CPD · Engineering

Selecting a cable to the standard

Choosing a cable size using current rating, voltage drop and protection. Verified by a named engineer before use.

about this course

What you will be able to do

L4 PractitionerAbout 4 hours (240 minutes) of CPDSelf-study plus role-playEngineering

Choosing a cable size using current rating, voltage drop and protection. Verified by a named engineer before use.

Calculate design current, given a load schedule, with correct units and no errors.
Select a cable, given tables and installation method, meeting current rating and voltage-drop limits.
Check protection, given a protective device, that it protects the cable.
Record the calculation, given a template, so another engineer can verify it.

Task, condition and standard

Task

Calculate the design current for a three-phase load, select a cable size and type that meets current rating and voltage-drop limits, and record the calculation so another engineer can verify it

Condition

Given a load schedule, sample rating tables, an installation method and a calculation template, in a 4-hour practical session with three practice loads

Standard

Calculate current with correct units and no errors, show a corrected rating above the design current, keep voltage drop within the stated limit, cite the clause and table, and complete a sheet ready for checking

These courses teach process, communication and recording. They give no clinical, legal, tax, financial or engineering sign-off. A named subject expert must review every line, and you must check your own professional body's current CPD rules, before you rely on a certificate.

sessions

8 modules

Module 1 Load, current and the safety margin 35 min

Three-phase current is I = P / (sqrt3 x V x pf).
Keep units clear: watts, volts, then amps.
A margin covers heat, grouping and ageing.

Worked example. 22 kW at 400 V, power factor 0.85: I = 22,000 / (1.732 x 400 x 0.85) = 22,000 / 588.9 = about 37.4 A. A sample 25 percent margin (your standard and a qualified person set the real one) would give 37.4 x 1.25 = 46.7 A. The later steps use 37.4 A as the design current and correct the table rating instead.

Practise: Practise with a case card or template for "load, current and the safety margin": calculate design current, given a load schedule, with correct units and no errors.

Standard to meet. Calculate the design current for a three-phase load such as 22 kW at 400 V, given a load schedule, using the correct formula and units, and state why a safety margin on current is applied.

Module 2 Voltage drop and tolerance 40 min

Drop in volts = mV per amp per metre x current x length / 1000.
Compare it with the limit on the sheet, in volts or percent.
Over the limit: choose a larger size or shorten the run.

Worked example. Case card 2: 22 kW load, 60 m from the board, 16 mm2 at 2.5 mV/A/m (sample table). Drop = 2.5 x 37.4 x 60 / 1000 = 5.6 V, or 1.4 percent of 400 V, under a sample 4 percent limit (16 V). If the sample limit were 1 percent (4 V), 25 mm2 at 1.6 mV/A/m gives 3.6 V, or 0.9 percent.

Practise: Practise with a case card or template for "voltage drop and tolerance": select a cable, given tables and installation method, meeting current rating and voltage-drop limits.

Standard to meet. Check voltage drop for a cable run, given its length and the limit on the sheet, and decide whether a larger size or a shorter run is needed to stay within tolerance.

Module 3 Reading the sample table 35 min

Find the size row, then the installation-method column.
Multiply the tabulated rating by every correction factor.
The corrected rating must be above the design current.

Worked example. Sample table: 10 mm2 = 52 A, 16 mm2 = 69 A. Temperature factor 0.87 x grouping factor 0.80 = 0.696. 10 mm2: 52 x 0.696 = 36.2 A, below 37.4 A, so it fails. 16 mm2: 69 x 0.696 = 48.0 A, above 37.4 A, so it passes (it is also above the 46.7 A sample margin figure). All table values here are worked-example figures.

Practise: Practise with a case card or template for "reading the sample table": check protection, given a protective device, that it protects the cable.

Standard to meet. Interpret the sample rating table, given the installation method and temperature, and apply the correction so the corrected rating is shown to be above the design current.

Module 4 Choosing size and type, citing the clause 40 min

Pass current first, then voltage drop, then short-circuit withstand.
Pick the smallest size that passes every check.
Write the clause and table number beside the choice.

Worked example. Choose 16 mm2: corrected rating 48.0 A is above 37.4 A, and the drop of 5.6 V (1.4 percent) is within the sample 4 percent limit. Record the sample table and clause reference (Table B, clause 7 on the sample sheet), and note that short-circuit withstand is still to be checked.

Practise: Practise with a case card or template for "choosing size and type, citing the clause": record the calculation, given a template, so another engineer can verify it.

Standard to meet. Select a cable size and type, given tables and an installation method, meeting current rating and voltage-drop limits, and cite the clause and table number that support the choice.

Module 5 Calculation sheet 30 min

Every line has an input, a unit and a result.
Cite the clause so a checker can find the rule.
Another competent person must be able to verify without asking.

Practise: Practise with a case card or template for "calculation sheet": record the calculation, given a template, so another engineer can verify it. Use a new case this time (round 2).

Standard to meet. Record the calculation on the template, with inputs, units, results and clause, so another competent person can verify it without asking questions.

Module 6 Three practice loads 40 min

Work each load from current to drop to clause.
Use that load's own length and method.
Check units before moving to the next load.

