Electrical Standards Reference

Why three standards exist at all

The differences are historical rather than technical. Each region developed its supply system independently, and the standards grew around the infrastructure already in the ground. The United States settled on a 120/240 V split-phase supply at 60 Hz, the UK on 230 V single-phase at 50 Hz, and Australia and New Zealand on 230 V at 50 Hz with their own earthing conventions.

That single difference in supply voltage cascades through everything. At half the voltage, twice the current is needed for the same power, which drives larger conductors, different protective device ratings and different voltage drop limits. It is the reason a circuit that is perfectly compliant in one jurisdiction can be seriously undersized in another.

How they differ in approach

Beyond voltage, the standards differ in philosophy. The NEC is prescriptive: it tells you what is permitted, largely through tables, and compliance means finding the right row. BS 7671 is more calculation-led, expecting you to derive a result from correction factors and then verify it against disconnection times. AS/NZS sits between the two, offering both tabulated selections and a calculation route.

• Conductor sizing. The NEC works from ampacity tables adjusted for temperature and conduit fill. BS 7671 works from the relationship between design current, protective device rating and cable capacity, with correction factors applied first.
• Voltage drop. The NEC treats 3% for branch circuits as a recommendation rather than a requirement. BS 7671 sets 3% for lighting and 5% for other uses as a limit you must satisfy.
• Earthing. The NEC uses a grounded-conductor system with equipment grounding conductors sized from Table 250.122. BS 7671 recognises TN-S, TN-C-S and TT arrangements, each with its own implications for fault loop impedance. AS/NZS uses the MEN system, which behaves like TN-C-S but with its own rules.
• Terminology. The same component often has three names. What the NEC calls a grounding conductor, BS 7671 calls a protective conductor. A breaker is an overcurrent protective device is a circuit protective device.

Which one applies to you

Jurisdiction decides, not preference. The NEC applies across the United States, though it is adopted state by state and local amendments are common, so the edition enforced in your area may not be the latest published one. BS 7671 applies in the UK and is referenced widely in Ireland and parts of the Middle East and Caribbean. AS/NZS 3000 applies in Australia and New Zealand.

If you work across borders, the safest habit is to select the standard explicitly for every calculation rather than relying on whichever was used last. Every Voltix calculator that behaves differently between standards asks you to choose one, precisely because the correct answer under one set of rules can be non-compliant under another.

How to use these references

Each reference page sets out the articles, tables and rules the calculators draw on, so you can check the reasoning rather than trusting an output. That matters most when a result is close to a limit, when a job is being inspected, or when you need to explain a cable size to someone else.

These pages summarise the standards, they do not replace them. Standards are revised periodically, amendments change specific requirements between editions, and local authorities may impose additional rules. For notifiable or contested work, always verify against the current published edition that applies in your jurisdiction.