Introduction #

How to size solar DC cable: compute design current from module Isc × parallel strings, apply your project’s continuous-current factor (often ~1.25× on Isc), screen conductor mm² for that ampacity, then verify voltage drop on the actual run length. Use the Cable Size Calculator for the mm² ladder and the Voltage Drop Calculator for the run.

Best for: commercial PV DC string / combiner feeder screens.

Not ideal for: AC service laterals only (still use cable-size, but different current source), or replacing NEC/IEC tabulated ampacity with a blog rule of thumb alone.

Design current for PV DC #

Conclusion: DC cable current follows fault/short-circuit capability of the array, not Imp marketing watts.

I_design ≈ Isc × N_parallel × CF

where CF is the continuous-current factor from your code/OEM (commonly ~1.25 for PV planning).

Input Tool
Isc, Voc, series/parallel Series Parallel Calculator
MPPT current gate MPPT Sizing / How to Size a Solar Charge Controller
mm² screen Cable Size Calculator

CTA: After you have I_design, enter amps in the Cable Size Calculator.

Ampacity then voltage drop #

Conclusion: Passing ampacity does not guarantee acceptable DC drop on long roof runs.

  1. Screen mm² from I_design (Cu/Al ladder).
  2. Measure one-way length to combiner / MPPT.
  3. Check % drop at operating current (often Imp × parallels for drop; keep Isc-based size for ampacity).
  4. Upsize if drop exceeds project limit (e.g. 2–3% DC—set by your standard).

Use the Voltage Drop Calculator after the mm² candidate.

Worked example (numbers) #

Given: Module Isc 11 A, Imp 10.2 A, 3P strings, CF 1.25, Cu cable, 480 V-class string voltage elsewhere already checked.

Step Result
Array Isc 11 × 3 = 33 A
I_design 33 × 1.25 = 41.25 A → screen at 42 A
Cable screen Enter ~42 A in Cable Size → next mm² step on the Cu ladder
Drop check Use Imp × 3 = 30.6 A operating current over actual length in Voltage Drop

If drop fails, increase mm² even when ampacity already passed.

AC vs DC feeders #

Conclusion: Inverter AC output uses AC line current (kW ÷ √3 ÷ V); DC strings use Isc-based current. Do not paste AC amps into DC string sizing.

AC side continues with Breaker Size and the same cable tool on AC amps—see Grid Tie Solar Protection Requirements.

Common mistakes #

  1. Sizing DC cable from Imp only without Isc × CF ampacity.
  2. Ignoring parallel strings.
  3. Skipping voltage drop on long roof homeruns.
  4. Using residential AWG charts without project derating (ambient, grouping).
  5. Assuming the MPPT input lug size replaces cable ampacity engineering.

Next steps #

  1. Layout current: Series Parallel → MPPT Sizing.
  2. mm² screen: Cable Size Calculator.
  3. Drop: Voltage Drop Calculator.
  4. Hub: Solar calculator · Electrical hub.
  5. Array context: How to Size a Commercial Solar Array.

Assumptions and disclaimer #

Continuous-current factors, temperature ratings, and installation methods are code- and project-specific. This guide is a planning screen, not a substitute for IEC/NEC tables or PE review.

FAQ #

How do I size solar DC cable?

Compute Isc × parallel strings × continuous-current factor, screen mm² for that amperage, then verify voltage drop on the run length.

Should I use Isc or Imp?

Use Isc-based current (with factor) for ampacity. Imp × parallels is often used for voltage-drop operating checks.

What continuous-current factor is common?

Many PV plans use about 1.25× on Isc. Confirm the edition of your wiring code and OEM instructions.

Which calculator should I use?

The Cable Size Calculator for mm² screening (see the PV DC note), then Voltage Drop for length.

Is AC inverter cable sized the same way?

No—AC uses inverter output current at AC voltage. DC strings use PV Isc topology. Both can use the cable tool with the correct amps.