Equipment and manuals from North America can give wire sizes in AWG and cite the NEC. The NEC is NFPA 70, and this page cites the 2017 edition. This page shows what converts to AS/NZS practice and what does not.
AWG in square millimetres
NFPA 70-2017 110.6 gives conductor sizes in American Wire Gage (AWG) or in circular mils. AWG is a geometric series. A smaller gauge number is a larger wire.
The defining formula of the gauge gives the diameter of gauge n: d = 0.127 mm × 92^((36 − n) / 39). The area follows from the diameter. AmpSize calculated these rows from the formula. They are not from a table in any standard.
| AWG | Diameter | Area | Next metric size up |
|---|---|---|---|
| 14 | 1.628 mm | 2.08 mm² | 2.5 mm² |
| 12 | 2.053 mm | 3.31 mm² | 4 mm² |
| 10 | 2.588 mm | 5.26 mm² | 6 mm² |
| 8 | 3.264 mm | 8.37 mm² | 10 mm² |
| 6 | 4.115 mm | 13.30 mm² | 16 mm² |
| 4 | 5.189 mm | 21.15 mm² | 25 mm² |
No AWG size is equal to a metric size. For example, 12 AWG is 3.31 mm², between 2.5 mm² and 4 mm².
Why the rating does not convert
NFPA 70-2017 gives conductor ampacity, its word for current rating, in Article 310. Those values are for the conductor types and the conditions that the NEC names.
AS/NZS 3008.1 rates metric cables for Australian or New Zealand conditions. Its tables list metric sizes only. So you cannot take an NEC ampacity and apply it to a metric cable. Convert the size if you must compare, then size the metric cable again from AS/NZS 3008.1.
Take a 20 A circuit in New Zealand, single phase at 230 V and 30 m long. The cable is multicore copper with PVC insulation, enclosed in air at 30 °C. The engine selects 2.5 mm², rated at 23 A. Open this circuit in the calculator. The page on current-carrying capacity and derating explains how that rating changes with the installation.
Voltage drop: a note against a requirement
NFPA 70-2017 210.19(A) has an informational note, No. 4, on voltage drop. It says a branch circuit with no more than 3 % drop at the farthest outlet gives reasonable efficiency. It gives 5 % for the feeder and the branch circuit together.
NFPA 70-2017 90.5(C) says informational notes are for information only. They are not enforceable requirements. So the NEC 3 % and 5 % figures are guidance.
AS/NZS 3000:2018 Cl 3.6.2 is a requirement. The drop from the point of supply to any point must not exceed 5 %. The guide to Clause 3.6.2 shows how to split that budget.
The 20 A circuit above has a drop of 4.25 % on 2.5 mm². That meets the 5 % requirement. With a 3 % budget for the final subcircuit, the maximum length on that cable is 21.16 m. Open the drop at a 3 % limit. On 4 mm², the same circuit has a drop of 2.66 %.
Imperial sizes are not AWG
Older installations in New Zealand and Australia can have imperial conductors. Their names give the wire count and the wire diameter in inches. For example, 7/.029 is seven wires of 0.029 inch. That is a different system from AWG.
AS/NZS 3000:2018 Table I1 gives ratings for existing imperial conductors. The imperial cable tool calculates the area from the wires. For 7/.029 it gives 2.98 mm², and the nearest metric size is 2.5 mm². Table I1 allows a circuit-breaker of up to 32 A for this size, unenclosed with V75 insulation. Open 7/.029 in the imperial cable tool.
What to remember
- A gauge converts to an area. Use the formula above, or the imperial cable tool for imperial sizes.
- A rating does not convert. Size the metric cable from AS/NZS 3008.1 for the real site.
- The NEC 3 % figure is a note. The AS/NZS 3000 5 % limit is a requirement.
This page is a design aid. Verify every value against the current edition of the standard.