Voltage Drop Calculator
Find voltage loss in a wire from length, current, and conductor size.
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Formulas
Vdrop = 2 × I × ρ × L / AFactor of 2 accounts for the return path.
Why Voltage Drop Matters
NEC recommends max 3% for branch circuits, 5% total. Copper resistivity: 0.0175 Ω·mm²/m. Aluminum: 0.0283.
How to Calculate Voltage Drop
Voltage drop is the loss in voltage that occurs as current flows through the resistance of a conductor. Over a long cable run, this loss can leave equipment receiving less voltage than it needs, causing dim lighting, overheating motors, or unreliable electronics. The calculator uses the standard single-phase formula:
Vdrop = 2 × I × ρ × L / AThe factor of 2 accounts for both the outgoing and return conductors in a single-phase circuit. I is current in amps, ρ (rho) is the conductor resistivity, L is the one-way cable length, and A is the cross-sectional area of the conductor.
Because both the supply and return wire carry current, the electron path is twice the one-way length. That is why a 30 m cable run is treated as 60 m of conductor when calculating loss.
Worked Examples
A 15 A load sits at the end of a 30 m copper cable with a 2.5 mm² cross-section. Using copper resistivity ρ = 0.0175 Ω·mm²/m:
Vdrop = 2 × 15 × 0.0175 × 30 / 2.5 = 6.3 V
On a 230 V supply that is a 2.7% drop — just within the 3% branch-circuit guideline.
Doubling the conductor to 4 mm² in the same run:
Vdrop = 2 × 15 × 0.0175 × 30 / 4 = 3.9 V (1.7%).
Increasing cross-sectional area is the most effective way to reduce drop, because Vdrop is inversely proportional to area.
Acceptable Voltage Drop Limits
Most electrical codes set recommended maximums to protect equipment performance. The figures below are common design targets.
| Standard | Branch circuit | Feeder + branch total |
|---|---|---|
| NEC (USA) | 3% | 5% |
| IEC / EN (lighting) | — | 3% |
| IEC / EN (other uses) | — | 5% |
The NEC values are recommendations rather than hard requirements, but exceeding them risks poor performance and wasted energy as heat in the cable.
Conductor Resistivity Reference
| Material | Resistivity (Ω·mm²/m) | Notes |
|---|---|---|
| Copper | 0.0175 | Standard for most wiring |
| Aluminium | 0.0283 | ~60% the conductivity of copper |
| Silver | 0.0159 | Best conductor, rarely economical |
Aluminium conductors need a larger cross-section than copper to carry the same current with equal drop, which is why copper dominates residential wiring.
Frequently Asked Questions
Why is there a factor of 2 in the formula?
Current flows out to the load through one conductor and returns through another. The total conductor length the current travels is twice the one-way cable run, so the drop is doubled. For three-phase circuits the multiplier is √3 instead of 2.
How much voltage drop is too much?
As a rule of thumb, keep branch circuits under 3% and the total system under 5%. Beyond that, motors run hot, LED drivers may flicker, and sensitive electronics can misbehave.
Does voltage drop waste energy?
Yes. The dropped voltage is dissipated as heat in the cable. A 5% drop at full load means roughly 5% of the delivered power is lost in the wiring.
How do I reduce voltage drop?
Increase the conductor cross-section, shorten the run, reduce the current, or switch to a higher supply voltage. Increasing area gives the largest improvement per unit cost.