Voltage Drop — Complete Guide

1. What is voltage drop?

Voltage drop is the reduction in electrical voltage that occurs as current flows through a conductor. Every cable — no matter how good — has electrical resistance. When current flows through that resistance, energy is lost as heat, and the voltage available at the far end of the cable is lower than at the source.

The physics is straightforward: resistance opposes current flow, and overcoming that resistance requires energy. The more current flowing, the longer the cable, and the smaller the cross-section of the conductor, the more voltage is lost along the way. This is an unavoidable physical consequence of Ohm's Law — it cannot be designed away, only managed.

Voltage drop is expressed either as a voltage (e.g. "5.2V drop") or as a percentage of the nominal supply voltage (e.g. "2.3% drop on a 230V circuit"). Standards use the percentage form because it scales with the supply voltage and gives a meaningful indication of the impact on connected equipment.

2. Why voltage drop matters

Equipment is designed to operate within a specific voltage range — typically ±10% of the nominal supply. When voltage at the load falls below the lower limit, a range of problems occur:

• Electric motors draw higher current at lower voltage to maintain torque. This increases operating temperature and shortens motor life. In severe cases, motor protection trips or the motor fails to start at all.
• LED lighting dims, flickers, or shifts colour temperature when supply voltage drops. Constant-current LED drivers have a compliance voltage range — fall outside it and the driver will not regulate correctly.
• Electronic equipment — switchmode power supplies, computers, PLCs — typically have wider input ranges, but poor voltage regulation on the supply can cause instability, reboots, or protection faults.
• Heating elements produce less heat at lower voltage, as power output is proportional to V². A 10% voltage reduction produces roughly 19% less heat output.
• Cable overheating — when a motor draws excess current due to low voltage, the cable itself runs hotter. Over time this degrades cable insulation and creates a fire risk.

Beyond equipment performance, excessive voltage drop also represents energy waste. The voltage dropped across the cable is dissipated as heat — in a large installation with poorly sized cables, this can represent a significant and ongoing energy cost.

3. Allowable limits by region

Every major electrical standard sets a maximum allowable voltage drop. These limits are defined from the point of supply (typically the main switchboard) to the most remote point of the installation.

4. The voltage drop formula

The standard engineering formula for voltage drop is derived from Ohm's Law and the resistivity of the conductor material. There are two forms — one for single phase and DC, one for three phase:

The factor of 2 in the single phase formula accounts for the fact that current travels down the active conductor and returns via the neutral — two conductors, each of length L. For three phase, the √3 factor (approximately 1.732) accounts for the phase relationship between the three conductors, which partially cancel each other.

To convert to a percentage: VD% = (VD / nominal voltage) × 100. For Australian single phase: VD% = (VD / 230) × 100.

5. Single phase worked example

Let's check what 4mm² gives us:

6. Three phase worked example

7. DC circuits

DC voltage drop uses the same formula as single phase AC: VD = (2 × L × I × ρ) / A. The ×2 factor applies because current travels out on the positive conductor and returns on the negative.

DC applications where voltage drop is particularly critical include:

• 12V and 24V LED strip lighting — at 12V, even a 5% drop is only 0.6V, but LED strips are sensitive to voltage and will visibly dim or shift colour. Most LED strip manufacturers recommend keeping drop under 3%, and under 2% for colour-critical applications.
• Solar and battery systems — cables between panels, battery banks, and inverters carry high DC currents. Undersized cables waste energy as heat and reduce system efficiency.
• EV charging — DC fast chargers operate at voltages up to 1000V DC with very high currents. Even small percentage drops represent significant power loss.
• 24V control circuits — PLC and control system wiring requires stable voltage for reliable operation.

8. Motor starting voltage drop

When an electric motor starts — particularly with a direct-on-line (DOL) starter — it draws a very large inrush current, typically 5 to 7 times the full load current. A 15A motor may draw 75–105A for the first few seconds of starting. This momentary surge causes a significant voltage drop across the supply cables.

The starting voltage drop calculation uses the same formula as single phase or three phase, but substitutes the starting current for the running current:

AS/NZS 3000 requires that the voltage at any point during motor starting does not cause other equipment to malfunction. As a practical guide, most designers aim to keep the starting voltage drop below 3% at the main switchboard. Where DOL starting causes unacceptable voltage drop, options include soft starters, star-delta starters, or variable frequency drives (VFDs), which all reduce the starting current.

