Solar calculators that do the whole check
Most free wire calculators check voltage drop and stop there. This one also checks ampacity, applies the ambient-temperature and bundling derates, and sizes continuous circuits at 125% — then tells you which limit actually governed.
Why two checks, not one
A conductor has to satisfy two independent limits. Ampacity is how much current it can carry without overheating — that is a fire question, and it gets worse in hot attics and in bundles. Voltage drop is how much voltage you lose over the run — that is a performance question, and it dominates on long low-voltage DC runs. On a 5 ft battery cable ampacity governs; on a 100 ft 12 V run to a shed, voltage drop can force a conductor four sizes larger than ampacity alone would suggest. A calculator that checks only one of them will confidently give you the wrong answer roughly half the time.
Which code edition is this based on?
The embedded tables follow the NEC values in common use: Table 310.16 for allowable ampacity, Table 310.15(B)(1) for ambient correction, Table 310.15(C)(1) for bundling, Chapter 9 Table 8 for conductor resistance, and 690.7 for cold-temperature voltage. Code editions and local amendments differ — always confirm against the edition your inspector enforces.
Why is my string limited to fewer panels than I expected?
Because panels make more voltage when they are cold. A module rated 40 V open-circuit at 25 °C produces about 44 V at −10 °C. Multiply that across a string and a system that looks fine on the datasheet can exceed the inverter's maximum on the first frosty morning. This site applies the cold correction before counting modules, which is why it will often allow fewer panels per string than a naive calculator.
Can I rely on this for a real installation?
Use it to plan, to sanity-check a quote, and to understand the trade-offs. Do not use it as the sole basis for an installation. Wiring errors cause fires, and DC arc faults are unforgiving. Have the design reviewed by a licensed electrician or solar installer who can inspect the actual site.
🔌 Wire Size Calculator
Checks ampacity and voltage drop, with the derates applied — then shows every size so you can see why.
📉 Voltage Drop Calculator
Drop across a known conductor size, for DC or AC.
🥶 String Sizing & Cold Voc Check
Panels make more voltage when cold. This is the check that keeps a string under the inverter's limit in January.
📋 Load Audit
List what you actually run. Everything else on this site starts from this number.
🔋 Battery Bank Sizer
Rated capacity is not usable capacity — depth of discharge, cold and inverter losses all take a share.
☀️ Solar Array Sizer
How many watts of panel it takes to actually refill what you use.
🎛️ Charge Controller Sizer
MPPT and PWM are limited by different things, so they size differently.
🔁 Inverter Sizer
Continuous rating covers what runs; surge rating covers what starts.
💰 Solar Payback
Year by year, with electricity prices rising and panels slowly degrading.
🧮 Appliance Running Cost
What a device costs to run, per day, month and year.