Cable 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 complete guide for the full map, or continue with AWG wire gauge data and home electrical safety. For the arithmetic, open the Wire Gauge (AWG) or Electrical Power Calculator.

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) and Electrical Power Calculator 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
• Read next: pid controller explained: proportional-integral-derivative in practice
• Also in this track: single-phase vs three-phase power: the real differences
• Continue with: power factor explained: why real power is not apparent power
• Calculate as you go: power calculator · transformer turns ratio tool · cable sizing helper
• 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.

Extended Application Notes

This section expands the practical application of cable sizing guide: current, voltage drop and installation beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.

Failure Analysis in Depth

The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. Ignoring grouping factors in shared trunking Sizing long outbuilding runs by ampacity alone Letting a 32 A breaker "protect" a 20 A cable. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.

Pre-Build Checklist

Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.

What Comes Next

Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.

Theory in Practice, Extended

The theory section of cable sizing guide: current, voltage drop and installation deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.

Component Sourcing and Substitution Notes

Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.

Instrumentation for This Design

Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.

Documentation Template for This Build

Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.

When in doubt on long runs, upsize one gauge step: the copper costs once, while the voltage drop costs energy on every ampere-hour the cable ever carries. Undersized cables are a permanent tax; oversized cables are a one-time fee.