Wire Size Calculator

What the wire size calculator does

A wire must be thick enough that the voltage lost along it stays within an acceptable limit. This calculator sizes a conductor by voltage drop, from the system type, voltage, allowable drop, current, distance, material, and temperature, giving the cross-sectional area and diameter needed.

Below is what sizing a wire means, the equation behind it, why long runs need thicker wire, and a worked example.

How to use it

1. Choose the system and enter the source voltage and the allowable voltage drop as a percentage.
2. Pick the conductor material, then enter the current, one-way distance, and maximum wire temperature.
3. Press Calculate for the required cross-sectional area and diameter, or Reset to clear them.

What sizing a wire means

Every wire has some resistance, and as current flows through it, a little voltage is lost along the way, turning into heat. Over a short distance this loss is negligible, but over a long run it can become significant, leaving too little voltage at the far end to run equipment properly. Sizing a wire means choosing a conductor thick enough that this voltage drop stays within an acceptable limit, because a thicker wire has less resistance and so loses less voltage.

The right thickness depends on several things: how much current flows, how far the wire runs, what it is made of, and how much voltage drop you are willing to tolerate. A higher current or a longer run means more voltage lost, calling for a thicker wire, while a more conductive material or a tighter voltage-drop limit also affects the choice. This calculator brings these together to find the cross-sectional area and diameter a conductor needs to keep the voltage drop within your chosen percentage, which is a core calculation in electrical installation.

The equation it uses

The cross-sectional area needed to hold the voltage drop to a target comes from Ohm's law applied to the wire's resistance:

A = (2 × I × ρ × L) ÷ V_drop

Here A is the cross-sectional area, I is the current, ρ is the resistivity of the conductor material, L is the one-way distance, and V_drop is the allowable voltage drop. The factor of two accounts for the current travelling out and back along the circuit. For three-phase systems the factor changes to reflect the different geometry. The calculator works out the resistivity at the wire's temperature, applies this relationship, and converts the area into a diameter, giving the minimum conductor size for your voltage-drop limit.

Why long runs need thicker wire

The most important driver of wire size for long circuits is the distance. Because the voltage drop is proportional to the length of the run, doubling the distance doubles the voltage lost for a given wire, which is why a conductor that is perfectly adequate over a short distance becomes too thin over a long one. To keep the same voltage drop over a longer run, you need a thicker wire with proportionally less resistance per unit length.

This is why running power to a distant outbuilding, a well pump, or a far corner of a site often calls for surprisingly thick cable, much thicker than the current alone would suggest. A common target is to keep the voltage drop below about three percent for a branch circuit, and meeting that over a long distance can force the wire up several sizes. The calculator makes this trade-off concrete, showing how the required area grows with distance, so you can choose a conductor that delivers usable voltage all the way to the load rather than losing too much along the journey.

Copper, aluminium, and temperature

What the wire is made of matters, because different metals have different resistivities. Copper is the most common conductor and a very good one, with low resistivity, while aluminium is cheaper and lighter but resists current more, so an aluminium wire must be thicker than a copper one to achieve the same voltage drop. The calculator includes both, letting you compare them or size whichever you intend to use.

Temperature also plays a role, because the resistivity of metals rises as they heat up. A wire carrying current warms, and warmer wire has more resistance, which increases the voltage drop slightly. The calculator accounts for this by adjusting the resistivity to the maximum wire temperature you specify, giving a more realistic figure than assuming the wire stays cool. Together, the choice of material and the operating temperature shape the resistivity that goes into the sizing, and the calculator handles both so the result reflects the conductor as it will actually run.

Units, precision, and ampacity

The calculator takes the voltage in volts, the allowable drop as a percentage, the current in amperes, the distance in metres, and the temperature in degrees Celsius, and returns the cross-sectional area in square millimetres and the diameter in millimetres. It sizes the wire by voltage drop, which governs long runs. A complete design must also check ampacity, the wire's ability to carry the current without overheating, which governs short runs, and should follow the conductor and temperature rules of the applicable electrical code. The larger of the voltage-drop and ampacity requirements is the one that controls the final choice.

A worked example

Suppose a single-phase copper circuit at 120 volts carries 20 amperes over a one-way distance of 30 metres, with an allowable voltage drop of 3 percent and the wire at 20 degrees Celsius.

The allowable voltage drop is 3 percent of 120 volts, or 3.6 volts. The required cross-sectional area works out to about 5.6 square millimetres, which corresponds to a diameter of about 2.67 millimetres. A longer run or a tighter voltage-drop limit would push this area up, calling for a thicker conductor to keep the voltage loss within bounds.

Questions people ask

How do you size a wire by voltage drop?

Use A = (2 × I × ρ × L) ÷ V_drop, where the area grows with the current, the resistivity, and the distance, and shrinks as the allowable voltage drop increases.

Why does a long run need thicker wire?

Because voltage drop is proportional to distance. Doubling the run doubles the voltage lost, so a thicker, lower-resistance wire is needed to keep the drop within the target.

Why does aluminium need to be thicker than copper?

Because aluminium has higher resistivity, it loses more voltage for the same size. An aluminium wire must be thicker than a copper one to achieve the same voltage drop.

Is voltage drop the only thing that sets wire size?

No. The wire must also carry the current without overheating, its ampacity, which governs short runs. The final size is whichever requirement, voltage drop or ampacity, is larger.

References

A quick note on where this comes from. Voltage-drop wire sizing rests on Ohm's law and resistivity, standard in OpenStax's University Physics, with the practical voltage-drop targets and ampacity rules from the National Electrical Code, published by the National Fire Protection Association. The HyperPhysics link is worth a quick click to confirm it lands where you expect.

1. OpenStax, University Physics Volume 2, Section 9.3, Resistivity and Resistance. https://openstax.org/books/university-physics-volume-2/pages/9-3-resistivity-and-resistance
2. National Fire Protection Association (NFPA), NFPA 70 National Electrical Code, voltage drop and ampacity. https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
3. HyperPhysics, Resistivity and Conductivity. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html