When a load current flows through a power cable, the conductor's resistance pulls the voltage down. Adjust the load current, cable length, conductor cross-section and wiring method to see the voltage drop, percentage drop, power loss and load-end voltage update in real time, and find a conductor size that stays within the limits.
Parameters
Load current I
A
Current the cable carries
Cable length L
m
One-way distance from supply to load
Conductor cross-section A
mm²
Cross-section of one copper conductor
System voltage V
V
Supply-side voltage
Wiring method
Changes the voltage-drop coefficient k
Results
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Conductor resistance (one-way) (Ω)
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Voltage drop (V)
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Percentage drop (%)
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Power loss (W)
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Load-end voltage (V)
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Voltage-drop rating
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Wiring circuit — voltage-drop animation
Current flows from the supply on the left to the load on the right, and the voltage declines along the cable. The fall is the voltage drop; the colour rising from the cable is power lost as heat.
Voltage drop V_drop and one-way conductor resistance R. rho: copper resistivity (0.0172 Ohm*mm2/m), L: cable length, A: conductor cross-section, I: load current. The coefficient k is 2 for a single-phase two-wire circuit and sqrt(3) for a three-phase circuit.
$$\delta = \frac{V_{drop}}{V}\times100\,[\%]$$
Percentage voltage drop delta — the drop as a fraction of the system voltage V. About 2% is the guideline for a final circuit, about 5% for the total installation.
Power loss P_loss (power dissipated as heat in the cable, with m = 2 for single-phase and 3 for three-phase) and load-end voltage V_load. The drop falls as the conductor cross-section A is increased.
What is the Cable Voltage Drop Simulator?
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I always thought a wire just carries electricity straight through. Does "voltage drop" mean the voltage actually decreases inside the cable?
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It does. A cable is not an ideal "zero-resistance wire" — even copper always has a small but real resistance. Run a current through it and the voltage falls by "current times resistance", exactly as Ohm's law demands. So even if the supply is 200 V, the load at the far end of a long cable might only see 190 V. That difference is the voltage drop.
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A difference of about 10 V doesn't sound like much... is that actually a problem?
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It can be a real headache. Equipment is designed to run at its rated voltage. If the voltage is too low, motors lose torque and run hot, lamps dim, and electronics misbehave or reset. On top of that, the lost voltage is dissipated as heat inside the cable — electricity you paid for vanishes just warming up the wire. Try stretching "cable length" on the left and watch the percentage drop climb fast.
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You're right — just making it longer pushes the drop past 5% and it turns red. How do I bring it down?
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The most effective move is to make the conductor thicker. The conductor resistance R = rho*L/A is inversely proportional to the cross-section A, so a thicker wire cuts both the resistance and the voltage drop. Look at the "Percentage drop vs conductor cross-section" chart below — you will see a curve that falls steeply as you go thicker. When voltage drop becomes a problem on a long run, the field rule of thumb is to step up to the next conductor size first.
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There's a wiring method choice between single-phase and three-phase. Does that change the voltage drop too?
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It does. In a single-phase 2-wire circuit the current goes out and back through two conductors, so the drop is 2*I*R. A three-phase 3-wire circuit gives sqrt(3)*I*R, dropping the coefficient from 2 to about 1.73. So under the same conditions a three-phase circuit has a smaller drop. Switch the wiring method and the rating number changes accordingly. This efficiency is one reason three-phase is used for high-power transmission and distribution.
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I see — so voltage drop is really about "how to choose the wire".
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Exactly. Wire sizing is decided by both "ampacity" (will the wire overheat?) and "voltage drop" (will the equipment work properly?). For short runs ampacity usually governs, but for long runs voltage drop often becomes the deciding factor first. That is why wiring design always includes this voltage-drop check to confirm the size.
Frequently Asked Questions
First find the one-way conductor resistance R = rho*L/A, where rho is the copper resistivity (about 0.0172 Ohm*mm2/m), L is the cable length and A is the conductor cross-section. The voltage drop depends on the wiring method: for a single-phase 2-wire circuit the current flows down one conductor and back the other, so V_drop = 2*I*R; for a three-phase 3-wire circuit V_drop = sqrt(3)*I*R. The percentage drop is V_drop divided by the system voltage, expressed as a percentage, and this tool compares it with the allowable limits.
As a common guideline, a final (branch) circuit should stay within about 2%, and the total drop from the origin of the installation to the load within about 5%. This tool rates a percentage drop below 2% as good, 2-5% as acceptable, and above 5% as excessive and in need of improvement. Excessive voltage drop causes motors to lose torque, lamps to dim and electronics to misbehave, so the basic cure when the limit is exceeded is to increase the conductor cross-section.
The most reliable fix is to increase the conductor cross-section A (use a thicker cable). The conductor resistance R = rho*L/A is inversely proportional to the cross-section, so doubling A halves both the resistance and the voltage drop. Other options are to route a shorter cable run to cut the length L, to split the load current I, or to raise the system voltage. The voltage drop is proportional to L and I and inversely proportional to A, so longer runs always need thicker conductors.
