Nominal current
In
The rated of a protective device, such as the 32 A marked on a 32 A . On an adjustable device, it's the current setting. It isn't what the circuit draws: for protection it has to be at least the and no more than the cable's capacity, written Ib ≤ In ≤ Iz.
Using Nominal 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 Nominal current beyond the definition?
- Explain what Nominal 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 Nominal 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
- MCB Types B, C and DInstallation
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.
An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.
Compare related terms
Open a meaning to compare its role with Nominal current, or follow its full entry.
MCB — Miniature Circuit Breaker
An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.
Read the MCB entryDesign current — Ib
The current a circuit is expected to carry in normal use. For a heater or shower it's just the power divided by the voltage, but a motor's input has to be worked back from its efficiency and power factor 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.
Read the Design current entryCurrent-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 entryOverload
An overcurrent in an otherwise electrically sound circuit, arising when the connected load exceeds the relevant rating. Several high-power appliances on one extension lead can cause an overload without a short circuit. Overload can cause overheating. An RCCB provides residual-current protection only; an RCBO combines that function with overload and short-circuit protection.
Read the Overload entryA definition is a start. Using it in a real question is where it sticks.