hide the other nineteen results. Windows and Unix line endings are both accepted, and the live tally under the output shows the circuit count, how many exceed the limit, and the worst drop found.
Everything runs locally in your browser — cable runs and load details for a project you haven't published yet never leave your device. Copy the result, download it as a .txt file, or send the output back into the input to adjust and recalculate.
FAQ
How is voltage drop calculated?
From the conductor resistance (resistivity of copper or aluminum, adjusted for temperature, times length over cross-section) and the circuit current. Single-phase and DC use a round-trip factor of 2 for the outgoing and return conductor; three-phase uses √3 for the line-to-line drop.
What drop percentage should I design for?
There is no universal legal limit, but 3% for a lighting circuit and 5% for the total run from supply to load are common guideline figures — check the wiring rules that apply where you are before finalizing a design.
What format does the circuit list use?
One circuit per line: name;length;cross-section;current, separated by semicolons — for example "Pump;40;4;12". Length and cross-section are read in whichever units you select in the options.
Does the suggested cross-section guarantee the circuit is safe?
No — it only shows the next standard size that would bring the voltage drop under your chosen limit. Current-carrying capacity, breaker sizing and installation method still need checking against the applicable wiring regulations.
Is my circuit data uploaded anywhere?
No. The calculation runs entirely in your browser — your circuit list and project details never leave your device.