12.3 Submains Definitions & Sizing

Submains Definitions & Sizing

Quick Answer: A submain is a circuit from the main switchboard (or from one distribution board) to another distribution board. Size it to the maximum demand of the portion it supplies, using diversity where allowed, then verify CCC, voltage drop and fault protection. Carry a protective earthing conductor with the submain so the sub-board earth bar bonds back to the main earthing system — do not fit a MEN link on the sub-board.

Definition — Topology, Not Cable Size

Candidates often think “big cable = submain”. Wrong. Classification is about what the circuit feeds:

A short 6 mm² run from MSB to a laundry DB is still a submain. A long 35 mm² consumer mains run to the MSB is still consumer mains. Thickness does not rename the circuit.

Why Submains Exist

Practical installations distribute protection and control:

• House MSB feeds a garage DB, shed DB, or upper-floor DB.
• Commercial MSB feeds tenancy boards or plant boards.
• Construction and temporary supplies use distribution assemblies fed by submains.

Submains let you keep origin protection manageable while placing local protective devices near loads. They also create the earthing and neutral topology that Section 12.4 polices strictly.

Demand and Diversity on Submains

Portion demand

Calculate maximum demand for the part of the installation supplied by that submain, not the whole site MD, unless the submain truly carries everything (rare if other circuits leave the MSB).

Example pattern: whole dwelling MD = 70 A on consumer mains; garage submain supplies only garage power and lighting with a calculated portion demand of 28 A after diversity → submain sized from ~28 A upward (then CCC/Vd/fault), not automatically 70 A.

Diversity

Appendix C / demand methods often already embed diversity for domestic load groups. For commercial and multi-board arrangements, further diversity may apply between load groups on a submain depending on the method used. Exam expectation:

• State which loads the submain supplies.
• Apply the correct demand/diversity method for those loads.
• Do not double-count diversity illegally, and do not ignore diversity where the method provides it.

Trap: Using the sum of downstream breaker ratings as submain current with no demand method — oversizing wastefully or, worse, still getting Vd wrong because length was ignored.

The Same Four Engineering Checks

1. Demand of the supplied portion (with diversity as applicable).
2. CCC for the installation method of the submain route (in roof space, underground to shed, clipped in garage, etc.).
3. Voltage drop — remember consumer mains have already used part of the budget; submain + finals must fit in what remains.
4. Fault / protection — submain protective device at the origin (usually MSB) must protect the submain cable and support required disconnection times for distribution circuits (often discussed in the 5 s distribution-circuit disconnection context, distinct from many 0.4 s final-subcircuit cases — confirm tables in AS/NZS 3000).

Protective device location

The submain OCPD is typically at the supply end (MSB). The downstream DB then has its own circuit-breakers/RCDs for final subcircuits. Do not leave a long submain unprotected because “the finals have breakers”.

Conductors in a Submain — Including Earth

A compliant submain to a MEN installation distribution board generally includes:

• Active conductor(s).
• Neutral conductor.
• Protective earthing conductor connecting the DB earth bar back to the main earthing system.

At the distribution board:

• Neutrals land on a neutral bar (no MEN link).
• Earths land on an earth bar (no MEN link).
• Bars remain separate.

Omitting the submain earth and “just bonding N–E at the shed board” is one of the most dangerous wrong answers in the entire licence syllabus.

Single-Phase and Three-Phase Submains

Garages and sheds are often single-phase; plant rooms and commercial tenancies may be three-phase. For three-phase submains:

• Assess per-phase demand correctly.
• Size neutral per the applicable rules for the load balance and harmonic considerations where relevant to the question.
• Keep earth continuous regardless of phase count.

Worked Capstone Sketch Narrative

Drawing shows:

• Energex underground service to meter panel (POS / consumers terminals).
• Consumer mains to house MSB (MEN link here).
• Submain A to garage DB.
• Submain B to rear shed DB.
• Finals from each board.

Assessment tasks typically include:

1. Name each cable run correctly.
2. Calculate portion MD for Submain B (shed tools + lights).
3. Select cable for method (e.g. underground to shed) with CCC + Vd.
4. Show earth bar at shed without MEN link.
5. Identify that an illegal N–E link at the shed would parallel the submain neutral with the earth conductor back to the MSB.

Common Sizing Mistakes

• Sizing every submain to full installation MD “to be safe” without checking Vd and cost — not always wrong electrically if CCC/Vd pass, but often wrong relative to the demand method the question required.
• Ignoring voltage drop on a 40 m shed run after spending the budget on consumer mains.
• Using indoor CCC for an underground submain.
• Forgetting earth continuity to the DB.
• Treating a submain as a final subcircuit and applying only 30 mA RCD logic without origin OCPD cable protection reasoning (RCDs on finals remain required where mandated, but they do not redefine the submain).

Coordination with Chapters You Already Studied

• Ch 6 Maximum demand — portion demand and diversity.
• Ch 4–5 Cable selection & Vd — tables and drop calculations.
• Ch 7 Protection — device rating and disconnection.
• Ch 9 MEN — earth return path remains via MSB MEN link; submain earth is a PEC, not a second MEN.

Bridge to Separation Rules

Section 12.4 focuses on the non-negotiable rule that makes submain earthing safe: neutral and protective earth remain separate downstream of the MEN link. If you understand submain topology, the illegal-link failure modes become obvious rather than memorised slogans.