Voltage Drop: The 100m Workshop Cable — NEC's 3% Rule vs the World
A 32A single-phase cable run across a 100m workshop. NEC flags it as non-compliant at 3%, while BS 7671 and AS/NZS pass it at 5%. See why voltage drop limits matter more than you think.
Voltage drop is the silent killer of electrical installations. The cable passes the current rating check, the protection device coordinates, the fault loop impedance is fine — but the equipment at the end of a long run doesn't work properly because it's seeing 210V instead of 230V. Each standard handles voltage drop differently, and the limits vary dramatically.
The Scenario
A workshop extension with a long cable run:
- Load: 32A single-phase (7.4kW at 230V)
- Cable: 4mm² copper, PVC insulated
- Length: 100 metres
- Voltage: 230V single-phase
- Installation: Clipped direct to surface (Method C)
- Circuit type: Final sub-circuit to workshop machinery
This is a realistic scenario — workshops, farm buildings, and remote plant rooms routinely have 80-120m cable runs.
Side-by-Side Results
Scenario
32A single-phase, 100m, 4mm² Cu PVC, 230V, Method C
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
mV/A/m value (4mm²) | 10.8 mV/A/mTable 30 | 11.0 mV/A/mTable 4D2B | 11.0 mV/A/mTable B.52.16 | 10.5 mV/A/m#10 AWG equivalentCh.9, Table 9 |
Calculated voltage drop (V) | 13.8V10.8 × 32 × 100/1000 × 2 | 14.1V11.0 × 32 × 100/1000 × 2 | 14.1V11.0 × 32 × 100/1000 × 2 | 13.4V10.5 × 32 × 100/1000 × 2 |
Voltage drop (%) | 5.8%13.8 / 240 × 100 | 5.8%14.1 / 242.5 × 100 | 5.8%14.1 / 242.5 × 100 | 5.6%13.4 / 240 × 100 |
Allowable limit (final circuit) | 5%PASS at 3% sub-main + 2% finalAS/NZS 3000, Cl 3.6.2 | 5%3% lighting / 5% otherTable 4Ab, Note | 4%Recommended totalIEC 60364-5-52, 525 | 3%Branch circuit (recommended)NEC 210.19(A)(1) Note 4 |
Verdict | FAIL5.8% > 5% — need 6mm² | FAIL5.8% > 5% — need 6mm² | FAIL5.8% > 4% — need 6mm² | FAIL5.6% > 3% — need 10mm² (#6 AWG) |
Minimum cable to pass VD | 6mm²VD = 3.6% < 5% ✓ | 6mm²VD = 3.7% < 5% ✓ | 6mm²VD = 3.7% < 4% ✓ | 10mm² (#6 AWG)VD = 2.1% < 3% ✓ |
Why NEC Is So Much Stricter
The 3% Rule
NEC 210.19(A)(1) Informational Note 4 recommends:
- 3% maximum voltage drop on any branch circuit
- 5% maximum total from service entrance to utilization equipment
While technically a recommendation (not a requirement), most US jurisdictions and engineering firms treat the 3% limit as mandatory. The reasoning is sound: equipment designed for 120V/240V operation has tighter voltage tolerances than 230V equipment.
Recommendation vs Requirement
NEC's voltage drop limits are in Informational Notes — technically advisory, not enforceable code. But try submitting a design with 4.5% branch circuit voltage drop to a US plan review office. It will be rejected.
Different Reference Voltages
The standards calculate percentage voltage drop against different reference voltages:
- AS/NZS: 240V phase-to-neutral (Australian nominal)
- BS 7671: 230V −6% to +10% (i.e., 216V to 253V, with 242.5V midpoint)
- IEC 60364: 230V nominal
- NEC: 120V/240V (US nominal)
The lower US nominal voltage (120V for single-phase circuits) means a given absolute voltage drop represents a larger percentage, making the NEC limit effectively even tighter for 120V circuits.
The Cost Impact
For this 100m workshop scenario:
| Standard | Cable Size | Copper Weight (100m) | Approximate Cost |
|---|---|---|---|
| AS/NZS 3008 | 6mm² | 10.7 kg | $85 |
| BS 7671 | 6mm² | 10.7 kg | $85 |
| IEC 60364 | 6mm² | 10.7 kg | $85 |
| NEC | 10mm² (#6 AWG) | 19.0 kg | $150 |
NEC requires 76% more copper for the same circuit. Over a large project with many long runs, this adds up significantly.
When Voltage Drop Really Matters
Motor Starting
The 5.8% steady-state drop in our scenario would be much worse during motor starting. A 32A motor drawing 6× starting current (192A) through the same cable would see:
Starting Voltage Drop
Vd_start = 5.8% × (192 / 32) = 34.8%
The motor would see only 150V — far below the 80% minimum that most motors require to start. This is why voltage drop matters: steady-state operation might be acceptable, but transient conditions expose the problem.
Sensitive Equipment
Modern electronics, VFDs (Variable Frequency Drives), and IT equipment often require voltage within ±10% of nominal. A 5.8% steady-state drop, combined with a 5% supply variation, means the equipment could see voltage 10.8% below nominal — outside its operating range.
The Hidden Failure Mode
Voltage drop doesn't trip breakers or blow fuses. Equipment simply malfunctions — motors run hot, VFDs fault out, lighting dims, and contactors buzz. These symptoms can persist for months before anyone traces them back to an undersized cable.
Practical Guidance
For Long Runs (>50m)
Voltage drop almost always governs over current rating for runs above 50 metres. The cable sized for current capacity at 32A (4mm²) is rarely adequate for voltage drop at length.
For Multi-Standard Projects
If your project must satisfy multiple standards:
- Use NEC limits even for IEC/BS designs if the end-use equipment is US-manufactured (designed for tighter voltage tolerance)
- Use IEC 60364's 4% limit as a sensible middle ground for international projects
- Never rely on the full 5% allowance — leave margin for future load growth and conductor ageing
The 1% Rule of Thumb
For quick estimates: every 1% of voltage drop on a 230V circuit represents about 2.3V. On a 100m run at 32A:
Quick Estimate: Cable Size for 3% Target
A_min = 2 × I × L × ρ / (Vd_max × V) = 2 × 32 × 100 × 0.0183 / (0.03 × 230) ≈ 17mm² → use 16mm²
This matches the NEC requirement more closely than any of the look-up table results — because it targets 3%.
Key Takeaways
- NEC is the strictest — 3% branch circuit limit vs 5% in BS/IEC/AS standards
- All four standards fail at 4mm² — voltage drop governs over current rating for this 100m run
- NEC requires 10mm² where others need 6mm² — 76% more copper
- Voltage drop limits are not arbitrary — they protect motor starting and sensitive equipment
- For international projects, the 3-4% range is the safest design target regardless of which standard applies
Related Resources
- Northeast Blackout: Long Feeder Voltage Drop — How a 200m feeder voltage drop cascaded into a grid collapse
- Why Your Cable Sizing Is Wrong: The Reactive Component — The X sinφ term most engineers forget above 25mm²
- Motor Voltage Drop: Startup vs Running — Why motor starting current changes the voltage drop game
- Cable Sizing: The 50m Office Feeder — Same scenario, four standards, different cable sizes
- Cable Derating: 12 Cables in a Tray at 40°C — NEC's 90°C advantage when cables are grouped
- View all worked examples →
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Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.