For the same current and the same conductor resistance, a single-phase 2-wire circuit has the current going out and back through two conductors, so V_drop = 2*I*R, while a three-phase 3-wire circuit gives V_drop = sqrt(3)*I*R (about 1.73*I*R). In other words, under identical conditions a three-phase circuit has a smaller voltage drop, with a coefficient ratio of sqrt(3)/2 about 0.87. This efficiency in voltage drop and conductor usage is one reason three-phase is widely used for high-power transmission and distribution.
Real-World Applications
Building electrical design: The feeders from the substation to each floor's distribution board, and the branch circuits from there to lighting, outlets and air conditioners, are all sized after a voltage-drop calculation. On long feeders the ampacity is often comfortable while the voltage drop forces a one-size-larger conductor, so design drawings always record the voltage-drop study results.
Industrial power wiring: Power cables to large motors, pumps and compressors carry heavy currents, so their voltage drop is large. Motors in particular draw several times their rated current at start-up; if the voltage sags badly at that instant the motor may fail to start or other equipment may misbehave. Conductors are sized while accounting for this start-up voltage drop.
Solar and renewable energy: On the DC cables from solar panels to the inverter, and on the long runs from outdoor wind turbines, voltage drop translates directly into lost generated energy. To deliver the generated electricity without waste, longer runs use thicker conductors and the percentage drop is kept low by design.
Quick estimates and design checks: A resistance-only model like this tool is useful in the early stages of wiring design to quickly judge "will this conductor size keep the voltage drop acceptable?". Detailed design adds the cable's inductance (reactance) on long high-voltage runs, but the practical approach is to first use this estimate to home in on a conductor size.
Common Misconceptions and Pitfalls
A common belief is that "voltage drop is determined by resistance alone". This tool uses a resistance-only model at unity power factor, which gives sufficient accuracy for short-to-medium low-voltage runs. But on long runs or high-voltage lines, the cable's inductance contributes an inductive reactance to the drop as well. Moreover, when the load power factor is below 1 (inductive loads such as motors), the reactive part of the drop can no longer be ignored. For long, high-current, low-power-factor jobs, re-check with a formula that accounts for both resistance and reactance.
Next, the assumption that "conductor resistance is constant regardless of temperature". The resistivity of copper rises with temperature — it differs by about 20% between 20°C and 70°C. As current heats the cable itself, the resistance rises and the voltage drop and power loss grow further, a vicious circle. This tool uses the representative value rho = 0.0172 Ohm*mm2/m (roughly 20°C); for cables carrying heavy current continuously, it is safer to study with the resistivity at the operating temperature.
Finally, the idea that "as long as the voltage drop is fine, the wire can be thin". Wire size is set not only by voltage drop but also by ampacity. Pushing a current beyond the ampacity overheats the wire even if the voltage drop is small, degrades the insulation and, in the worst case, leads to a fire. The rule for wire selection is to choose a size that satisfies both ampacity and voltage drop; this tool is the instrument for checking the latter. Ampacity depends on the cable type, installation method and ambient temperature, so always verify it separately.
How to Use
Enter load current in amperes (typical range: 10–200 A for industrial feeders) using the currentNum input or slider.
Set cable length in meters (one-way distance from source to load; e.g., 50 m for a substation feeder) via cableLengthNum or cableLengthRange.
Specify conductor cross-sectional area in mm² (common values: 10, 16, 25, 35, 50, 70, 95, 120 mm² for copper or aluminium) using conductorAreaNum.
Input system voltage in volts (e.g., 400 V three-phase, 230 V single-phase, 11 kV medium voltage) via systemVoltageNum.
Read real-time outputs: one-way conductor resistance, voltage drop in volts, percentage drop relative to system voltage, power loss in watts, actual voltage at the load end, and compliance against applicable voltage-drop rating (typically 3% for feeders, 5% for branch circuits per IEC 60364).
Worked Example
A 400 V three-phase distribution circuit supplies a motor 45 m away with 80 A demand. Copper cable cross-section is 35 mm² (resistivity ρ = 0.0175 Ω·mm²/m). One-way conductor resistance: R = (0.0175 × 45) / 35 = 0.0225 Ω. Voltage drop: ΔV = 80 × 0.0225 = 1.8 V. Percentage drop: (1.8 / 400) × 100 = 0.45%. Power loss: P = 80² × 0.0225 = 144 W. Load-end voltage: 400 − 1.8 = 398.2 V. Result meets the 3% feeder criterion (0.45% < 3%).
Practical Notes
Aluminium cable (ρ = 0.0278 Ω·mm²/m) produces ~59% higher resistance than copper for identical geometry; upsize cross-section accordingly to maintain voltage-drop budget.
Long runs (>100 m) or high currents (>150 A) in small cables (10–16 mm²) risk exceeding 5% drop; pre-calculate before installation to avoid motor starting issues and lighting flicker.
Three-phase circuits reduce effective voltage drop by factor of √3 compared to single-phase; account for this when mixing load types on the same feeder.
Power loss heats the cable; verify thermal rating (ampacity) independently—voltage drop and thermal capacity are separate sizing constraints per AS/NZS 3008 or BS 7671.