The wire voltage drop calculator is designed to determine voltage losses in electrical conductors and optimise cable sizing for power distribution systems. Understanding cable voltage drop calculations is fundamental for electrical engineers working with power systems, building wiring, and industrial installations. The cable voltage drop calculation formula, which is a variation of Ohm's Law, is used to perform cable size and voltage drop calculations for DC, single-phase AC, and three-phase AC circuits, ensuring equipment receives adequate voltage.
For DC circuits, where only resistance is a factor, the formula is:
Vdrop= 2ILRc
Where I is the current in amps, L is the length of the wire, and Rc is the resistance per unit length.
For AC circuits, the calculation must also account for reactance (Xc), with the total opposition to current flow being impedance (Zc).
The Cable Voltage Drop Calculation Formula
The formulas for AC systems are:
For single-phase AC:
For three-phase AC:
where I is current, L is cable length, Rc is conductor resistance, and θ is the power factor angle.
If you don’t know the actual resistance per unit length, you can simply select the material type and diameter; the calculator will automatically do that for you.
Example of Voltage Drop in Cable Calculator
Calculate the voltage drop for 12V LED strips drawing 5A through 10m of 2.5mm² copper wire.
For copper at 20°C: ρ = 0.0175 Ω·mm²/m
Rc = ρ/A = 0.0175/2.5 = 0.007 Ω/m
DC voltage drop:
Vdrop = 2 × I × L × Rc
Vdrop = 2 × 5 × 10 × 0.007
Vdrop = 0.7V
Percentage drop = (0.7/12) × 100% = 5.8%
Voltage at load = 12 - 0.7 = 11.3V
Calculate the voltage drop for a 230V AC circuit, 20A load, 30m run, 4mm² copper wire, power factor = 0.9.
For 4mm² copper:
Rc = 0.0175/4 = 0.004375 Ω/m
Xc ≈ 0.00008 Ω/m (typical for residential wiring)
θ = arccos(0.9) = 25.84°
cos(θ) = 0.9, sin(θ) = 0.436
Vdrop = 2 × I × L × (Rc×cos(θ) + Xc×sin(θ))
Vdrop = 2 × 20 × 30 × (0.004375×0.9 + 0.00008×0.436)
Vdrop = 2 × 20 × 30 × 0.00397
Vdrop = 4.76V
Percentage = (4.76/230) × 100% = 2.07%
Frequently Asked Questions on Cable Calcs Voltage Drop
⇥ What's an acceptable voltage drop?
Industry standards typically recommend:
- 3% for branch circuits
- 5% total from service to load
- 2-3% for motor circuits
- Critical electronics may require <1%
⇥ Why multiply by 2 for DC and single-phase AC?
Current flows through two conductors - the "hot" wire to the load and the return/neutral back to the source. Both contribute to the voltage drop.
⇥ Which wire size should I use?
Wire must satisfy both current capacity (ampacity) and voltage drop requirements. Increase the size of the drop if it is excessive.
⇥ How does temperature affect voltage drop?
Conductor resistance increases with temperature, which in turn increases the voltage drop.
⇥ Why is reactive voltage drop usually ignored?
For most power wiring, Xc << Rc, making the reactive drop negligible. It becomes significant only for:
- Large conductors (>500 MCM)
- High frequencies
- Bundled conductors
- Long parallel runs
Related Calculators for Power Distribution and Efficiency
Accurate voltage drop calculations are essential for safe and efficient electrical installations. These related tools help you size conductors, evaluate power loss, and ensure stable voltage delivery in your wiring and cabling systems.
Enter any two known circuit parameters in the below Ohm's law calculator and calculate the remaining two values according to Ohm's Law.
The Resistivity Calculator allows use to easily calculate the resistivity of a component like a wire or coil, based on its resistance value, its length, and its cross-sectional area.
This Energy Consumption Calculator or Electricity Consumption Calculator allows you to calculate the energy/electricity consumed by your home appliances, or industrial loads.
