Cable Size Calculator Walkthrough: Sanity Check Your Sparky in Two Minutes

Can you check your electrician's cable size recommendation yourself? Yes. Our free cable size calculator, embedded below, runs the same AS/NZS 3008 tables professionals use. Enter the load, the run length and how the cable will be installed, and in about two minutes you will see the compliant size, the voltage drop and why your sparky quoted the cable they did.

⚡ Quick Answers: the cable size calculator

• What does the calculator do? It selects a compliant cable size from the AS/NZS 3008 data for your load, run length and installation method, and shows the working.
• Is a bigger cable than I expected a rip off? Almost never. Long runs need bigger cable to control voltage drop, even when the current rating alone says a smaller cable would carry the load.
• What do I need before I start? Three things from your quote or plan: the load in amps or kilowatts, the cable route length, and how the cable will be run.
• Is it accurate? It is built directly on the AS/NZS 3008 tables, the same source professional software uses. Site specific factors still belong to your electrician.
• Can I keep a record? Yes, the Download PDF button produces a two page cable sizing report you can save or send to your electrician.
• Does it replace an electrician? No. It sanity checks a design; it does not survey your site, and the final call always follows a site assessment.

1. Grab three details before you start

Everything the calculator needs is usually sitting in your quote. First, the load: the circuit's current in amps, or the appliance rating in kilowatts. Second, the run length: the actual metres of cable from the switchboard to the end of the circuit, following the route the cable takes, not the straight line distance. Third, the installation method: whether the cable is buried in conduit, clipped to a surface, run through a ceiling, and so on.

If the quote does not state these, that is worth a question in itself. A good electrician can tell you all three without hesitation.

2. Enter the load

Type the current, or the kilowatt rating and let the tool convert. Choose single phase or three phase to match the circuit; if you are not sure which supply you have, our guide to your point of attachment and service fuse shows where to look, and anything three phase related is covered in our upgrade service.

3. Enter the run length

This is the input people underestimate, and it is the one that changes results the most. Electricity loses a little voltage in every metre of cable, so a 45 metre run to a garage, shed or granny flat can demand a size or two more than the same load at 10 metres. It is the exact effect behind the weak power point at the end of a long backyard run, and it is why granny flat connections get their cables sized carefully rather than copied from the main house.

4. Choose how the cable is installed

A cable buried in the ground sheds heat differently to one clipped in free air or pulled through an insulated ceiling, and AS/NZS 3008 rates each arrangement differently. Pick the option matching your job; if you genuinely do not know, ask your electrician which method they quoted. The difference is not cosmetic, it changes how much current the same cable can safely carry.

5. Read the result

The calculator returns the recommended size along with the numbers behind it: the cable's current capacity against your load, the voltage drop over your run, the maximum run length that size could support, and the matching minimum earth conductor. It also shows the comparison across all sizes, so you can see exactly where your run length pushed the answer up a size. If you want the theory behind those columns, our plain English AS/NZS 3008 explainer covers it, and the deeper professional method lives in how to size a cable to AS/NZS 3008.

6. Download the report

The Download PDF button generates a two page cable sizing report with your inputs, the checks and the full comparison table. Save it with your renovation paperwork or send it back to your electrician with your questions; it turns a vague conversation into one about specific numbers.

7. What the calculator cannot decide

The tool assumes standard conditions. It does not know that your cable shares a conduit with three others, runs through ceiling insulation, or crosses a patch of ground that bakes all summer, all of which derate a cable and can push the size up further. That is real design work, done on site. Treat the calculator's answer as the floor, and treat an electrician who quotes above it as someone who has probably seen your ceiling space.

This tool is one of several free ones on the site, from the postcode network checker to the maximum demand calculator; the full list with worked examples is in every free tool on our site.

How High Demand Electrical helps

We size and install cables every day, from single circuits to complete consumer mains installations, power supply upgrades and underground service mains. If the calculator raises a question about your quote, ours or anyone else's, we are happy to walk through the sizing with you, and if a load increase is behind the new cable, we can advise whether it triggers a notification of service work with your network.

Cable Size Calculator FAQs (Sydney)

Why did my electrician quote a bigger cable than the calculator shows?

Usually because of site conditions the calculator assumes away: grouped cables, thermal insulation, high ambient temperatures or planned future loads. Bigger than calculated is normal and often smart. Smaller than calculated is the answer that deserves questions.

Is the calculator based on Australian standards?

Yes, it is built on the AS/NZS 3008 cable data used across the industry in Australia and New Zealand, with earth sizing aligned to the wiring rules.

Does it handle three phase circuits?

Yes. Select three phase and the voltage drop and capacity calculations adjust automatically. It covers the common copper cable configurations used in residential and light commercial work.

Can I use it for a new shed, granny flat or EV charger run?

That is exactly what it is best at, because those long runs are where voltage drop bites. Put in the true route length and you will immediately see why long distance circuits cost more per metre than short ones.

Check your quote, then check it with us

Get a cabling or supply upgrade quote or call 0402 559 777. Every job is confirmed with a site assessment before scope and pricing are locked in, which is precisely the part no calculator can do.