Three regions, three philosophies. The United States works to the National Electrical Code (NEC, NFPA 70), the UK and much of the Commonwealth to BS 7671 (the IET Wiring Regulations), and Australia and New Zealand to AS/NZS 3000 (the Wiring Rules). They protect the same things - people and property from electric shock and fire - but they use different voltages, different sizing methods, different earthing arrangements and a different vocabulary. This comparison maps them against each other so you can move between them with confidence.
Jurisdiction and legal basis
The NEC is published by the National Fire Protection Association and is updated on a three-year cycle. It is not itself law, but it is adopted (often with amendments) by states and municipalities, where it becomes enforceable. BS 7671 is a British Standard maintained jointly by the IET and BSI; it is non-statutory but is the recognised means of meeting the legal duties of the UK's Electricity at Work Regulations. AS/NZS 3000 is a joint Australian/New Zealand standard that is called up by state and territory electrical safety legislation, making compliance effectively mandatory.
Voltage and frequency
This is the most visible difference. The US distributes a nominal 120 V single-phase to outlets and 240 V split-phase / 208 V or 480 V three-phase for larger loads, all at 60 Hz. The UK and Australia/New Zealand use 230 V single-phase and 400 V three-phase at 50 Hz. Higher voltage means lower current for the same power, which is why UK/AU-NZ final circuits often use smaller conductors than their US counterparts for an equivalent load.
Conductor size units
The NEC sizes conductors in AWG (American Wire Gauge) and, above 4/0, in kcmil (thousands of circular mils). BS 7671 and AS/NZS size conductors in mm² cross-sectional area. There is no neat conversion factor - a rough guide is that 2.5 mm² is close to 13 AWG, 6 mm² to 9 AWG, and 16 mm² to 5 AWG. Use the wire gauge calculator (AWG) to convert between gauge, area and resistance.
Cable sizing methodology
All three start from a tabulated ampacity and apply correction factors, but the framing differs:
- NEC works from Table 310.16, selects a temperature column constrained by the terminal rating (110.14(C)), then applies ambient correction and a bundling adjustment, and finally the 125% continuous-load uplift. See the NEC wire sizing guide.
- BS 7671 uses the Ib ≤ In ≤ Iz chain with correction factors Ca (ambient), Cg (grouping), Cc (rewirable fuse / burial) and Ci(thermal insulation), referenced to an installation “reference method.” See the BS 7671 cable sizing guide.
- AS/NZS 3008 uses a closely related correction-factor method to BS 7671, with its own current-carrying-capacity and derating tables tuned to Australian/New Zealand conditions.
Earthing and grounding systems
Terminology diverges sharply here. The US speaks of grounding (the equipment grounding conductor and the grounded/neutral conductor), with system grounding rules in NEC Article 250. The UK/AU-NZ speak of earthing and classify supplies by system type: TN-S, TN-C-S (PME) and TT. A US “equipment grounding conductor (EGC)” is broadly the equivalent of a UK circuit protective conductor (CPC). Size protective conductors for either system with the earthing & grounding calculator.
Protective devices and shock protection
The NEC relies on the GFCI(ground-fault circuit interrupter, typically 4–6 mA trip) for personal shock protection and the AFCI for arc faults. BS 7671 and AS/NZS use the RCD(residual current device), commonly 30 mA for additional protection, increasingly built into an RCBO that combines overcurrent and residual-current protection in one module. The underlying principle - detect the small imbalance that signals current leaking to earth - is the same.
Voltage drop limits
The NEC offers recommendations (3% branch, 5% total) in informational notes rather than mandatory limits. BS 7671 sets recommended limits of 3% for lighting and 5% for other circuits; AS/NZS recommends 5% total from the point of supply. Whatever the standard, the maths is the same - see voltage drop explained and check any circuit with the voltage drop calculator.
Side-by-side summary
| Topic | NEC (US) | BS 7671 (UK) | AS/NZS (AU/NZ) |
|---|---|---|---|
| Nominal voltage | 120 / 240 V | 230 / 400 V | 230 / 400 V |
| Frequency | 60 Hz | 50 Hz | 50 Hz |
| Conductor size unit | AWG / kcmil | mm² | mm² |
| Ampacity table | Table 310.16 | Appendix 4 | AS/NZS 3008 |
| Sizing condition | 125% continuous | Ib ≤ In ≤ Iz | Correction-factor method |
| Protective term | Grounding (EGC) | Earthing (CPC) | Earthing |
| Shock protection | GFCI / AFCI | RCD / RCBO (30 mA) | RCD (30 mA) |
| Voltage drop guidance | 3% / 5% (advisory) | 3% lighting, 5% power | 5% total |
A common vocabulary
Switching standards is often more about translation than new engineering. A few equivalences worth memorising:
- Breaker / MCB- US “circuit breaker,” UK “MCB” (miniature circuit breaker).
- Panel / consumer unit / switchboard - the distribution board, by region.
- Receptacle / socket-outlet / GPO - the wall outlet (US / UK / AU-NZ).
- Conduit / trunking / conduit - raceways for containment.
- Grounded conductor / neutral - the current-carrying return; the grounding conductor is the protective earth.
Work in any standard with Voltix
Most Voltix calculators let you pick your standard, so you can apply the right tables without switching tools. Size NEC circuits with the wire size calculator and UK/AU-NZ circuits with the cable sizing calculator; check fill with the conduit fill calculator; confirm voltage drop with the voltage drop calculator; and size protective conductors with the earthing & grounding calculator. Whatever code you work to, the physics underneath is the same - and so are the tools.