The voltage drop calculator above estimates how much voltage a cable run loses along the way. Every conductor has resistance, so current flowing through it produces a drop that never reaches the load, and on a long run at a high current that loss becomes significant. The tool reports the drop in volts, as a percentage of the nominal system voltage, as the voltage actually arriving, and as the power being turned into heat inside the cable.
Arb Digital builds free tools that state their sources rather than hiding them. The only resistivity figures offered here are the published copper and aluminium values at 20 °C cited below, and no ampacity or current-carrying-capacity table appears anywhere on this page. That is deliberate: current ratings depend on insulation type, installation method, grouping, ambient temperature and the rules of your jurisdiction, and a generic table would be dangerous rather than helpful. Voltage drop is one input to conductor sizing. A licensed electrician sizes the final run against the wiring rules that apply where the work is being done.
What This Voltage Drop Calculator Does
It computes conductor resistance from first principles and applies Ohm's law to it. OpenStax University Physics Volume 2, section 9.3 on resistivity and resistance, gives the relationship as R = ρL/A and tabulates resistivity at 20 °C as 1.68 × 10−8 Ω·m for copper and 2.65 × 10−8 Ω·m for aluminium. Those two numbers are the only material data this calculator uses, and they are the ones in the dropdown.
The result is a resistive estimate. Real alternating-current circuits also have inductive reactance, which adds to the impedance and therefore to the drop, and the contribution grows with conductor spacing and with size. For small conductors on ordinary domestic runs the resistive term dominates comfortably. For large feeders it does not, and a proper calculation uses published impedance figures for the specific cable rather than a resistivity from a physics table.
The conductor size can be given three ways. Square millimetres is what most of the world specifies cable in. AWG is converted using the standard geometric definition, where each gauge step changes the diameter by a fixed ratio. Thousand circular mils is used for larger North American conductors, and one circular mil is the area of a circle one thousandth of an inch across.
How to Use It
- Choose the system type. Single-phase and DC both count the outbound and return conductors, so the tool doubles the run length. Three-phase uses a factor of the square root of three.
- Enter the one-way length. Measure from the supply to the load along the actual cable route, including vertical drops and slack, not the straight-line distance.
- Enter the design current, not the breaker rating. The current that actually flows is what produces the drop. A 32 A circuit carrying 10 A drops less than a third as much.
- Set the nominal system voltage. This is used only to express the drop as a percentage, which is the figure wiring rules are written in terms of.
- Treat the answer as an estimate. It ignores reactance, connection resistance and the rise in resistivity with temperature, all of which push the real figure higher.
The Formula: How Voltage Drop Is Calculated
Conductor resistance is R = ρL/A, where ρ is resistivity in ohm metres, L is the conductor length in metres and A is the cross-sectional area in square metres. For a single-phase or DC circuit the current travels out and back, so the total resistance is twice the one-way figure and the drop is V = 2IRone-way. For a balanced three-phase circuit the line-to-line drop is √3 × IRone-way.
Work the defaults. A 10 mm² copper conductor 30 m long has a one-way resistance of 1.68 × 10−8 × 30 ÷ 0.00001 = 0.0504 Ω. Carrying 32 A on a single-phase circuit the drop is 2 × 32 × 0.0504 = 3.226 V, which on a 230 V system is 1.40 per cent. The load sees 226.77 V, and the cable dissipates 32² × 0.1008 = 103.2 W as heat along its length.
That last figure is the one people overlook. Just over a hundred watts is being turned into warmth inside the wall for as long as the load runs, which is both an energy cost and a reason the cable and its surroundings get warm. Halving the drop by doubling the conductor area also halves that loss.
What the Rules Actually Say, and Where
Voltage drop limits are set by wiring regulations, and they differ by jurisdiction. In the United States the governing document is NFPA 70, the National Electrical Code, which NFPA publishes through its free-access viewer. The Code's informational notes recommend that branch-circuit conductors be sized so that voltage drop does not exceed three per cent, with the combined drop across feeder and branch circuit not exceeding five per cent, for reasonable efficiency of operation.
The status of those figures matters. In the NEC, informational notes are explanatory material and are not enforceable requirements in the way that code articles are, so a three per cent drop is a design recommendation rather than a violation threshold. Local amendments can and do change that, and some jurisdictions adopt the limits as mandatory.
Outside the United States the framework is different again. Installations designed to IEC 60364 and its national implementations set their own voltage drop limits, often with different figures for lighting and for other uses, and those limits are the ones that apply. This calculator does not encode any jurisdiction's limit, and it deliberately does not tell you whether a result passes or fails. It gives you the percentage; which percentage is acceptable is a question for the rules in force where the installation is, and for the electrician responsible for it.
Why This Page Publishes No Ampacity Table
The most common request for a tool like this is a table saying which cable size carries which current. This page does not provide one, and neither does the breaker size calculator elsewhere on the site, for the same reason in both cases.
Current-carrying capacity is not a property of the conductor alone. The same copper cable has a very different rating depending on the insulation's temperature class, whether it runs in free air, in conduit, in insulation or buried, how many current-carrying conductors are bundled together, the ambient temperature, and any derating the applicable rules impose. A single number stripped of those conditions is not a simplification, it is a hazard, because the conditions that reduce the rating are exactly the ones a non-specialist is least likely to notice.
