Voltage Drop Calculator
Check that your cable run stays within the allowable voltage drop limit. Supports single-phase 230V and three-phase 400V circuits using conductor resistance data from AS/NZS 3008.
Calculator Inputs
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Voltage drop is the silent killer of electrical installations. A cable might be perfectly sized for current-carrying capacity but still deliver under-voltage to the load because the run is too long. Appliances run hot, motors stall on startup, and LED drivers flicker. AS/NZS 3000 caps total voltage drop at 5% from the point of supply to the final sub-circuit, and this calculator checks your design against that limit.
What is voltage drop and why does it matter?
Every cable has electrical resistance. When current flows through that resistance, some voltage is lost as heat along the length of the conductor. The longer the cable run and the higher the current, the more voltage is lost before it reaches the load.
On a 230V supply, 5% voltage drop means the load only sees 218.5V. That might sound acceptable, but consider that the supply voltage at your meter box is not always a perfect 230V to begin with. If the street supply is sitting at 225V on a hot afternoon (common in fringe-of-grid areas), and you lose another 5% through your cable run, the load sees 213.75V. That is getting into the territory where motors struggle, compressors cycle on their overloads, and sensitive equipment malfunctions.
This is why AS/NZS 3000 Clause 3.6 sets a hard limit. The 5% allowance covers the total drop from the supply point to the furthest outlet on the installation. In practice, sparkies often split this as roughly 2% on the sub-mains and 3% on the final sub-circuit — but the standard does not mandate that split, it just requires the total to stay under 5%.
Single-phase vs three-phase voltage drop
The calculation differs because of how current returns in each system.
Single-phase: Current travels out on the active conductor and returns on the neutral. The voltage is dropped across both legs of the circuit — so the route length is effectively doubled. This is why the formula uses a multiplier of 2.
Three-phase: In a balanced three-phase system, the neutral carries minimal current (theoretically zero when loads are equal across all three phases). The voltage drop formula uses √3 (approximately 1.732) as the multiplier instead of 2, and the reference voltage is 400V line-to-line rather than 230V line-to-neutral.
Three-phase circuits inherently have lower percentage voltage drop than single-phase circuits for the same cable size and current, because the 400V base is higher and the √3 multiplier is smaller than 2. This is one of the practical advantages of three-phase distribution on longer runs.
Worked example — checking voltage drop on a shed sub-main
You are running a sub-main from the main switchboard in a house to a detached shed 90 metres away. The shed has a 32A single-phase sub-board. You have selected 6 mm² TPS copper cable based on the current-carrying capacity calculation.
Step 1 — Gather values:
- Phase: Single phase, 230V
- Load current: 32A
- Cable: 6 mm² V-90 PVC copper
- Route length: 90m (one way)
- mV/A·m Table value for 6 mm²: 7.53 (from AS/NZS 3008, already factors in neutral loop)
Step 2 — Calculate voltage drop:
Using the single-phase lookup value, we calculate the absolute voltage drop:
Convert to a percentage of the 230V supply voltage:
Result: 9.43% — this blows through the 5% limit by a wide margin. The cable is undersized for this route length.
Step 3 — Try next sizes up:
Let's check 10 mm² TPS (Table mV/A·m of 4.49):
Drop (%) = (12.93 / 230) × 100 = 5.62%
Still slightly over 5%. Let's try 16 mm² TPS (Table mV/A·m of 2.82):
Drop (%) = (8.12 / 230) × 100 = 3.53%
Final result: 16 mm² passes at 3.53% drop. Even though 6 mm² is rated for 34A current-carrying capacity, the 90-metre route length forces you up to 16 mm² just to meet voltage drop limits. This is a classic example of voltage drop governing cable size rather than thermal current ratings.
Quick Reference Table
Below is a lookup table displaying the maximum route length (in metres) to maintain a maximum 5% voltage drop (11.5V) on a 230V single-phase copper circuit using standard V-90 PVC cables.
| Cable Size | 10A Load | 16A Load | 20A Load | 25A Load | 32A Load |
|---|---|---|---|---|---|
| 1.5 mm² | 40 m | 25 m | 20 m | — | — |
| 2.5 mm² | 67 m | 42 m | 33 m | 27 m | — |
| 4 mm² | 107 m | 67 m | 53 m | 43 m | 33 m |
| 6 mm² | 160 m | 100 m | 80 m | 64 m | 50 m |
| 10 mm² | 267 m | 167 m | 133 m | 107 m | 83 m |
| 16 mm² | 427 m | 267 m | 213 m | 171 m | 133 m |
Note: These are approximate values for single-circuit runs at reference ambient. Actual mV/A·m values from AS/NZS 3008 may vary slightly.
When voltage drop — not current — determines cable size
On short runs (under 15–20 metres), current-carrying capacity almost always governs cable size. The voltage drop is negligible over such short distances.
On long runs (30+ metres for sub-mains, 40+ metres for final sub-circuits), voltage drop takes over as the controlling factor. You end up selecting a cable that is way larger than what the current demands, purely to keep the voltage within limits at the far end.
This is especially common with:
- Shed and granny flat sub-mains on rural properties
- Farm installations where the pump shed is 80+ metres from the switchboard
- Commercial car park lighting circuits running 50+ metres from the distribution board
- Solar array DC runs on large roof areas (governed by AS/NZS 5033's tighter 3% limit)
If your voltage drop result pushes you up two or more cable sizes, consider whether the sub-board location can be moved closer to the supply, or whether a separate supply point (second meter) is more economical than running oversized copper.
What about voltage drop in three-phase motor circuits?
Motor circuits need special attention because motors draw significantly more current during startup (typically 6–8× their rated running current for direct-on-line starts). While the voltage drop calculation is based on running current for steady-state compliance, a large startup current surge can cause a temporary voltage dip that trips undervoltage protection or dims lighting circuits sharing the same sub-main.
For motor circuits on long runs, some sparkies calculate voltage drop at both running current and starting current to check that the starting dip is tolerable. If it is not, soft starters or variable speed drives (VSDs) can reduce the starting current and solve the voltage drop problem without upsizing the cable.
Frequently Asked Questions
Frequently asked questions about AS/NZS 3000 voltage drop rules