Final subcircuit
A final subcircuit is the last leg of wiring between switchboard and consuming load. AS/NZS 3000 RCD and isolation rules are written in terms of this category.
Ask Chalkline about this →A final subcircuit is the last leg of wiring between a switchboard and the load or socket-outlet it serves. In residential wiring, every lighting circuit, every general-purpose outlet (GPO) ring or radial, every dedicated cooktop circuit, every oven circuit, every hot-water service circuit is a final subcircuit. The term is defined in AS/NZS 3000:2018 Section 1.4 and is the unit the RCD and isolation rules in Section 2.6 are written around.
Why the term matters for builders:
A builder signing off on a switchboard layout, reviewing the sparky’s circuit schedule, or approving an Electrical Compliance Certificate needs to recognise:
- The RCD coverage rule: AS/NZS 3000 requires each final subcircuit to be RCD-protected (residential). The Standard limits no more than three final subcircuits per RCD in a domestic switchboard (Section 2.6).
- The minimum-two-RCDs rule: a domestic switchboard typically requires at least two RCDs so that loss of one RCD doesn’t kill all power (e.g. fridge + lighting on the same RCD = a fault kills the lighting).
- Isolation: each final subcircuit must be individually isolatable at the switchboard. One MCB or RCBO per circuit.
Common categories of final subcircuit (residential):
| Subcircuit | Typical rating | Cable |
|---|---|---|
| Lighting (general) | 6 A / 10 A | 1.5 mm² |
| GPOs (general) | 16 A / 20 A | 2.5 mm² |
| Cooktop | 25 A / 32 A / 40 A | 4 mm² or 6 mm² |
| Oven | 16 A / 20 A | 2.5 mm² |
| Hot water service | 16 A / 20 A | 2.5 mm² |
| Air-conditioning unit | 16 A to 32 A | 2.5 mm² to 6 mm² |
| EV charger | 32 A to 40 A | 6 mm² + |
Distinction from related circuit terms:
- Submain: the wiring between the main switchboard and a sub-switchboard (e.g. shed, granny flat, second-storey distribution board). Not a final subcircuit.
- Mains: the wiring from the supply point to the main switchboard. Not a final subcircuit.
- Final subcircuit: the last leg, switchboard-to-load.
For builders:
- Read the sparky’s circuit schedule at handover. Confirm each final subcircuit is on an RCBO or covered by an RCD on its MCB pole, and that the switchboard has at least two RCDs.
- Check the load grouping on each RCD. Don’t put lighting and fridge on the same RCD; a fridge fault during a holiday kills the lighting and wastes hours of fridge contents.
- Spec EV chargers as their own final subcircuit with dedicated RCBO. AS/NZS 3000 doesn’t allow piggybacking a high-current EV charger on a shared circuit.
Also known as: final circuit, consumer subcircuit, load circuit.
Category: Electrical / wiring.
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Last updated: 2026-05-15. Verified: 2026-05-15.