How to Size Solar DC Cable: PV Current, Drop & Example
Introduction #
How to size solar DC cable: compute maximum circuit current from module Isc × parallel strings × continuous-current factor, then size conductors to the greater of (a) that current after conditions-of-use ampacity adjustments and (b) the continuous-load conductor factor required by your adopted PV article (often a second 1.25× on many NEC 690.8 paths). Screen mm² for that ampacity, then verify voltage drop. Always confirm the adopted code edition and OEM string instructions.
Open solar cable size calculator (PV preset) →
Use the Cable Size Calculator (loads the PV DC string · 27.5 A preset: Isc 11 A × 2P × 1.25) 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_max ≈ Isc × N_parallel × CF₁
I_cond_screen ≈ max(I_max after conditions of use, I_max × CF₂) when your adopted code uses a continuous conductor factor (many NEC 690.8(B)(1) paths use CF₁ = CF₂ = 1.25 → about 1.56× Isc × N before other adjustments).
Do not collapse both factors into a single “~1.25 design current” unless your AHJ/OEM explicitly uses a one-step method.
| 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 the conductor screening amps (not Imp alone), open the Cable Size Calculator — solar / PV DC preset.
Ampacity then voltage drop #
Conclusion: Passing ampacity does not guarantee acceptable DC drop on long roof runs.
- Screen mm² from the conductor screening current.
- Measure one-way length to combiner / MPPT.
- Check % drop at operating current (often Imp × parallels for drop; keep Isc-based size for ampacity).
- Upsize if drop exceeds the project DC % limit (do not invent a universal 2–3%).
Use the Voltage Drop Calculator after the mm² candidate.
Worked example (numbers) #
Given: Module Isc 11 A, Imp 10.2 A, 3P strings, CF₁ = CF₂ = 1.25 (illustrative NEC 690.8(A)+(B)(1)-style path—confirm your edition), Cu cable.
| Step | Result |
|---|---|
| Array Isc | 11 × 3 = 33 A |
| I_max (CF₁) | 33 × 1.25 = 41.25 A |
| Conductor screen (CF₂ on I_max) | 41.25 × 1.25 = 51.56 A → screen ≈ 52 A |
| Cable screen | Enter ~52 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 line current for three-phase active power is I ≈ P ÷ (√3 × V × PF) (prefer inverter nameplate output current when available). DC strings use Isc-based ampacity. 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 #
- Collapsing two continuous-current factors into one ~1.25× when your code path requires both I_max and conductor factors.
- Sizing DC cable from Imp only without Isc-based ampacity.
- Ignoring parallel strings.
- Skipping voltage drop on long roof homeruns.
- Using residential AWG charts without project derating (ambient, grouping).
- Assuming the MPPT input lug size replaces cable ampacity engineering.
Next steps #
- Layout current: Series Parallel → MPPT Sizing.
- mm² screen: Cable Size Calculator.
- Drop: Voltage Drop Calculator.
- Hub: Solar calculator · Electrical hub.
- 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?
Confirm continuous-current factors in your adopted PV article (many NEC 690.8 paths use 1.25× for I_max and a second continuous factor for conductors). Do not assume a single 1.25× replaces both steps.
Which calculator should I use?
The Cable Size Calculator (opens with the PV DC string Quick preset) for mm² screening, then Voltage Drop for length. This Guide is the method page—not a separate solar-cable-only tool.
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.