GCC Solar Academy

Voltage drop is one of the simplest calculations in PV electrical design, but it is also one of the most frequently oversimplified. A professional check needs more than current and cable length. The result depends on conductor resistance, conductor temperature, system configuration, power factor and, for AC circuits, conductor reactance.

1. Start with the circuit

First identify whether the circuit is DC, single-phase AC or three-phase AC. For a two-conductor DC circuit, current travels out and back, so the effective conductor length is twice the one-way route. Single-phase AC follows the same two-conductor principle. Three-phase circuits use the √3 relationship for a balanced three-phase load.

2. DC voltage drop

A simplified DC relationship is ΔV = 2 × I × L × R, where I is current in amperes, L is one-way length in kilometres and R is conductor resistance in ohms per kilometre. If resistance is given at 20°C, it should be adjusted when the conductor operates at a materially different temperature.

3. AC voltage drop

For an AC circuit, the resistive component is influenced by power factor and the reactive component by sinφ. A common engineering approximation for single-phase circuits is ΔV = 2IL(Rcosφ + Xsinφ). For balanced three-phase circuits, a common approximation is ΔV = √3IL(Rcosφ + Xsinφ).

4. Why conductor temperature matters

Copper and aluminium resistance increases with temperature. A conductor with a resistance quoted at 20°C will therefore produce a larger voltage drop at an elevated operating temperature. This is particularly relevant in rooftop PV where cables can be exposed to high ambient temperature and roof heat.

5. Percentage drop

The percentage is ΔV% = ΔV / system voltage × 100. Percentage is useful because a 4 V drop means something very different on a 48 V battery circuit than on a 400 V AC feeder.

6. Worked example

Assume a 400 V three-phase circuit, 40 A, 80 m one-way length, copper resistance of 1.15 Ω/km at 20°C, reactance 0.08 Ω/km, power factor 0.95 and conductor temperature 70°C. The resistance is first adjusted for temperature, then the three-phase equation is applied. The result should be compared with the project’s specified voltage-drop limit.

7. Common mistakes

• Using one-way length instead of the correct circuit relationship.
• Using DC formulas on three-phase AC circuits.
• Ignoring power factor and reactance on AC feeders.
• Using 20°C resistance for a hot rooftop conductor without checking temperature.
• Choosing a cable solely from ampacity while ignoring voltage drop.
• Applying a generic percentage limit without checking the project standard or utility requirement.

8. Design workflow

1. Determine the actual design current.
2. Determine circuit type and voltage.
3. Get representative R and X data from the cable manufacturer or approved design table.
4. Adjust resistance for operating temperature where appropriate.
5. Calculate voltage drop and percentage.
6. Compare with the design criterion.
7. Increase conductor size if required and re-check ampacity, protection and installation constraints.