5.1 Voltage Drop (mV/A·m) & the 5% Limit

Voltage Drop (mV/A·m) & the 5% Limit

Quick Answer: Under AS/NZS 3000 Clause 3.6, voltage drop from the point of supply to any point of consumption must not exceed 5% of nominal voltage. Calculate drop with Vd = (L × I × Vc) / 1000, taking Vc (mV/A·m) from the correct AS/NZS 3008 table for cable type and single-phase or three-phase arrangement.

Why Voltage Drop Is a Capstone Critical Skill

Cable selection is not finished when the conductor “looks big enough” for current. Even a cable that satisfies current-carrying capacity after derating can still fail the Wiring Rules if the voltage at the load is too low. Low voltage causes motors to draw higher current and run hot, luminaires to underperform, electronic supplies to misbehave, and — critically for assessment — a non-compliant installation under AS/NZS 3000 Clause 3.6 (voltage drop), which sits in the same selection theme as Clause 3.5 cable selection principles.

The capstone is open-book. Assessors expect you to find the 5% rule, apply the millivolt method with AS/NZS 3008 tables, and show units and working. Memorising a single “rule of thumb” cross-section without checking drop is a classic written-paper failure mode.

The Mandatory Limit: 5% Point of Supply to Point of Consumption

Clause 3.6 requires that the voltage drop between the point of supply (POS) and any point of the installation at which equipment is connected for use (point of consumption) shall not exceed 5% of the nominal voltage of the consumer’s mains / installation supply.

Translate that into volts before you compare numbers:

Point of supply is where the distributor’s network ends and the consumer’s electrical installation begins (for example, at the service connection / network boundary as defined for that installation). Point of consumption is where utilisation equipment is connected. The 5% figure is therefore an end-to-end budget covering consumer mains, submains and final subcircuits in series — not a free 5% on every segment independently.

The Working Formula: Vd = (L × I × Vc) / 1000

Australian trade calculation practice (and AS/NZS 3008 tabulated values) uses:

Vd = (L × I × Vc) / 1000

Where:

Reading Vc from AS/NZS 3008

AS/NZS 3008 publishes Vc values that already embed the conductor resistance (and, where tabulated, reactance effects for a.c.) for defined cable constructions, conductor materials, and circuit arrangements. Your job on the capstone is not to derive resistivity from first principles — it is to:

1. Identify cable type (for example V-90 PVC/PVC copper, XLPE, aluminium).
2. Identify conductor size (mm²).
3. Select the single-phase or three-phase value that matches the circuit.
4. Confirm whether the table basis matches the installation (multi-core vs single-core arrangements where the standard distinguishes them).

Using a three-phase Vc on a single-phase circuit (or the reverse) is a frequent error. Single-phase values account for the go-and-return path in the tabulated mV/A·m figure; three-phase values are for three-phase circuits. Length L remains the one-way route length in metres in both cases when you use the matching table column.

Quick worked fragment

A single-phase final subcircuit: L = 28 m, I = 16 A, Vc = 7.3 mV/A·m (illustrative table value — always take the real figure from your permitted AS/NZS 3008 edition).

Vd = (28 × 16 × 7.3) / 1000 = 3.27 V (approximately).

Against a 230 V supply, 5% = 11.5 V. This segment alone is fine — but you must still add drop on consumer mains and any submain that feeds it before claiming Clause 3.6 compliance.

Allocating Drop Along the Path (Design Practice vs Compliance)

Designers often budget voltage drop roughly as follows on longer routes:

Why mention 2.5% at all? Because if you “spend” nearly all 5% on long consumer mains, final subcircuits have almost no room left — especially long garage, shed, pool plant or air-conditioning runs. Capstone questions may ask you to check mains alone against a design target or to verify the total drop. Read the stem: design guidance is not a licence to ignore the overall 5%.

Choosing Current I for the Calculation

Unless the question states otherwise, calculate voltage drop at the design current Ib of the circuit (after diversity where maximum demand methods have already fixed Ib for mains/submains). Do not casually substitute the protective device rating In unless the assessment item explicitly requires a drop check at In — In is for coordination with Iz, not automatically the load used in Vd.

For multi-load final subcircuits, use the design current of that circuit. For a dedicated motor circuit, use the motor full-load current (and be aware starting voltage dip is a separate motor-starting consideration outside the steady Clause 3.6 continuous-drop check).

Percentage Form (Useful Cross-Check)

After you have Vd in volts:

% drop = (Vd / Vnominal) × 100

Example: 4.6 V drop on 230 V → (4.6 / 230) × 100 = 2.0%. That segment consumes 2.0 percentage points of the 5.0% installation allowance.

Capstone Open-Book Mindset for Clause 3.6

1. Locate Clause 3.6 (and related selection notes around Clause 3.5) in AS/NZS 3000 — confirm the 5% wording rather than relying on memory alone.
2. Convert 5% to volts for the circuit voltage.
3. Pull Vc from AS/NZS 3008 with the correct phase arrangement.
4. Compute Vd for each series segment; sum segments from POS to the critical load.
5. If total Vd exceeds the limit, increase conductor size (lower Vc), shorten route if design allows, or revise circuit arrangement — then re-check CCC because a larger cable changes both drop and current capacity.

Voltage drop is a calculation discipline: formula, table value, units, and the end-to-end 5% story. Master that pattern before moving to selection order in Section 5.2.