Voltage drop is different in kind. It is a straightforward consequence of resistivity, length, area and current, and it can be estimated honestly from published physical constants without reference to installation conditions. That is why this calculator computes it and stops there. The output tells you whether a run is long enough to cause a voltage problem; it says nothing about whether the cable is thermally adequate, and those two questions have to be answered separately. Where they interact, the wire size calculator approaches the sizing question from the conductor end.
The Things That Make the Real Drop Larger
Three effects all push the same way, and none of them is in the calculation above. The first is temperature. Resistivity rises as a conductor warms, and a cable working near its rated temperature is appreciably more resistive than the 20 °C figure used here. A run computed at the tabulated value and then loaded heavily will drop more than predicted.
The second is connections. Every termination, junction, connector and switch contact adds a small resistance in series with the cable. Individually they are milliohms; across a long run with several joints they add up, and a loose or corroded connection can dwarf the entire cable resistance while also becoming a heat source in its own right. A measured drop far larger than the calculated one is a strong signal to inspect the terminations rather than the cable.
The third is reactance, which affects alternating current only. Conductor inductance adds an impedance term that the resistive calculation ignores entirely, and the contribution grows with conductor size and separation. For large feeders the reactive part can be a substantial fraction of the total, which is why cable manufacturers publish impedance figures rather than expecting anyone to work from resistivity. Power factor interacts with this too: a load drawing reactive current pushes more current through the same conductors for the same real power delivered.
How This Differs From the Adjacent Electrical Tools
The boundary in one sentence: this page computes the voltage lost along a conductor because of its own resistance, while the voltage divider calculator computes an intentional split between two resistors. A cable run is an unintentional voltage divider, with the cable as the upper resistor and the load as the lower one, which is why the two pages share a formula but answer different questions.
The Ohm's law calculator relates a single voltage, current and resistance without any geometry, and the electrical power calculator handles power, current and voltage relationships. The resistance converter rescales resistance units. For what the wasted heat costs over a year, the electricity bill calculator turns watts into money, and the length converter handles run lengths that arrive in the wrong unit.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Entering the round-trip length — measure one way, because the tool already doubles it for single-phase and DC circuits.
- Using the breaker rating as the current — the drop depends on the current that actually flows, which is usually well below the protective device rating.
- Assuming a small drop means the cable is adequate — voltage drop and thermal capacity are separate questions, and passing one says nothing about the other.
- Ignoring connections — terminations and joints add resistance the calculation does not see, and a bad one can exceed the whole cable.
- Applying a resistive estimate to a large feeder — reactance becomes significant at larger sizes, and manufacturer impedance data is the right source there.
Related Free Tools From Arb Digital
For conductor sizing from the other direction, see the wire size calculator and the breaker size calculator. The Ohm's law calculator and the electrical power calculator cover the underlying relationships, and the voltage divider calculator covers deliberate voltage division. Rescale values with the resistance converter or the length converter, cost the wasted energy with the electricity bill calculator, and browse the full free online tools hub for everything else.
Frequently Asked Questions
One way, measured along the actual cable route. The calculator doubles it automatically for single-phase and DC circuits, and applies the square root of three for balanced three-phase, so entering the round trip would double the answer wrongly.
From the resistivity table in OpenStax University Physics Volume 2, section 9.3, which lists copper at 1.68 times ten to the minus eight ohm metres and aluminium at 2.65 times ten to the minus eight, both at 20 degrees Celsius. No other material is offered.
That depends on the wiring rules in force where the installation is. In the United States the National Electrical Code's informational notes recommend three per cent on a branch circuit and five per cent overall, though informational notes are explanatory rather than enforceable. Other jurisdictions set their own limits.
Because current-carrying capacity depends on insulation type, installation method, grouping, ambient temperature and local derating rules, none of which a generic table can capture. Publishing one stripped of those conditions would be misleading, so this tool computes voltage drop only.
It will usually be larger. Resistivity rises as the conductor warms, every termination adds resistance, and alternating-current circuits also carry an inductive component this resistive calculation ignores. Treat the result as a lower bound.
The effective options are a larger conductor area, a shorter route, a lower current, or a higher supply voltage for the same power. Changing the protective device does nothing at all, because it does not alter the resistance of the run.
Because its resistivity is about 58 per cent higher, so the same cross-section drops correspondingly more voltage. Aluminium conductors are commonly used in larger sizes to compensate, which is a design decision made against the applicable wiring rules.
No. It answers one question out of several, and only as an estimate. A licensed electrician sizes the final run against the wiring rules that apply where the work is being done, taking thermal capacity, protection and installation conditions into account alongside voltage drop.
This tool is provided for educational and estimating use only. It is not electrical design advice and it does not address conductor ampacity, overcurrent protection, insulation rating, installation method or any code compliance requirement. Electrical work must be designed and carried out by a licensed electrician in accordance with the wiring regulations applicable in your jurisdiction.
