mV/A/m
millivolts per amp per metre
The -drop figure in a cable table: the millivolts lost for each amp along each metre of run. Multiply it by the and the one-way length, then divide by 1,000 to get volts. Three-phase circuits have their own figure, and that answer is judged against 400 V rather than 230 V.
Using mV/A/m 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 mV/A/m beyond the definition?
- Explain what mV/A/m 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 mV/A/m 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 reduction in voltage along a current-carrying circuit caused by its impedance. Load current, conductor size, length and, in AC circuits, reactance and power factor can affect it. Equipment must receive a suitable voltage, so voltage-drop requirements are checked separately from a cable's thermal current rating.
Voltage drop
Read the Voltage drop entryCompare related terms
Open a meaning to compare its role with mV/A/m, or follow its full entry.
Voltage drop
The reduction in voltage along a current-carrying circuit caused by its impedance. Load current, conductor size, length and, in AC circuits, reactance and power factor can affect it. Equipment must receive a suitable voltage, so voltage-drop requirements are checked separately from a cable's thermal current rating.
Read the Voltage drop 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 entryThree-phase supply
A supply with three line conductors whose voltages reach their peaks one after another. On a standard 400/230 V system there's 400 V between any two lines and 230 V from any line to neutral. It's used for bigger loads such as motors and large boards, and the lines are identified by brown, black and grey.
Read the Three-phase supply entryA definition is a start. Using it in a real question is where it sticks.