Voltage Drop Calculator
This voltage drop calculator finds the voltage lost along a copper or aluminum cable using the voltage drop formula, then checks it against the allowed limit. Enter the current, the run length, and the conductor size and the voltage drop calculator returns the drop in volts, the percentage, and a pass or fail. It handles single phase and three phase and doubles as a dc voltage drop calculator.
Results
The Voltage Drop Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.
What the voltage drop calculator does
Current flowing through a cable meets resistance, and that resistance turns some of your supply voltage into heat before it reaches the load. This voltage drop calculator measures that loss with the voltage drop formula so you know how much voltage reaches the load. As a volt drop calc it reports the loss in volts and as a percentage, then compares it to the limit for the circuit, which is the check a voltage drop calculation table is built around.
The voltage drop formula
The voltage drop formula multiplies current by the round-trip resistance of the run:
For three phase the equation for voltage drop uses √3 in place of 2:
Here R is the resistance per unit length for the conductor and L is the one-way length. The voltage loss formula is the same physics whether you call it drop or loss, and the voltage drop calculator evaluates it and divides by the supply voltage to give the percentage.
Voltage drop formula example
A 25 A load, 40 m away, on 4 mm² copper at 230 V single phase. Using the voltage drop formula with copper resistance of about 0.0045 Ω/m:
That fails a 3% limit, so the voltage drop calculator flags it and you step up a size. Enter these values and the volt drop calc returns the same result with a pass or fail.
How to use the voltage drop calculator
Enter the load current, the one-way run length, the conductor material, and the cable size, then choose single phase, three phase, or DC. The voltage drop calculator applies the voltage drop formula for that arrangement and returns the drop in volts and percent. Switch to DC and it works as a dc voltage drop calculator; the same computation of voltage drop applies, only the phase factor changes.
Reading the voltage drop calculator result
The drop in volts is what the load loses; the percentage is how you judge it. A common target is 3% for the final circuit and 5% total from the source, though your local code sets the real figure. If the computation of voltage drop exceeds the limit, the fix is a larger conductor, a shorter run, or a higher supply voltage. The voltage drop calculator shows both volts and percent so you can see how much margin a size change buys.
When to use the voltage drop calculator
- Checking a long feeder against code with the voltage drop formula
- Sizing DC runs for solar or battery with the dc voltage drop calculator
- Comparing copper and aluminum in one voltage drop calculation table
- Verifying a cable choice as a cable voltage drop calculator before install
Common voltage drop formula mistakes
- Using the round-trip length. Enter the one-way run; the voltage drop formula already accounts for both conductors.
- Wrong phase factor. Single phase uses 2, three phase uses √3 in the equation for voltage drop.
- Ignoring material. Aluminum resistance is higher, so the computation of voltage drop must use the right conductor.
Tips for accurate results
- Enter the true one-way length so the voltage drop formula sizes the full round trip.
- Keep the final circuit under about 3% and the whole path under 5%.
- For DC and long runs, size up early; the dc voltage drop calculator will show why.
Why the voltage drop formula uses the round trip
The commonest slip with any volt drop calc is length. Current has to travel out to the load and back, so the voltage drop formula counts both conductors, which is where the factor of 2 comes from in single phase and DC. Enter the one-way run length and let the tool double it; enter the round-trip length by mistake and the computation of voltage drop comes out twice too high. Three phase is different because the currents in the three conductors are out of step, so the equation for voltage drop uses √3 rather than 2, and the drop per conductor is lower for the same power.
Fixing a failing voltage drop calculation table
When a result fails, you have three levers. Increasing the conductor size lowers resistance and is the usual fix, and a voltage drop calculation table makes the effect obvious: each step up roughly cuts the drop in proportion to the cross-section. Shortening the run helps directly, since drop is proportional to length, though the route is often fixed. Raising the supply voltage is the most powerful lever because the same watts flow at less current, which is exactly why long DC runs suffer and why a dc voltage drop calculator so often demands heavy cable. Weigh the copper cost of sizing up against the energy wasted as heat over the life of the circuit.
Voltage drop calculation table
Approximate single-phase voltage drop per 100 m at 20 A on copper, a quick voltage drop calculation table from the calculator:
| Cable size | Resistance (Ω/m) | Drop at 20 A over 100 m |
|---|---|---|
| 2.5 mm² | 0.0074 | 29.6 V |
| 4 mm² | 0.0046 | 18.4 V |
| 6 mm² | 0.0031 | 12.4 V |
| 10 mm² | 0.0018 | 7.2 V |
| 16 mm² | 0.0012 | 4.8 V |
Voltage Drop Calculator FAQ
What is the voltage drop formula?
The voltage drop formula is Vdrop = 2 × I × R × L for single phase or DC, and √3 × I × R × L for three phase. The voltage drop calculator applies the equation for voltage drop and converts the result to a percentage.
What voltage drop is acceptable?
A common target is 3% for the final circuit and 5% overall, but your local code sets the limit. The volt drop calc compares your result to the figure you enter and flags a fail.
Why is DC voltage drop worse?
Low-voltage DC has little supply to spare, so the same loss is a bigger percentage. The dc voltage drop calculator uses the single-phase form of the voltage loss formula but the low voltage makes the percentage climb fast.
Does aluminum drop more than copper?
Yes. Aluminum has higher resistance, so for the same size the computation of voltage drop gives a larger loss. Enter the material and the voltage drop calculator uses the correct resistance.