ProcirelCable Sizing Guide: Current, Voltage Drop and Installation
Cable size is a chain of three checks. Ampacity for heat, derating for how the cable is installed. Voltage drop for the load at the end of the run. Any one of them can size the cable; the answer is the largest that satisfies all.
> At a glance: 8 minute guide · part 9 of 10 in the electrical engineering complete guide track · includes a worked example and a quick-reference table.
## Ampacity and derating
Here is the working theory in one pass. Base current ratings assume a single circuit in free air at 30 °C. Grouped conduits, thermal insulation and ambient heat all reduce capacity grouping factors can cut ratings by half. Derating tables matter more than the copper cross-section printed on pride.
| C | o | p | p | e | r | | s | i | z | e | | | | | | | | | | |
| — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — |
| B | a | s | e | | r | a | t | i | n | g | | ( | a | p | p | r | o | x | . | ) |
| T | y | p | i | c | a | l | | u | s | e | | | | | | | | | | |
| 1.5 mm² | 15–20 A | Lighting circuits | | | | | | | | | | | | | | | | | | |
| 2.5 mm² | 20–25 A | Socket radial | | | | | | | | | | | | | | | | | | |
| 4 mm² | 25–32 A | Showers, short sub-mains | | | | | | | | | | | | | | | | | | |
| 10 mm² | 50–60 A | Cookers, small supplies | | | | | | | | | | | | | | | | | | |
| 25 mm² | 100–110 A | Domestic service tails | | | | | | | | | | | | | | | | | | |
## Voltage drop budgets
Resistance × current × length eats voltage along the run. Rules of thumb cap drop at ~3–5 % to the furthest load. Long garden and outbuilding runs are usually sized by drop, not heat a cable perfectly rated for amps can still brown out a motor at sixty metres.
## The professional method
Calculate design current, apply derating factors, choose a conductor meeting tabulated ampacity, then verify the millivolt drop over the actual route. Finally, check the breaker protects the conductor (In ≤ Iz). The cable, breaker and load are one system.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Determine design current and breaker rating
2. Apply grouping and temperature derating to pick size
3. Calculate voltage drop over the real route
4. Confirm the breaker protects the chosen conductor
### Worked example
A 6 kW shower at 230 V draws 26 A. Six metres of 6 mm² is comfortable. The same load at forty metres needs 10–16 mm² purely for voltage drop, not heat. Run the numbers yourself with the Wire Gauge (AWG) and the result should agree to within rounding.
> Practical note from the bench. Procirel wiring diagrams include a sizing table for every run length, design current, chosen size and the governing criterion (heat or drop).
## Pitfalls that cost real hardware
– Ignoring grouping factors in shared trunking
– Sizing long outbuilding runs by ampacity alone
– Letting a 32 A breaker “protect” a 20 A cable
## Key takeaways
– Ampacity and derating the foundation of this guide; revisit it if any measurement here surprises you.
– Voltage drop budgets the foundation of this guide; revisit it if any measurement here surprises you.
– The professional method the foundation of this guide; revisit it if any measurement here surprises you.
## Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the electrical engineering complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
## What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. The Wire Gauge (AWG) / Electrical Power Calculator open in a tab. Determine design current and breaker rating before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Ampacity and derating |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Pitfalls that cost real hardware |
## How this fits the electrical engineering complete guide track
This guide is one stop in the structured learning path. Start from the [electrical engineering complete guide](/tutorial/electrical-engineering-complete-guide) pillar page for the full map, or continue with [AWG wire gauge data](/tutorial/wire-gauge-awg) and [home electrical safety](/tutorial/home-electrical-safety). For the arithmetic, open the [Wire Gauge (AWG)](/tools/wire-gauge-awg) or [Electrical Power Calculator](/tools/electrical-power).
## Frequently asked questions
Aluminium or copper?
Aluminium serves large feeds cheaply with larger cross-sections; copper dominates below 16 mm² for termination reliability.
How much voltage drop is allowed?
Common guidance: ~3 % lighting, ~5 % total to the furthest point local codes set exact numbers.
Is there a calculator for this?
Yes the [Wire Gauge (AWG)](/tools/wire-gauge-awg) and [Electrical Power Calculator](/tools/electrical-power) tools run the formulas from this guide instantly, client-side, with no signup.
## Your next step in this track
– The complete electrical engineering guide: [Electrical Engineering complete guide](/tutorial/electrical-engineering-complete-guide)
– Read next: [pid controller explained: proportional-integral-derivative in practice](/tutorial/pid-controller-explained)
– Also in this track: [single-phase vs three-phase power: the real differences](/tutorial/single-phase-vs-three-phase)
– Continue with: [power factor explained: why real power is not apparent power](/tutorial/power-factor-explained)
– Calculate as you go: [power calculator](/tools/electrical-power) · [transformer turns ratio tool](/tools/transformer-turns-ratio) · [cable sizing helper](/tools/wire-gauge-awg)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.
## Experience notes
Inrush from motors or supplies. Curve C breakers tolerate it, and sizing follows the datasheet.
Isolated low-voltage work, yes. Mains and panel work needs a qualified electrician, every time.
## How to revisit this guide
Second readings work best with a purpose. Pick one section from Ampacity and derating,Voltage drop budgets,The professional method and rebuild only that part at the bench, predicting each value before measuring. Prediction errors mark exactly which concept needs the next pass, and the linked electrical calculators resolve any arithmetic doubt in seconds. Keep the marked sections in your notebook: after a month of builds, that list becomes your personal Electrical syllabus.