6mm and 4mm Cable Amp Ratings Explained (With the Calculator to Prove It)

What is the amp rating of 6mm and 4mm cable? As a working guide for copper twin and earth in an Inner West Sydney home, 6mm carries around 44 amps clipped to a surface and about 39 amps in conduit, while 4mm carries around 34 amps and 30 amps in the same situations. There is no single number, because AS/NZS 3008 rates the same cable differently depending on how it is installed. That is why two charts, or two electricians, can quote you different figures and both be right.

1. The quick reference numbers

These are the current carrying capacities from AS/NZS 3008 for copper two core sheathed cable (the flat twin and earth used in most Inner West Sydney homes), installed in air as a single circuit. The first figure in each cell is standard PVC insulated cable, the second is 90°C rated cable such as XLPE.

Those figures assume one circuit on its own, at the standard reference temperature, with nothing packed around the cable. Real installations rarely look like that, which is where the next section comes in.

2. Why "it depends on installation method" is the real answer

A cable's rating is a heat limit. The number in the table is the current at which the conductor sits at its maximum safe operating temperature, so anything that traps heat around the cable lowers what it can carry.

Three conditions do most of the damage in older Inner West homes. Grouping is the first: when circuits run bunched together they share heat, and two touching circuits drop to about 80% of their rating, three to about 70%. This shows up often in the weatherboard cottages and workers' terraces around Leichhardt, Annandale and Petersham, where roof spaces are tight and cables end up bunched together during a retrofit. Ambient temperature is the second, because a western Sydney roof space in January runs well above the standard's reference temperature. Thermal insulation is the third and biggest, since a cable fully surrounded by ceiling batts can lose half or more of its rating. Our AS/NZS 3008 cable sizing guide covers the derating logic in more detail.

3. Why some charts say 6mm is good for 50 amps or more

You will find charts online quoting 6mm at 53A and 4mm at 42A. Those are the 90°C figures. Standard Australian twin and earth is V-90 insulated, and the standard allows it to be worked to the higher temperature ratings only where the cable is not subject to mechanical pressure that could deform hot insulation. Many electricians size to the more conservative 75°C figures for exactly that reason, especially for cables clipped under floors or pulled tight through frames. Neither approach is wrong. The point is that the higher number comes with conditions attached, and it is the installer's job to confirm they are met.

4. Voltage drop can overrule everything above

Current rating is only the first of two checks. AS/NZS 3000 limits the total voltage drop across an installation to 5%, and on a long run the resistance of the cable eats into that budget quickly. A 6mm cable that comfortably passes the current check on a 10 metre run to a cooktop can fail badly on a 40 metre run to a shed at the same load, and the fix is a bigger conductor, not a different breaker.

This is the check that surprises people, and it is why quotes for granny flats, sheds and rear garages in suburbs like Rozelle and Lilyfield often come back with 10mm or 16mm cable where the load alone suggested 6mm. Working through the voltage drop maths yourself, using the run length and load, takes about two minutes and can change the answer completely.

5. The circuits these sizes usually feed

In a typical Inner West Sydney home, 6mm copper turns up on cooktop and oven circuits, larger ducted air conditioning condensers, and 32A EV charger circuits where the run is short. 4mm commonly feeds electric hot water systems, smaller air conditioners and high demand kitchen circuits. This comes up often during renovations in Balmain, Newtown and Marrickville, where an EV charger or a new oven circuit pushes past what the original switchboard was built for. If you are planning new load like an EV charger or a heat pump, the cable is only part of the picture: the switchboard and the supply need to keep up too, which is where a switchboard upgrade or a power supply upgrade sometimes enters the conversation.

6. Work through the numbers yourself

Rather than trusting any single chart, work through your own numbers. Apply the AS/NZS 3008 method in two steps: check the current capacity for your installation method against the table above, then check voltage drop for your actual run length. Whichever check gives the smaller allowable size is the one that decides the result. The full method is explained in our guide to sizing a cable to AS/NZS 3008, and it is the same approach our licensed electricians use to check a run before it is ordered.

Why it's Level 2 ASP work

Sizing a subcircuit is standard licensed electrical work, but the same question on the supply side of your meter is not. Consumer mains, private power pole runs and anything between the network and your main switch must be sized and installed by an accredited Level 2 electrician. If your cable question is really a consumer mains question, that falls under consumer mains installation work rather than a straightforward subcircuit.

How High Demand Electrical helps

We size every run to AS/NZS 3008 against the real conditions on your property, not a generic chart, and as one of the Inner West's most trusted Level 2 ASPs we can handle the job from the street connection to the final subcircuit as one piece of work. Bring us your numbers and we will confirm them on site before anything is ordered.

Not sure what your run actually needs? Get a power supply upgrade quote or call 0402 559 777 and talk it through with a licensed local team, not a call centre. Every job needs a site assessment before final scope and pricing are confirmed.