MYTH: A 20% Voltage Drop Safety Margin Is Good Engineering Practice
Blanket safety margins on voltage drop waste money. A 100m motor run: designing to 3% instead of 5% means upsizing from 16mm² to 25mm² — $3,500 extra.
February 26, 2026
The Myth
"Always design for 80% of the allowable voltage drop limit. Better safe than sorry."
This sounds prudent. It's not. It's a blanket policy that treats every circuit the same, regardless of actual operating conditions, and it wastes significant money.
The Real Cost
AS/NZS 3000:2018 Clause 3.6.2 allows 5% voltage drop from the point of supply to the most distant point of utilisation. An engineer applying a 20% safety margin designs to 4% (80% of 5%).
Consider a 100m run to a 22kW, 3-phase motor at 400V:
| Design Target | Required Cable | Actual VD | Cost per metre | Total Cable Cost |
|---|---|---|---|---|
| 5% (standard limit) | 16mm² Cu | 4.7% | $8.50 | $850 |
| 4% (with 20% margin) | 25mm² Cu | 3.0% | $13.50 | $1,350 |
| 3% (over-conservative) | 35mm² Cu | 2.2% | $19.00 | $1,900 |
The 20% margin costs an extra $500 per run. The over-conservative 3% target costs $1,050 extra. Across a 500-cable industrial project:
- 20% margin blanket policy: ~$250,000 additional cable cost
- 3% target policy: ~$525,000 additional cable cost
That's real money redirected from the project budget into unnecessary copper.
Why Blanket Margins Are Wrong
The voltage drop calculation already has built-in conservatism:
- Resistance values are at maximum operating temperature (70°C for PVC, 90°C for XLPE). Actual operating temperature is usually lower, meaning actual resistance is lower
- Load current is the protective device rating or maximum design current, not the typical operating current. Most circuits operate at 60-80% of design capacity
- Cable length typically includes routing allowances. The actual electrical length is often shorter than the routed length used in calculations
Applying a 20% margin on top of these embedded conservatisms creates double-counting. The effective safety margin is often 40-50%, not 20%.
When Margins ARE Justified
Targeted margins make sense in specific situations:
- Motor starting: VD during DOL starting can be 3-6× the running VD. If the motor start VD exceeds 10-15%, other equipment on the same bus may malfunction. Additional margin for motor circuits is defensible
- Future load growth: If the cable serves a distribution board that will be loaded in phases, designing for future capacity is reasonable — but document the planned growth, don't use a blanket margin
- Long cable runs (>200m): Temperature variation and measurement uncertainty become more significant at longer distances
- Critical loads: Medical equipment, data centres, and process control where voltage stability directly affects operation
The Right Approach
Instead of blanket margins, use engineering judgment per circuit:
- Calculate voltage drop at actual expected operating load (not worst-case design current)
- If within 85-100% of the limit, check whether embedded conservatisms provide adequate margin
- Apply additional margin only where a specific technical justification exists
- Document the justification for any margin applied
Check your design: Calculate precise voltage drop with the Voltage Drop Calculator.
Frequently Asked Questions
What is the maximum allowable voltage drop?
AS/NZS 3000 Clause 3.3.4 allows 5% for lighting and heating, IEC 60364-5-52 allows 4% for lighting (3% recommended), BS 7671 allows 3% for lighting and 5% for other uses. Always check local regulations.
Does conductor temperature affect voltage drop?
Yes significantly. Copper resistance increases with temperature per IEC 60228 (α = 0.00393/°C). A cable at 10°C has ~20% lower resistance than at 90°C, affecting voltage drop calculations for cold-start conditions.
Related Articles
- Voltage Drop Calculator - Interactive calculator with standards compliance
Try It Yourself
Run the calculations from this article using our free calculators: