Submain and consumer mains voltage drop
A guide for allocating voltage drop across Australian consumer mains, submains and downstream circuits without turning the allocation into a final compliance decision.
When allocation matters
Submain and consumer mains checks are different from a single final-circuit voltage-drop calculation. The reviewer is usually asking how much of the project allowance is used upstream, and how much remains for downstream circuits or equipment runs.
The useful record separates segments. It shows the demand current, supply voltage, one-way route length, cable data and target for the upstream run, then keeps the downstream allowance visible. That prevents a low downstream result from hiding a long upstream route, or an upstream result from consuming too much of the total allowance.
Allocation sequence
- Identify the point of supply, main switchboard, distribution board and downstream loads involved in the record.
- Confirm the voltage and phase basis for the upstream run.
- Use demand current or another reviewed current basis that suits the submain or consumer mains record.
- Enter the installed one-way route length for the upstream segment.
- Enter cable data that belongs to the upstream candidate.
- Enter the total project target and downstream allowance used for this review stage.
- Review upstream percentage, downstream allowance and total allocation together.
Segment record table
| Field | What to record | Why it matters |
|---|---|---|
| Upstream segment | Point of supply, board, route and candidate cable | Separates submain pressure from final circuits |
| Current basis | Demand current, measured current or reviewed design value | Current controls voltage-drop arithmetic |
| Voltage basis | Single-phase or three-phase value used for percent | Percent is not repeatable without voltage |
| Cable data source | mV/A/m or R/X values for the segment | Data must match the upstream cable context |
| Downstream allowance | Percentage or voltage reserved for downstream runs | Shows whether the upstream route leaves room |
| Total allocation | Upstream result plus downstream allowance | Keeps the whole installation context visible |
Reading allocation pressure
If the upstream result is low and the total allocation has margin, the record can move forward for project review with the current, route and cable data source attached. If the upstream run consumes most of the allowance, the project team may need to review route length, board location, demand current, candidate cable data or downstream allowance before changing a conductor.
If the total allocation is above the entered target, do not decide the answer from one number. Name the pressure point. The issue might be a long consumer mains route, a submain candidate, downstream circuit allowance, voltage basis or demand current source.
Boundaries
- Do not use allocation arithmetic as a final installation decision.
- Do not mix current basis from one board with cable data from another segment.
- Do not hide downstream allowance when the record is meant to describe a total installation path.
- Do not copy controlled cable table values into the public worksheet.
- Do not ignore DNSP, local authority, manufacturer or project requirements.