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SOLARCALCKIT SUITE / Solar Wire Size & Drop Calculator

Solar Wire Size & Drop Calculator

NEC CH. 9 & IEC 60228 REFERENCE RESISTIVITY

Size DC conductors from current, one-way run length, and allowable voltage drop — with complete resistance data and standard cable cross-sections.

TOOLS: Solar Battery Battery Bank MPPT Sizing Wire Gauge Inverters Load Sizing
Circuit Parameters
Client-Side Engine
01 Circuit Demand & Length

Conductor current in amps.

One-way distance in metres (doubled for round-trip).

Nominal DC circuit voltage.

Standard practice is ≤3% for DC solar cable runs.

02 Conductor Material & Specific Check

Uses standard 20 °C resistivity values.

Enter standard mm² size to check its exact voltage drop.

How solar wire size is determined

A DC solar cable has to carry current without overheating (ampacity) and without wasting too much voltage (voltage drop). This solar wire size calculator sizes for voltage drop, which drives the cross-section for most solar runs.

Formulas

The voltage-drop math

Three deterministic equations from Ohm's Law and conductor resistivity.

Step 1 — Round-Trip Resistance

R = (2 × L × ρ) ÷ Area

Round-trip length (2 × L) accounts for positive and negative conductors.

Step 2 — Voltage Drop

V_drop = I × R  ·  Drop% = (V_drop ÷ V_sys) × 100

Voltage lost as heat across the conductor run.

Frequently asked questions

How do I calculate solar wire size?

For DC solar cable, size for voltage drop: required area (mm²) = round-trip length × current × resistivity ÷ the allowed voltage drop. For example, 20 A over a 10 m one-way run on a 24 V system allowing 3% drop needs about 9.6 mm² of copper, so 10 mm² cable. Always confirm the result against ampacity and your local wiring rules.

What voltage drop should I allow for solar wiring?

A common guideline is 3% or less for the DC cable run, often split across the array-to-controller and controller-to-battery segments. The drop percentage is the voltage lost (I × R for the full round trip) divided by the system voltage.

How do I convert solar panel watts to amps for wire sizing?

Divide the array watts by the battery system voltage: 1000 W on a 24 V system is about 41.7 A, on a 48 V system about 20.8 A. Use that current, the one-way cable length, and your allowable voltage drop to find the conductor area from A = 2 × L × I × ρ ÷ V_drop.

Does this calculator check ampacity or code compliance?

No. It performs voltage-drop sizing only, which is one requirement among several. Ampacity depends on temperature ratings, ambient derating, conduit fill and terminations, and varies by adopted code (for example NEC 310 ampacity tables). Verify against your installation rules.

Why is the round-trip cable length used?

Current flows from the source to the load and back again, so both conductors contribute to the voltage drop. The resistance is calculated over twice the one-way distance (2 × L), which is why the formula uses the round-trip length.

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