Electrical
Cable sizing and voltage drop: AS/NZS 3008 explained
Sizing a cable is two calculations, not one. Get either wrong and the install either overheats or under-delivers. Here is how AS/NZS 3008 and the Wiring Rules fit together.
Sizing a cable looks like one question — "what size do I run?" — but it is actually two. The cable has to carry the load current without overheating, and it has to deliver that current to the far end without the voltage dropping too far. These are separate calculations with separate answers, and the cable you install has to satisfy both.
AS/NZS 3008.1.1 is the standard for the selection of cables. It works alongside AS/NZS 3000, the Wiring Rules, which sets the voltage drop limit. This guide explains both calculations, the derating factors that catch people out, and why the answer is always the larger of the two.
Calculation one: current-carrying capacity
Every cable has a current-carrying capacity — the maximum current it can carry continuously without its insulation exceeding its rated temperature. Exceed it and the insulation degrades, then fails, and a failed insulation in a wall cavity is how electrical fires start.
The headline capacity figures in AS/NZS 3008 are tabulated for defined reference conditions: a particular installation method, a particular insulation type, a particular ambient temperature, and the cable installed on its own. Real installations almost never match the reference conditions exactly — which is where derating comes in.
Derating: where the reference figure stops applying
Derating factors reduce the tabulated capacity to account for the conditions the cable is actually installed in. Ignore them and you size off a number that does not apply to your job. The main factors are:
- Installation method — a cable clipped to a surface in free air sheds heat far better than the same cable buried in thermal insulation.
- Grouping — cables bunched together warm each other; the more circuits in a group, the lower each cable's effective capacity.
- Ambient temperature — a cable in a hot roof space carries less than the same cable at the reference temperature.
- Thermal insulation — a cable run through bulk ceiling insulation can lose a substantial part of its rated capacity because the insulation traps the heat the cable produces.
Thermal insulation is the one that most often catches people. A 2.5 mm² cable that comfortably carries a 20 A circuit clipped in free air may not carry the same load once it is run through a fully insulated ceiling. The cable did not change — the conditions did, and the derated capacity is the number that matters.
You apply the relevant derating factors to the tabulated figure, and the result is the actual current-carrying capacity for your installation. That figure has to be at least the protective device rating for the circuit.
Calculation two: voltage drop
A cable can be perfectly capable of carrying the current and still be the wrong size, because of voltage drop. Every metre of cable has resistance, and current through resistance loses voltage. By the time the supply reaches the far end of a long run, it can have dropped far enough that equipment runs poorly, motors struggle to start, and lights dim.
AS/NZS 3000 limits the total voltage drop, from the point of supply to any point in the installation, to a defined percentage of the nominal voltage. That budget has to cover the consumer mains, any submains, and the final subcircuit combined — it is a single allowance shared across the whole path, not a fresh allowance for each segment.
Voltage drop is driven by three things: the current, the length of the run, and the cross-sectional area of the conductor. You cannot change the current or, usually, the length — so the lever you have is conductor size. Long runs are where voltage drop, not current capacity, decides the cable size.
Why the larger answer wins
You now have two candidate cable sizes: the size needed to carry the current after derating, and the size needed to keep voltage drop within the limit. The cable you install is whichever of those is larger.
On a short run feeding a heavy load, current-carrying capacity usually decides it. On a long run feeding a modest load — a shed at the back of a property, a pump a long way from the board — voltage drop usually decides it, and it can push the cable a size or two above what the current alone would suggest.
The mistake is to do only the first calculation, size off current capacity, and discover on commissioning that the far end is starved of voltage. By then the cable is in the wall. Doing both calculations before you order cable is the cheap way to avoid the expensive fix.
Getting the numbers right on site
The arithmetic is not hard, but it is fiddly, and the inputs — installation method, grouping, ambient temperature, run length, load — change job to job. That is exactly the kind of calculation that is fast to get slightly wrong with a table and a phone.
Standardsmate's calculator suite covers cable size, voltage drop and maximum demand against AS/NZS 3008 and AS/NZS 3000, with the derating factors built in. You enter the conditions for the job and get the size, with the standard and the assumptions stated, so you can size the run before you order the cable rather than after.
Key takeaways
- Sizing a cable is two calculations: current-carrying capacity and voltage drop. The cable must satisfy both.
- Tabulated capacity figures assume reference conditions — derating for installation method, grouping, temperature and insulation gives the real capacity.
- Thermal insulation is the derating factor most often missed, and it can take a meaningful bite out of a cable's capacity.
- AS/NZS 3000 caps total voltage drop across the consumer mains, submains and final subcircuit combined — one shared budget.
- The cable you install is the larger of the two answers; long runs are usually decided by voltage drop, not current.