Understanding your voltage drop result
The percentage drop is compared against the commonly used design guidance below. These thresholds are advisory engineering targets (cited, for example, in NEC informational notes); local codes set the binding requirements for any installation.
| Drop (% of supply) | Assessment | Typical design reading |
|---|---|---|
| 3% or less | Within guidance | Meets the common branch-circuit design target |
| 3% – 5% | Review | May be acceptable for feeder + branch combined; consider a larger conductor |
| Above 5% | High | Generally regarded as excessive; increase cross-section or shorten the run |
- Resistivity values are 20 °C engineering figures; conductors running warm under load have higher resistance and slightly larger drop.
- The three-phase formula gives the line-to-line drop for a balanced load; unbalanced loads and power factor effects are not modeled.
- The 3%/5% figures are advisory design guidance, not legal limits. Conductor sizing for a real installation is governed by the local electrical code (ampacity, protection, installation method) and must be handled by a qualified electrician.
What is voltage drop?
Voltage drop is the reduction in voltage along a conductor caused by its own resistance. Every cable is itself a resistor: by Ohm's law, current flowing through that resistance converts part of the supply voltage into heat, so the equipment at the far end receives less voltage than the source provides. The longer and thinner the cable and the higher the current, the larger the drop.
Conductor resistance is determined by the material's resistivity, the run length and the cross-sectional area: R = ρ × L ÷ A. This calculator uses widely published 20 °C engineering values of resistivity — 0.0172 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum — meaning aluminum of the same size drops about 64% more voltage than copper. Resistivity rises with temperature, so a heavily loaded, warm conductor drops slightly more than the 20 °C figure.
Excessive voltage drop causes dim lighting, motors that run hot and underperform, and nuisance faults in electronics. A widely cited design guidance — for example the informational notes in the US National Electrical Code (NFPA 70) — suggests limiting drop to about 3% on a branch circuit and about 5% for feeder and branch combined. These figures are advisory design targets, not universal legal limits; the binding requirements come from the local electrical code that applies to the installation, and any wiring work should be performed or verified by a qualified electrician.
How to use this voltage drop calculator
- Select the conductor material — copper or aluminum.
- Select the circuit type: DC, single-phase AC (both use the two-conductor out-and-return path) or three-phase AC (√3 line factor).
- Enter the system voltage, the load current in amperes, the one-way cable length in metres and the conductor cross-section in mm².
- Read the voltage drop in volts, its percentage of the supply voltage, and the voltage remaining at the load; the color band compares the percentage with the common 3%/5% design guidance.
The formula behind voltage drop
For DC and single-phase circuits the current travels out and back, so the resistive path is twice the one-way length: Vd = 2 × ρ × L × I ÷ A. For balanced three-phase circuits the line-to-line drop uses the √3 factor instead of 2: Vd = √3 × ρ × L × I ÷ A. Here ρ is resistivity in Ω·mm²/m, L the one-way length in metres, I the current in amperes and A the cross-section in mm².
Worked example: a single-phase 230 V circuit carrying 16 A over 25 m of 2.5 mm² copper. Vd = 2 × 0.0172 × 25 × 16 ÷ 2.5 = 5.5 V. That is 5.5 ÷ 230 = 2.39% of the supply, leaving about 224.5 V at the load — within the common 3% branch-circuit guidance.
Common mistakes
- Entering the round-trip cable length: the formula already doubles the one-way length for DC and single-phase circuits, so entering the round trip doubles the result.
- Confusing line-to-line with line-to-neutral quantities in three-phase work — the √3 formula here gives the line-to-line drop for a balanced load.
- Sizing a conductor on voltage drop alone: ampacity, overcurrent protection and installation method are code requirements that can demand a larger size than the drop calculation suggests.
- Using AWG gauge numbers as mm² values — 2.5 mm² is not AWG 2.5; convert the gauge to cross-sectional area first.
- Assuming aluminum behaves like copper: at equal size aluminum drops about 64% more voltage, which is why aluminum runs are typically sized larger.
Perguntas frequentes
What is an acceptable voltage drop?
A widely used design guidance, cited for example in informational notes of the US National Electrical Code (NFPA 70), suggests limiting voltage drop to about 3% on a branch circuit and about 5% for feeder and branch circuits combined. These are advisory efficiency targets, not universal legal limits — the electrical code applicable in your location sets the binding requirements.
How do I calculate voltage drop for a cable?
For DC or single-phase circuits: Vd = 2 × ρ × L × I ÷ A, where ρ is resistivity (0.0172 Ω·mm²/m for copper at 20 °C), L the one-way length in metres, I the current in amperes and A the cross-section in mm². For example, 16 A over 25 m of 2.5 mm² copper drops 2 × 0.0172 × 25 × 16 ÷ 2.5 = 5.5 V.
Why does three-phase use √3 instead of 2?
In a balanced three-phase system the return current of each phase flows through the other phases rather than a dedicated return conductor, so the effective voltage drop between lines works out to √3 (about 1.732) times the single-conductor drop instead of 2 times. This makes three-phase distribution more efficient for the same conductor size.
Does voltage drop differ between copper and aluminum?
Yes. At 20 °C, aluminum's resistivity (0.0282 Ω·mm²/m) is about 64% higher than copper's (0.0172 Ω·mm²/m), so an aluminum conductor of the same cross-section drops correspondingly more voltage. In practice aluminum conductors are chosen one or two sizes larger than the copper equivalent to compensate.
What happens if voltage drop is too high?
Equipment receives less voltage than it was designed for: lights dim, resistive heaters deliver less output, motors draw more current and run hotter for the same mechanical load, and sensitive electronics may reset or malfunction. The lost voltage is dissipated as heat in the cable, which also wastes energy.
Can I use this calculator to size wiring for my house?
No — it is an educational estimation tool. Conductor sizing for real installations is governed by local electrical codes, which cover ampacity, overcurrent protection, installation method and derating in addition to voltage drop. Fixed wiring work must be performed or certified by a qualified electrician in most jurisdictions.
Referências
- NFPA 70. National Electrical Code — informational notes on voltage drop (3% branch circuit / 5% combined design guidance).
- IEC 60228. Conductors of insulated cables — standard conductor classes and resistance values.
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019 — units of resistance and resistivity.