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Before the details, the important part.
Who can do electrical work in the UK?
Electrical installations in the UK should follow BS 7671, the IET Wiring Regulations. The IET describes it as the national standard for electrical installations in the UK. The current edition is BS 7671:2018+A4:2026 (Amendment 4); the IET says the transition period for Amendment 2 (BS 7671:2018+A2:2022) ended on 15 October 2026.
In homes in England and Wales, Part P of the Building Regulations also applies. Approved Document P says new work should be carried out in accordance with BS 7671, and it lists the work that is notifiable:
- installing a new circuit;
- replacing a consumer unit;
- any addition or alteration to existing circuits in a special location, such as the space around a bath or shower.
Electrical Safety First explains that, apart from some minor tasks, electrical work in a home must either be reported to the local authority building control body or carried out by a registered electrician who is a member of a Government approved scheme. It also says homeowners and landlords must be able to prove the work meets Part P, or they commit a criminal offence. The IET notes that people who are not registered with a self-certification scheme, including DIYers, need to notify or submit plans to a building control body, unless the work is non-notifiable. GOV.UK lists the approved competent person schemes for electrical work in homes, including NAPIT and Certsure.
Electrical Safety First adds that Scotland uses its own Building Standards system, and that Northern Ireland currently has no equivalent statutory requirement.
So, plainly: a voltage drop calculator helps with one design check. It does not make anyone competent to do electrical work, and it does not replace a registered, qualified electrician. If the work is notifiable, use a registered electrician or notify building control.
How do you calculate voltage drop?
The Regs blog sets out the formula used with the BS 7671 tables:
Voltage drop (V) = (mV/A/m) x Ib x L ÷ 1,000
- mV/A/m is the voltage drop per amp per metre for your cable type, size and installation method. It comes from the current carrying capacity and voltage drop tables in Appendix 4 of BS 7671.
- Ib is the design current of the circuit, in amps.
- L is the length of the cable run, in metres.
To get a percentage, divide the result by the supply voltage and multiply by 100.
Voltage drop calculator worked example
Say the table gives 18 mV/A/m for your cable, the design current is 20 A and the run is 20 m on a 230 V single phase supply:
- 18 x 20 x 20 ÷ 1,000 = 7.2 V
- 7.2 ÷ 230 x 100 = about 3.13%
That would be within 5% for a general circuit, but over the 3% guide for lighting. The figures here only show the arithmetic. Always read the mV/A/m value from the correct table for the actual cable and installation method.
What is the maximum voltage drop allowed under BS 7671?
According to myCableEngineering’s summary of Appendix 4, the recommended maximum voltage drops are:
| Supply | Lighting | Other uses |
|---|---|---|
| Public low voltage supply | 3% | 5% |
| Private low voltage supply | 6% | 8% |
These values are informative. The same source points out that the needs of the connected equipment and the total drop from the origin of the installation to the load should also be considered.
Questions people ask
Is voltage drop the only check a circuit needs?
No. It is one part of cable selection. Current carrying capacity, protection and other requirements in BS 7671 all apply, and new work should be inspected and tested.
Can I do my own electrical work?
Some minor work is not notifiable. For notifiable work in England and Wales, such as a new circuit or a new consumer unit, use a registered electrician or notify building control first.
Which edition of BS 7671 is current?
The IET says BS 7671:2018+A4:2026 is now the current edition.
Where Electrician Calculator helps, and where it does not
Electrician Calculator is our Android app for quick on site checks. You pick UK or US rules once. With UK rules it covers voltage drop (with the 3% lighting and 5% other uses limits), a cable sizer, cable derating and current capacity; with US rules it covers voltage drop, a wire sizer, wire ampacity, conduit fill and box fill. Results show their working with method notes, and the app leaves a case out rather than guessing when it is not covered. With UK rules there is no conduit fill, because the app does not include the IET On-Site Guide cable factor tables, and current capacity leaves out 90°C thermosetting and aluminium cables. For UK cables above 16 mm², voltage drop uses the resistive value only and does not apply reactance or power factor. The UK mode is labelled BS 7671:2018+A4:2026 (Amendment 4). Every calculator is free; a one-time Pro unlock adds an unlimited cable sizer, saved results by job and PDF job reports.
What it is good for: a fast second check of your own sums, or a quick look at whether a longer cable run is likely to need a bigger size.
What it will not do: it is a quick checking tool. It does not replace the wiring regulations, the full code book or your own judgement on site. It cannot inspect, test or certify anything, and it does not replace a registered electrician. If you are not qualified, do not use it to plan electrical work yourself.
Questions or bugs: Email us.
Sources
- BS 7671 18th Edition Wiring RegulationsIET electrical.theiet.org
- Approved Document P (2013 edition)GOV.UK assets.publishing.service.gov.uk
- Part P of the Building RegulationsElectrical Safety First electricalsafetyfirst.org.uk
- Certification schemesIET electrical.theiet.org
- Competent person schemesGOV.UK gov.uk
- BS 7671 voltage dropmyCableEngineering mycableengineering.com
- Calculating voltage dropThe Regs the-regs.co.uk