Design current
Ib
The a circuit is expected to carry in normal use. For a heater or shower it's just the power divided by the , but a motor's input has to be worked back from its and first. It's where cable sizing starts: the device's rated current must be at least Ib, and the cable's capacity at least the device's rating.
Using Design current in context
Reviewed
A distinction worth remembering
A useful revision comparison is a short cable in open air against the same load supplied over a long route through insulation. Installation conditions, voltage drop and fault protection can change the selection. Learn the meaning of each input before treating a calculation as a design decision.
Questions to take further
- How can I revise Design current beyond the definition?
- Explain what Design current describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- Open University: relationships between electrical quantities — The relationship between voltage, current and resistance and the limits of an ohmic model.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- IET: earthing and bonding FAQs — Protective bonding, earthing arrangements and loop impedance.
Where Design current fits in the learning path
The Design Calculations section covers these units:
- Sizing a Cable, Every Factor
- Volt Drop and Zs in Depth
- Fault Current, the Adiabatic Check and Device Choice
- Submains, Three-Phase Boards and Demand
Curriculum mapping: C&G 2365 Unit 305.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- Cable SizingInstallation
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Cable Selection — quiz previewPlus · Installation
3. Return to the meaning
After reading or practising, explain the idea in your own words, then check what you remembered.
Recall a related term
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
The current a conductor can carry continuously under stated installation conditions without exceeding its specified temperature limit. Cable tables give values for defined reference conditions, with adjustments where the actual conditions differ. Meeting the thermal-capacity check does not replace the other design checks, including protective-device coordination, voltage drop and fault protection.
Current-carrying capacity
Read the Current-carrying capacity entryCompare related terms
Open a meaning to compare its role with Design current, or follow its full entry.
Current-carrying capacity — Iz
The current a conductor can carry continuously under stated installation conditions without exceeding its specified temperature limit. Cable tables give values for defined reference conditions, with adjustments where the actual conditions differ. Meeting the thermal-capacity check does not replace the other design checks, including protective-device coordination, voltage drop and fault protection.
Read the Current-carrying capacity entryNominal current — In
The rated current of a protective device, such as the 32 A marked on a 32 A MCB. On an adjustable device, it's the current setting. It isn't what the circuit draws: for overload protection it has to be at least the design current and no more than the cable's capacity, written Ib ≤ In ≤ Iz.
Read the Nominal current entryTabulated current — It
The current a cable can carry under the reference conditions in BS 7671's Appendix 4 tables, before any adjustment. To choose a cable, divide the device's rating by the correction factors that apply, then find a cable whose tabulated figure is at least that. Multiply the chosen cable's It by installation factors such as Ca, Cg and Ci, and you've got its capacity as installed, Iz.
Read the Tabulated current entryMaximum demand
The assessed greatest simultaneous demand of an installation or part of it. Connected loads, their operating patterns and justified diversity allowances inform the assessment. It helps determine supply and distribution capacity; it is not a guarantee that actual demand can never be higher, and a change of use may require reassessment.
Read the Maximum demand entryDiversity
An allowance used when assessing which connected loads are likely to operate together and at what level. It can reduce the assessed maximum demand below the sum of all load ratings where the expected use justifies that reduction. It is not a rule that loads never run together; the designer must consider the installation and any applicable special requirements.
Read the Diversity entryA definition is a start. Using it in a real question is where it sticks.