Understanding Voltage Drop in AC and DC Circuits
Voltage drop occurs when electrical energy is lost as current flows through a conductor with resistance. This drop reduces the effective voltage reaching your load, which can lead to inefficient operation, dim lighting, or even equipment malfunction in sensitive electronics.
What Causes Voltage Drop?
Voltage drop depends on several factors:
- Current (I): Higher current increases voltage drop.
- Conductor Length (L): Longer wire runs create more resistance.
- Wire Size (A): Thicker wires (larger cross-section) reduce resistance.
- Material Resistivity (ρ): Copper conducts better than aluminum.
- System Type: Single-phase, three-phase, or DC configurations each have different return paths and loss behavior.
Acceptable Voltage Drop Limits
Most electrical standards, such as the NEC and IEC, recommend keeping voltage drop within these limits:
- Branch Circuits: ≤ 3%
- Feeders (total circuit): ≤ 5%
- DC Solar Wiring: Typically 2–3% to minimize energy loss
For example, if your system voltage is 230 V, a 3% drop corresponds to about 6.9 V — meaning your load should still receive at least 223 V.
Voltage Drop Formulas
The calculator above uses standard resistivity-based equations:
- DC & 1-Phase: Vdrop = 2 × ρ × L × I / A
- 3-Phase: Vdrop = √3 × ρ × L × I / A
Where:
- ρ: Resistivity of conductor (Ω·mm²/m)
- L: One-way cable length (m)
- I: Load current (A)
- A: Cross-sectional area (mm²)
Reducing Voltage Drop
Here are some practical ways to minimize voltage loss in your solar or electrical wiring:
- Use a thicker cable (larger cross-sectional area).
- Shorten wire runs when possible.
- Use copper instead of aluminum for better conductivity.
- Balance three-phase loads evenly.
- Increase system voltage (e.g., from 12 V DC to 24 V DC) to reduce current flow.
Why Voltage Drop Matters in Solar Systems
In solar PV systems, voltage drop directly affects system efficiency and battery charging. A 3% drop in a 12 V solar circuit can result in a 0.36 V loss — enough to prevent full battery charging or trigger inverter undervoltage warnings.
Carefully sizing your wiring minimizes wasted power and extends component lifespan, especially for long cable runs between panels, charge controllers, and batteries.
Quick Reference Values
| Material | Resistivity (Ω·mm²/m) |
|---|---|
| Copper | 0.0172 |
| Aluminum | 0.0282 |
Key Takeaway
Always check voltage drop when designing solar or electrical circuits — especially in low-voltage DC systems. Keeping losses below 3% ensures stable performance, efficient charging, and reduced heat buildup in conductors.