8.3 Voltage Drop Requirements & Worked Examples

Why Voltage Drop Matters

When current flows through real conductors, the conductors’ impedance produces a voltage difference between the supply end and the load end. That difference is voltage drop (VD). At the load, available voltage is lower than at the origin of the circuit.

Equipment performance

Motors may run hotter, draw more current, or fail to start under low voltage. Electronic power supplies, LED drivers, and control gear can misbehave. Heating appliances deliver less power ((P = V^2/R) thinking for resistive loads). Socket circuits at the far end of a long run can show dim lights or sluggish tools when heavily loaded.

Heating and related effects

Voltage drop in the cable is power lost as heat in the conductors ((I^2R) losses). Excessive design drop often signals undersized conductors for the length and current, which also stresses CCC and energy efficiency. VD limits in the Wiring Rules are therefore both a performance and a design quality control—not optional aesthetics.

Typical Overall Limits (Confirm in Your Edition)

For low-voltage installations, training and exam prep commonly use an overall voltage drop limit on the order of 5% from the point of supply to the final load, sometimes with guidance on how that total is shared between mains, submains, and final subcircuits.

Critical exam discipline:

• Treat “about 5% total” as the commonly taught benchmark, not a substitute for the exact clause, note, or table in the AS/NZS 3000 edition Aspeq permits for your sitting.
• Editions and amendments can refine percentages, application notes, and exceptions (e.g. certain equipment or utility interface conditions).
• Under open-book conditions, open the voltage-drop requirement when the stem asks for the limit—do not rely only on course memory.

How to think about the budget

If the overall limit is (V_{%}) (e.g. ~5%):

1. Compute allowable drop in volts: (V_{d,max} = V_{%} \times V_{nominal}) (e.g. 5% of 230 V ≈ 11.5 V phase-to-neutral thinking for single-phase examples).
2. Allocate drop across consumer mains + submains + final circuit so the sum stays within the overall limit.
3. Check the worst-case loaded path from point of supply to the furthest or heaviest relevant load.

Stems that give only a final subcircuit length still expect you to remember that upstream drop exists if the scenario includes long mains or submains.

Factors That Control Voltage Drop

Current (I)

Drop is proportional to load current (for a given impedance). Use the design current consistent with maximum demand or the circuit’s design load—not an arbitrary lighter figure unless the method allows.

Length (L)

Conductor length is the route length of the circuit conductors (go and return are embedded in single-phase formula forms that use route length carefully—follow the formula notes in AS/NZS 3008 / 3000 guidance you are using). Longer runs → more drop.

Conductor impedance (R, X)

Larger cross-sectional area → lower resistance → less drop. Conductor material (Cu vs Al), temperature, and cable construction matter. On AC, reactance can matter for larger cables and higher currents; many simple LV final-circuit estimates are resistance-dominated, but use tabulated mV/A·m or impedance data when the standard method requires it.

Power factor (pf)

For AC, voltage drop depends on the phase relationship between current and voltage. Lower power factor (more lagging industrial loads) can increase the magnitude of drop for the same current magnitude when reactive components are included. Exam qualitative point: pf is a real factor, not only “DC ohms.”

Simple Worked Example (Assumptions Stated)

Purpose: show the method, not a universal official calculator. Always prefer tabulated values and formulas from AS/NZS 3008.1.2 and your AS/NZS 3000 edition in real design and on the exam if numbers are required.

Assumptions

• Single-phase circuit, nominal 230 V.
• Design load current I = 20 A (continuous design current for this example).
• Route length L = 40 m from board to load.
• Approximate cable drop constant e = 3.5 mV/A·m (illustrative only—not a substitute for 3008 tables).
• Overall installation VD budget for this path segment treated as needing to stay within a ~5% overall story; we check this circuit’s contribution in volts.

Method

A common teaching form is:

[ V_d \approx \frac{e \times I \times L}{1000}\ \text{(volts)} ]

where (e) is millivolts per ampere per metre from cable data.

Substitute illustrative values:

[ V_d \approx \frac{3.5 \times 20 \times 40}{1000} = 2.8\ \text{V} ]

As a percentage of 230 V:

[ V_{d%} \approx \frac{2.8}{230} \times 100 \approx 1.2% ]

Interpretation: this final circuit alone uses about 1.2% in the example. If consumer mains and a submain already use 4% under load, the overall path may fail a ~5% total even though the final circuit “looks fine” in isolation. Conversely, if upstream drop is tiny but a long 1.5 mm² run carries high current, the final circuit can consume the entire budget.

Second glance — length doubles

Same cable and 20 A, L = 80 m:

[ V_d \approx \frac{3.5 \times 20 \times 80}{1000} = 5.6\ \text{V} \approx 2.4% ]

Still only the final segment—illustrates linear growth with length. If the stem’s limit is tight, upsizing (lower (e)) is the usual fix.

Exam presentation tips

• State assumptions (voltage, current, length, which mV/A·m value).
• Show V and % if asked.
• Never present a fabricated (e) as if it were from memory of 3008—on the real exam, open the table when numbers are required.

When Voltage Drop Governs over CCC

Current-carrying capacity answers: “Will this cable overheat at design current in this installation method?”
Voltage drop answers: “Will the load still see adequate voltage at this length and current?”

A cable can satisfy CCC and still fail VD on a long run. Then design options include:

1. Increase conductor CSA (most common “upsize for VD”).
2. Shorten the route or relocate the board.
3. Split load across additional circuits or a closer sub-board.
4. Reduce design current on that path (rarely available if the load is fixed).

Exam trap: “Cable is rated 20 A and breaker is 16 A, so length does not matter.” Length always matters for VD and often for earth-fault loop impedance.

AS/NZS 3008.1.2 Role (NZ Cable Selection)

AS/NZS 3000 sets installation requirements and voltage-drop limits/principles. AS/NZS 3008.1.2 is the companion standard for cable selection under New Zealand conditions—current-carrying capacity tables, installation methods, grouping, temperature, and the impedance / mV/A·m style data used to calculate voltage drop for practical cable types.

On the EWRB regulations prescription, 3008.1.2 is typically a B-level reference: you must know what it is for and when to reach for it, while deep table drills may sit in later cable-selection study. Cross-link in your mind:

Study drill

1. Flag voltage-drop requirements in your permitted AS/NZS 3000.
2. Flag where 3008.1.2 tables give mV/A·m or equivalent for a common twin-and-earth size.
3. Recalculate the worked example using a real table value once, writing every assumption.
4. For one long submain scenario, decide whether CCC or VD forces the larger cable.

Master the method and the two-standard split before memorising isolated millivolt figures. Numbers without the open book are less valuable than knowing which book and which check the stem is testing.