Skip to main content
ECalPro
Try Free

The 4% Voltage Drop Rule Is Not a Universal Standard — Here's What Each Code Actually Says

The widely cited 4% voltage drop limit exists in no electrical standard. AS/NZS 3000 allows 5%, BS 7671 recommends 3%/5%, IEC defers to national annexes, and NEC 210.19 is informational only. A quick-reference comparison for engineers working across jurisdictions.

2 min readUpdated March 12, 2026
Share:

Where the “4% Rule” Comes From

Ask ten electricians what the maximum voltage drop is, and at least six will say “4%.” It is repeated in training courses, rule-of-thumb guides, and contractor shorthand across the English-speaking world. The problem: no major wiring standard specifies 4% as a hard limit. It is a blended average of several different numbers from several different codes — none of which actually say 4%.

What Each Standard Actually Says

Here is the actual language from each code:

StandardVoltage Drop LimitReferenceStatus
AS/NZS 3000:20185% total (consumer mains + sub-mains + final sub-circuit)Clause 3.6.2Mandatory
BS 7671:2018+A23% lighting / 5% other (from origin to point of use)Appendix 4, Section 6.4Recommended, not regulation
IEC 60364-5-52Defers to national annex; suggests 3–5% in informative annexAnnex G (informative)Guidance only
NEC/NFPA 70:20233% branch circuit + 5% total210.19(A) Informational Note No. 4Not mandatory — informational note only

The NEC point is critical: the 3%/5% figures appear in an informational note, which per NFPA 70, Section 90.5(C) is “not enforceable as a requirement.” Yet entire cable schedules are designed to these numbers as if they were code. In the US, there is technically no NEC-mandated voltage drop limit — only equipment manufacturer requirements and engineering judgement.

Field note: At Batu Hijau, we ran 11 kV feeders over 4 km to the pit crusher. The IEC informative annex was irrelevant — we calculated voltage drop against motor starting requirements and transformer tap range. Standards give you a floor; your load tells you the ceiling.

Practical Takeaway

Stop citing “4%” as if it is a universal rule. Instead:

  • Know which standard governs your jurisdiction and cite the actual clause number.
  • Distinguish mandatory from advisory. AS/NZS 3000 Clause 3.6.2 is mandatory; NEC 210.19 Informational Note is not.
  • Calculate for the actual load. Motor starting, LED driver compatibility, and sensitive electronic equipment may impose tighter limits than any standard requires.

Try the Voltage Drop Calculator

Put this methodology into practice. Calculate results with full standard clause references — free, no sign-up required.

Or embed this calculator on your site
Calculate Voltage Drop

Frequently Asked Questions

Not among the four major international standards (AS/NZS, BS, IEC, NEC). Some local utility connection agreements or specific industrial specifications may stipulate 4%, but these are contractual requirements, not code mandates. The 4% figure appears to be a rounded average of the various 3% and 5% numbers across different standards.
Where the limit is mandatory (AS/NZS 3000), exceeding it is a code violation and may fail inspection. Where the limit is advisory (BS 7671 Appendix 4, NEC informational notes), exceeding it is not a violation but may cause practical issues: flickering lights, motor starting problems, or equipment malfunction. The standard limits exist to prevent these problems, not as arbitrary numbers.

Related Resources

Voltage Drop Calculator

Calculate voltage drop to the correct standard limit for your jurisdiction.

Read more

Cable Sizing Calculator

Size cables considering both current capacity and voltage drop constraints.

Read more