jCalc vs Our Free Cable Size Calculator: An Honest Comparison

Is there a free alternative to jCalc for sizing a cable to Australian standards? Yes. Our free cable size calculator, embedded below, sizes low voltage cables to AS/NZS 3008 with current capacity, voltage drop and earth sizing, and produces a downloadable PDF report. jCalc is excellent professional software with a much wider scope. Which one you should use depends on who you are and what the result is for.

⚡ Quick Answers: jCalc vs our free calculator

• Do they give the same cable size? On the runs we have tested side by side, yes. Both work from AS/NZS 3008, so a like for like input produces the same size.
• What is jCalc best for? Professional electrical design. It goes well beyond cable sizing and is built for engineers and contractors producing full designs.
• What is our calculator best for? Fast, free Australian cable sizing with a client ready PDF report, no account and no setup.
• What does ours cost? Nothing. No sign up, no trial clock, no locked features.
• What does neither tool replace? A licensed electrician's judgement on site conditions, derating and the final design.

1. What jCalc does well

Credit where it is due: jCalc is a serious professional toolset. It is web based, widely used by electrical engineers and contractors, and covers far more than cable sizing, with calculators that support full design work across multiple international standards. If you are producing engineering documentation for large projects all day, a professional package like jCalc earns its place, and nothing below is an argument against that. Its advanced features are aimed at exactly that professional market.

2. What our free calculator does

Our cable size calculator is deliberately narrower and completely free. It answers the question Australian sparkies and switched on homeowners actually ask: what size cable does this run need? You give it the load, the run length and the installation method, and it works through AS/NZS 3008 to return the minimum size, its current rating, the voltage drop over your run, the maximum run length for that size, and the minimum earth conductor to match, drawn from the Wiring Rules. Every result can be exported as a two page PDF report with the inputs, the checks and a comparison of nearby sizes, ready to attach to a quote or keep on file.

It is one of several free tools on the site, all listed in our free tools roundup, and there is a plain English walkthrough of the calculator if you want a guided first run.

3. The same job through both tools

The fairest test is a real one. We took a typical submain job: a 63 amp three phase supply on four core and earth PVC cable, run 40 metres in underground conduit, and put it through jCalc and through our calculator.

Both landed on the same answer: 16mm² copper. The detail matched too. Both report the 16mm² cable's current rating as 73 amps for that installation method, voltage drop over the 40 metre run at 6.1 volts, which is 1.53 percent, and a 6mm² minimum earth. Our PDF report presents the result in two pages against jCalc's three, with the same substance: inputs, compliance checks and the all sizes comparison table.

That is not a fluke, and it is not us being clever. Both tools are applying the same standard to the same inputs, which is exactly what you want. The difference is workflow, scope and price, not the maths.

4. Who should use which

Use jCalc, or software like it, if you are an engineer or contractor producing complete designs: coordinated protection, maximum demand across a whole installation, documentation for certifiers. That work needs professional tools and professional judgement, and for network level design you also want a Level 3 designer involved.

Use our free calculator if you want a quick, standards based answer for a single run: a homeowner sanity checking the cable size on a quote for a granny flat feed like the ones in our granny flat connection guide, a sparky wanting a client ready PDF without opening a subscription tool, or anyone weighing up why their electrician quoted a bigger cable than expected, which our cable sizing explainer unpacks. For the professional level math behind the standard, the deep dive is in how to size a cable to AS/NZS 3008.

5. What neither tool replaces

A calculator, free or paid, works from the inputs it is given. What it cannot see is your site: how the cable is actually grouped with others, how hot the roof space runs, soil conditions on a buried run, and the condition of the switchboard and consumer mains either end. Those judgements change real cable selections, which is why every result, from any tool, ends with a licensed electrician confirming the design before anything is installed. If the run supports a bigger project, our power supply upgrades team can assess the whole picture.

How High Demand Electrical helps

We built the calculator, we use the same standard on real jobs every week, and we install the results: consumer mains, submains to granny flats and sheds, and the switchboard work either end. Run your numbers below, download the PDF, and if you want the design confirmed on site, that is exactly what we do.

Cable Calculator FAQs (Sydney)

Why does the calculator sometimes recommend a bigger cable than the amp rating needs?

Voltage drop. On long runs the cable that carries the current comfortably still loses too much voltage over the distance, so the next size up wins. That is the most common surprise in cable sizing.

Is the PDF report acceptable for council or certifier paperwork?

It documents the sizing inputs and checks clearly, which supports quotes and records well. Formal design sign off still comes from the licensed electrician or engineer responsible for the job.

Does the calculator handle aluminium cable?

Yes, both copper and aluminium selections are supported, including the matching minimum earth sizes for each.

Can I share a calculation with my electrician?

Yes. Download the PDF report and send it over, inputs included, so they can see exactly what was assumed and correct anything that differs on site.

Try it yourself

The calculator is embedded below, or open the full cable size calculator page. If the run is part of a bigger job, call 0402 559 777. Every installation starts with a site assessment so we can confirm scope and pricing before work begins.