Worked example. Three loads at 400 V: A 11 kW, pf 0.80, I = 11,000 / (1.732 x 400 x 0.80) = 19.8 A; B 15 kW, pf 0.90, I = 15,000 / (1.732 x 400 x 0.90) = 24.1 A; C 22 kW, pf 0.85, I = 37.4 A. All three are practice numbers; the checker confirms the real values.

Practise: Practise with a case card or template for "three practice loads": record the calculation, given a template, so another engineer can verify it. Use a new case this time (round 2).

Standard to meet. Calculate and select the cable for three practice loads, each with its own length and installation method, with correct units and no errors.

Module 7 Assessment 10 min

Work alone, with the tables and the template.
Do not copy an old sheet without re-checking inputs.
Hand in a sheet that is ready for a checker.

Worked example. New load: 30 kW, 400 V, pf 0.85, 80 m. I = 30,000 / (1.732 x 400 x 0.85) = 50.9 A. Then correct the rating, check the drop, cite the clause and fill the sheet.

Practise: Practise with a case card or template for "assessment": record the calculation, given a template, so another engineer can verify it. Use a new case this time (round 2).

Standard to meet. Complete a full cable selection for a new load, from current to voltage drop to clause, with no calculation errors and a sheet ready for checking.

Module 8 Teach-back and peer check 10 min

Swap sheets with a partner and check the inputs first.
Recalculate the current before reading the cable choice.
Say what is wrong in one sentence, with the right value.

Worked example. A partner's sheet gives 31.8 A for the 22 kW load. Recalculation: 22,000 / (1.732 x 400) = 31.8 A, so the 0.85 power factor was left out. The correct value is 37.4 A, which is 5.6 A higher.

Practise: Practise with a case card or template for "teach-back and peer check": record the calculation, given a template, so another engineer can verify it. Use a new case this time (round 2).

Standard to meet. Evaluate a partner's calculation sheet by recalculating the current, find any wrong input or missing factor, and state the correct value with a one-sentence explanation.

practice

Worked example and case cards

Worked example (three-phase)

Load 22 kW, 400 V, power factor 0.85. Current I = P / (sqrt3 x V x pf) = 22,000 / (1.732 x 400 x 0.85) = about 37.4 A. Select a cable whose tabulated rating, after correction for installation method and temperature, is above 37.4 A; then check voltage drop is within the limit in your national standard. Cite the clause and table number.

Case card

three loads with lengths and installation methods.

Case card 2

A 22 kW load 60 m from the board, voltage drop limit stated on the sheet. Learners calculate and choose.

checks

10 check questions with model answers

Formula for three-phase current?

Model answer: P / (sqrt3 x V x pf). (Covered in module 1: Load, current and the safety margin. To pass that module: Calculate the design current for a three-phase load such as 22 kW at 400 V, given a load schedule, using the correct formula and units, and state why a safety margin on current is applied.)

Why correct the rating?

Model answer: Heat, grouping and method change what the cable can carry. (Covered in module 3: Reading the sample table. To pass that module: Interpret the sample rating table, given the installation method and temperature, and apply the correction so the corrected rating is shown to be above the design current.)

What else after current?

Model answer: Voltage drop and short-circuit withstand. (Covered in module 2: Voltage drop and tolerance. To pass that module: Check voltage drop for a cable run, given its length and the limit on the sheet, and decide whether a larger size or a shorter run is needed to stay within tolerance.)

Where is the rule?

Model answer: Your national standard; cite clause and table. (Covered in module 4: Choosing size and type, citing the clause. To pass that module: Select a cable size and type, given tables and an installation method, meeting current rating and voltage-drop limits, and cite the clause and table number that support the choice.)

Why a margin on current?

Model answer: Heat, grouping and ageing reduce safe capacity. (Covered in module 1: Load, current and the safety margin. To pass that module: Calculate the design current for a three-phase load such as 22 kW at 400 V, given a load schedule, using the correct formula and units, and state why a safety margin on current is applied.)

What if voltage drop is over the limit?

Model answer: Use a larger size or shorten the run. (Covered in module 2: Voltage drop and tolerance. To pass that module: Check voltage drop for a cable run, given its length and the limit on the sheet, and decide whether a larger size or a shorter run is needed to stay within tolerance.)

Where is the choice recorded?

Model answer: On the calculation sheet with the clause. (Covered in module 5: Calculation sheet. To pass that module: Record the calculation on the template, with inputs, units, results and clause, so another competent person can verify it without asking questions.)

Who checks the sheet?

Model answer: A second competent person. (Covered in module 5: Calculation sheet. To pass that module: Record the calculation on the template, with inputs, units, results and clause, so another competent person can verify it without asking questions.)

Can you copy last year's sheet?

Model answer: Only after re-checking the inputs. (Covered in module 7: Assessment. To pass that module: Complete a full cable selection for a new load, from current to voltage drop to clause, with no calculation errors and a sheet ready for checking.)

A partner's sheet shows 31.8 A for a 22 kW, 400 V, power factor 0.85 load. What went wrong?

