Line Losses
Line losses refer to the electrical energy lost as heat when current flows through conductors such as wires, busbars, and cables in a solar PV system. These losses occur due to the inherent electrical resistance of conductors and are an unavoidable part of any AC or DC electrical distribution system.
In solar design, controlling line losses is essential because excessive resistive loss reduces system efficiency, lowers inverter input voltage, affects power delivery, and can lead to overheating or code violations. Proper conductor sizing, shorter cable runs, correct string configurations, and voltage-drop calculations are critical to minimizing line losses.
Solar designers frequently account for line losses during system layout, stringing, and interconnection planning—often using specialized tools like the Voltage Drop Calculator and advanced engineering workflows within Solar Designing.
Key Takeaways
- Line losses are resistive losses that occur when electricity flows through conductors.
- They depend on current, wire length, wire size, temperature, and installation conditions.
- Excessive losses reduce energy yield, harm inverter performance, and cause voltage-drop issues.
- Proper wire sizing, optimized stringing, and smart layout planning are essential for minimizing losses.
- Tools like the SurgePV Voltage Drop Calculator and Solar Designing improve accuracy and efficiency.
What Are Line Losses?
Line losses are the amount of electrical energy dissipated as heat when electricity travels through conductors. They are also called:
- Resistive losses
- Copper losses (in copper wiring)
- I²R losses
These losses increase when:
- Wire length increases
- Wire size (cross-sectional area) decreases
- Current increases
- Temperature increases
Designers must understand and plan for line losses to maintain high system yield and meet NEC and equipment voltage requirements.
Related concepts include Stringing & Electrical Design, Voltage, and Inverter Sizing.
How Line Losses Work
Line losses follow the basic electrical formula:
Power Loss = I² × R
Where:
- I = current (amps)
- R = resistance (ohms)
1. High Current = Higher Losses
Because losses scale with the square of current, doubling current quadruples losses.
2. Resistance Depends on Wire Type & Size
Longer, smaller, or lower-quality conductors have higher resistance.
3. Temperature Increases Resistance
Hot environments or high loading increases conductor temperature and losses.
4. Voltage Drop Increases as Losses Increase
Excessive voltage drop can trigger inverter shutdowns or reduce output.
A well-designed system balances conductor sizing, current flow, and run length to maintain acceptable voltage drop and minimize loss.
Types / Categories of Line Losses
1. DC Line Losses
Occur on the DC side of the system, between:
- Modules and strings
- Strings and combiner boxes
- Combiner boxes and inverters
These are particularly important because DC voltage drop affects:
- String voltage at inverter input
- Maximum power point tracking (MPPT)
- Cold-weather voltage windows
2. AC Line Losses
Occur after inversion, typically between:
- Inverters and main service panels
- Main panels and grid interconnection points
AC line losses depend heavily on:
- Conductor length
- Phase configuration
- Load distribution
3. Secondary Resistive Losses
Additional minor losses from:
- Terminations
- Connectors
- Breakers
- Fuses
- Busbars
Though individually small, they matter in large systems.
How Line Losses Are Measured
Line losses are evaluated using:
Percentage Voltage Drop (%)
Most AHJs and industry best practices recommend:
- ≤ 3% total voltage drop (DC + AC combined)
- ≤ 2% is ideal for high-performance systems
Calculate using tools like the Voltage Drop Calculator.
Power Loss (Watts or kW)
Determined using conductor resistance and current.
Efficiency Impact (%)
Represents how much yield is lost due to wiring.
Temperature Rise
Important for conductor ampacity and NEC compliance.
Practical Guidance for Solar Designers & Installers
1. Keep conductor runs as short as possible
Longer runs = higher line losses.
2. Use larger conductor sizes for long distances
Upsizing wire significantly reduces resistive loss.
3. Increase system voltage where permitted
Higher voltage → lower current → lower I²R losses.
Relevant when planning stringing or inverter selection.
4. Maintain acceptable voltage drop
Use the Voltage Drop Calculator to stay within best-practice limits.
5. Balance DC strings carefully
See Stringing & Electrical Design.
6. Use proper connectors and terminations
Loose or corroded connections can add hidden resistance.
7. Plan cable routes during system layout
Tools like Solar Designing help reduce unnecessary cable length.
8. Verify wiring ampacity and derating
Particularly in hot environments where resistance increases.
Real-World Examples
1. Residential Rooftop System
A wiring run from a string inverter to the main panel is 90 ft.
Designer upsizes wire from 10 AWG to 8 AWG to reduce voltage drop from 3.5% to 1.8%.
2. Commercial PV System
Long DC homeruns on a warehouse roof create a 2.4% voltage drop.
Switching to a combiner-based design reduces wiring length and cuts losses nearly in half.
3. Utility-Scale Solar Farm
Engineers design high-voltage strings (1500V) to reduce current and minimize loss over 300+ ft cable runs.
.svg)