9. Choosing the right cable size

Cable selection for voltage drop compliance follows these steps:

1. Determine the load current — from the equipment nameplate, or calculate from power (I = P/V for single phase, I = P/(√3 × V × pf) for three phase).
2. Check the current-carrying capacity first — the cable must be rated for the load current under the actual installation conditions (ambient temperature, installation method, bundling). This is the ampacity check. AS 3008 Table 5 provides base ratings; derating factors apply for elevated temperature and bundling.
3. Calculate voltage drop for candidate cable sizes — use the formula or the calculator's comparison table to find the smallest cable that keeps drop within limits.
4. Select the larger of the two requirements — ampacity and voltage drop may point to different cable sizes. Always use the larger.

Common cable sizes and their typical single-phase voltage drop per amp per metre (mV/A/m), copper:

Cable Size	mV/A/m (approx)	Max current (clipped, 40°C)
1.5mm²	23.3	17.5A
2.5mm²	14.0	24A
4mm²	8.75	32A
6mm²	5.83	41A
10mm²	3.50	57A
16mm²	2.19	76A
25mm²	1.40	101A
35mm²	1.00	125A
50mm²	0.70	151A

To use this table: VD (volts) = mV/A/m value × current (A) × length (m) / 1000. The values above are approximate; use the calculator for precise results.

10. Reading AS 3008 cable tables

AS 3008.1.1 is the Australian standard for cable selection. It provides current-carrying capacity and voltage drop values for cables under defined installation conditions. Understanding how to read it is an essential skill for anyone sizing cables for Australian installations.

Current-carrying capacity tables

AS 3008 Table 5 gives the base current-carrying capacity for single-core and multi-core cables under reference conditions (40°C ambient for thermoplastic insulation, specific installation methods). If your installation conditions differ, derating factors apply:

• Ambient temperature — Table 27 provides derating factors for temperatures above 40°C. At 50°C, a cable rated at 32A might be derated to around 27A.
• Grouping — cables installed in groups derate each other because they cannot dissipate heat as freely. Table 22 provides grouping factors.
• Installation method — cables in conduit, on cable tray, buried direct, clipped to a surface, and in free air all have different current ratings.

Voltage drop tables

AS 3008 Appendix C provides voltage drop values in mV/A/m for common cable sizes and installation conditions. These values account for conductor resistance and, for AC circuits, the inductive reactance of the cable. For most sub-circuit calculations, resistance dominates and the simplified resistivity formula gives results within 1–2% of the full AS 3008 method. For large cables (25mm² and above) on long AC runs, the reactance component becomes more significant and the full AS 3008 method should be used.

11. How to reduce voltage drop

When a calculated voltage drop exceeds the allowable limit, there are several approaches to bring it within compliance:

• Increase cable cross-section — the most straightforward fix. Going up one cable size (e.g. 2.5mm² to 4mm²) reduces voltage drop by 37.5%. This is the most common solution for sub-circuits.
• Shorten the cable run — relocating a sub-board, switchboard, or distribution point closer to the load directly reduces voltage drop. Not always practical but worth considering in the design phase.
• Reduce load current — using more efficient equipment (higher efficiency motors, LED lighting instead of halogen) reduces current for the same useful output.
• Split into multiple circuits — dividing a long run into parallel circuits halves the current in each cable and reduces drop significantly.
• Use three phase instead of single phase — for large loads on long runs, converting from single phase to three phase supply reduces the voltage drop factor from 2× to √3× (1.732×), a 13% reduction for the same cable size. More importantly, the load current per phase is reduced by √3, giving a total drop reduction of about 42%.
• Use copper instead of aluminium — copper's lower resistivity (0.0175 vs 0.028 Ω·mm²/m) gives about 37% less voltage drop for the same cable size. Copper is standard for most building sub-circuits.

12. Common mistakes

• Using one-way length instead of cable length — the formula uses the one-way distance from source to load (L), not the total cable length. The factor of 2 (or √3 for three phase) accounts for both conductors. Using the total cable length instead of one-way length doubles the calculated drop.
• Ignoring the feeder drop — the 5% limit applies to the total from the point of supply, not just the sub-circuit. A main feeder dropping 2.5% only leaves 2.5% budget for sub-circuits.
• Not checking ampacity separately — voltage drop and current-carrying capacity are separate checks. A cable that passes the voltage drop calculation may still be undersized for the load current — especially on short, high-current circuits.
• Using rated current instead of actual current — calculate voltage drop using the actual expected running current, not the circuit protection rating. A 20A circuit breaker protecting a 10A load has 10A running through the cable.
• Forgetting temperature derating — cables installed in roof spaces, conduits in direct sun, or other hot environments must be derated. A cable rated at 32A at 40°C may only be suitable for 24A at 55°C.
• Applying AC cable tables to DC circuits — AS 3008 and NEC tables include an inductive reactance component for AC circuits that does not apply to DC. For DC, use the pure resistivity formula.