Model answer: The power factor was left out; with it the current is about 37.4 A. (Covered in module 8: Teach-back and peer check. To pass that module: Evaluate a partner's calculation sheet by recalculating the current, find any wrong input or missing factor, and state the correct value with a one-sentence explanation.)

glossary

Words used in this course

load, ampacity, voltage drop, clause, derating.

marking

How the practice and assessment are marked

Level	Meaning	Marks of 4
Secure	Meets the standard in the outcome every time, no prompts	4
Meeting	Meets the standard with one small slip, corrected when prompted	3
Developing	Meets part of the standard; needs a re-attempt	2
Not yet	Standard not met	1

Pass = average 3 or more, and no outcome at 1. You may re-attempt after feedback. The subject expert sets any stricter pass mark.

where next

Ladder L1 to L7, slide by slide with notes

Slide 1: L1 Basic: What a cable does. Speaker note: A cable carries current from the board to a load. At this first step the learner can say what current, voltage and load mean, and why a cable that is too small gets hot and can fail.
Slide 2: L2 Awareness: Why size matters. Speaker note: The learner can name the three checks behind a cable choice: current rating, voltage drop and protection. They know the national standard holds the rules and that a named engineer verifies the choice before use.
Slide 3: L3 Foundation: Current and the table. Speaker note: The learner can calculate current for a simple load and read a rating table with help. They follow the 22 kW worked example, including the power factor and the correction for installation method.
Slide 4: L4 Practitioner: Select and record a cable. Speaker note: This course sits here. The learner calculates design current, corrects the table rating, checks voltage drop, selects a size, cites the clause and table, and records every step so another engineer can verify it.
Slide 5: L5 Specialist: Long runs and groups. Speaker note: The specialist handles long runs, grouped cables, high ambient temperature and several loads on one feeder. They test two or three sizes, compare the results, and justify the final choice in writing.
Slide 6: L6 Advanced: Review other sheets. Speaker note: The advanced practitioner checks other engineers' sheets, finds missing factors and wrong inputs, and confirms that the chosen cable is protected by its device, including short-circuit withstand.
Slide 7: L7 Super expert: Own the method. Speaker note: The super expert maintains the calculation templates and teaches the method. They compare editions of the standard, set review rules for sheets, and mentor engineers to verify design work instead of copying old sheets.

This course sits at L4 Practitioner. Credits stack; aligned with, not accredited by, any framework.

jobs this leads to

Where this takes you

Electrical technician
Electrical design assistant
Junior electrical engineer
Estimator
Site electrician

assessment and certificate

How you are checked

Each outcome is checked against its standard with a checklist or a short test. A digital CPD record shows the hours. Keep evidence in My page. Aligned with common CPD practice; not accredited. Learners check their own body's rules.

ten locations and global

Your place

Use your own body's forms, units and currency. Local professional bodies set the rules for CPD hours; check them first.

India (Bengaluru)

Language: English, Hindi, Kannada. Personal data: Digital Personal Data Protection Act 2023. Credit and levels: National Skills Qualifications Framework (NSQF) and the national credit framework.

Netherlands (Eindhoven area)

Language: Dutch, English. Personal data: EU GDPR (Dutch: AVG), supervised by the Autoriteit Persoonsgegevens. Credit and levels: the Dutch qualification framework (NLQF) linked to the European Qualifications Framework.

UAE (Dubai and Abu Dhabi)

Language: Arabic, English. Personal data: Federal Decree-Law No. 45 of 2021 (personal data protection). Credit and levels: the emirate education and training regulators (for example KHDA in Dubai) and the national qualifications framework.

Brazil (Sao Paulo)

Language: Portuguese (Spanish and English support). Personal data: LGPD (Lei Geral de Protecao de Dados). Credit and levels: the national technical-education catalogue and the national S-system training bodies.

Canada (Toronto)

Language: English, French. Personal data: PIPEDA, with provincial rules where they apply. Credit and levels: the provincial colleges framework and the national credential-recognition guidance.

United States

Language: English, Spanish. Personal data: state privacy laws (for example California), and COPPA for children under 13. Credit and levels: state and community-college credit systems.

Singapore

Language: English, Mandarin, Malay, Tamil. Personal data: Personal Data Protection Act (PDPA). Credit and levels: the Singapore national skills frameworks and the national credit system.

Australia

Language: English. Personal data: the Privacy Act 1988 and the Australian Privacy Principles. Credit and levels: the Australian Qualifications Framework (AQF) levels.

Hong Kong (Eastern Asia)

Language: Chinese (Cantonese, Mandarin), English. Personal data: the Personal Data (Privacy) Ordinance. Credit and levels: the Hong Kong Qualifications Framework.

Georgia and Israel (Western Asia)

Language: Georgian, Hebrew, Arabic, English. Personal data: the personal data law of the country where the learner lives. Credit and levels: the national qualifications framework of that country.

Cayman Islands (Caribbean)

Language: English. Personal data: the Data Protection Act. Credit and levels: the local education and training framework.

Mexico

Language: Spanish, English. Personal data: the federal law on personal data held by private parties. Credit and levels: the national competency certification system and the education ministry levels.

Draft

Draft CPD course. It opens only after final checks. No fee is shown.

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