Wiring calculator

DC Cable Size Calculator

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The battery-to-inverter cable is the most dangerous run in the system. Size it for the current it carries and the length it runs. Enter your numbers and this tool checks both the 125% ampacity rule and a 2% voltage drop, then recommends the larger conductor.

Recommended copper cable

—

Enter your current and run length.

For 125% ampacity: —
For 2% voltage drop: —
Drop at this size: —

75°C copper, planning estimate. Derate for heat and bundling.

Continuous DC current

amps (A)

From the inverter datasheet (max continuous DC current). Not sure? A rough estimate is inverter watts ÷ battery volts ÷ 0.9.

System voltage

One-way run length

metres (m)

The distance from the battery to the inverter, one way. Keep it as short as you can.

Never connect an inverter to your house wiring without a transfer switch

Wiring an inverter's AC output into your home circuits without a listed transfer switch or a mechanical interlock can push power back onto the utility line. That can kill a lineman working on what they believe is a dead circuit, and it can destroy your inverter the moment grid power returns. Transfer equipment is mandatory under NEC 702.5 and the equivalent clause in IEC 60364. Plug loads directly into the inverter, or have a licensed electrician install a proper transfer switch. There is no safe DIY shortcut here.

More broadly, working with inverters, batteries, and mains wiring carries a real risk of electric shock, burns, and fire, and high-current DC battery banks are especially dangerous because arcs do not self-extinguish. This content is educational only. It is not a substitute for hands-on work by a licensed electrician who knows your local codes and can inspect your setup.

How this calculator works

Two independent checks decide a DC cable size, and you take whichever asks for the larger conductor.

Ampacity (125% rule). An inverter's DC feed is a continuous load, so the conductor must be rated to carry 125 percent of the continuous current without overheating. We compare your current against the 75°C ampacity of standard copper conductors.

Voltage drop (2%). Every metre of cable has resistance, so some voltage is lost on the way. We size so the drop stays under 2 percent of the system voltage over the round trip, using the resistance of copper. On long runs this is usually what forces a bigger cable.

The result is a planning figure at 75°C in normal conditions. In a hot battery enclosure, in direct sun, or with several cables bundled in conduit, the safe current drops and you may need to go larger. Always use pure stranded copper, never CCA, crimp your terminations, and fit a correctly rated DC fuse or breaker at the battery. Final selection is the licensed installer's call under your local code.

Do not know your current yet? Start with the inverter size calculator to find your inverter size and battery voltage first.

FAQ

Cable sizing questions

How do you size DC battery cable?

Two checks decide it. First, the conductor must carry 125 percent of the continuous current (the NEC continuous-load rule). Second, it must keep voltage drop under about 2 percent over the run. Whichever check demands the larger conductor is the one you follow. This tool runs both.

Why does run length matter so much?

Voltage drop rises with both current and length. A short run is usually limited by ampacity, so a modest cable is fine. A long run at high current can need a much larger conductor purely to hold voltage drop down, even though a smaller one would carry the current.

Can I use copper-clad aluminium (CCA)?

No. Use pure stranded copper for battery and inverter DC runs. CCA has higher resistance and is not suitable for these high-current connections. Terminations should be crimped, not soldered, because a soldered joint can creep and fail under heat and vibration.

Is this a substitute for an electrician?

No. This is a planning tool using standard 75 degrees C copper values. Heat, conduit bundling, and your local code can all require a larger conductor. Final cable selection is the licensed installer’s call under your authority having jurisdiction.

Written by Engr. Syed Farrukh Anwar

Registered Professional Electronics Engineer (PEC ELECTRO/15141)

A licensed practitioner with boots-on-the-ground experience across South Asia and the Middle East, designing and commissioning high-yield solar arrays and backup systems for some of the harshest thermal environments on Earth. InverterWise exists to give you the straight engineering facts, minus the sales talk.

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