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Module Lessons
Electrical Principles & Diagnostics
Ohm's Law & Electrical Power
Protection Against Electric Shock
Electrical Statutory Regulations
BS 7671 & IET Guidance Documents
Reading Electrical Plans & Diagrams
Consumer Units & Protective Devices
Lighting & Power Circuits
Earthing, Bonding & ADS
Electrical Accessories
Current-Using Equipment & Specialist Controls
Effects of Electric Shock on the Body
Tool & Equipment Safety Classes
The Safe Isolation Procedure, Step-by-Step
Hand Tools for Electrical Work
Cable Types
BS 7671 Termination Requirements (Section 526)
Terminal & Connection Types
Cable Glands & Enclosure Entry
Terminating Flexes & Single-Core Cables
Terminating Flat Profile (Twin & Earth) Cable
Installing Main Protective Bonding
Fitting & Labelling BS 951 Bonding Clamps
Electrical Test Instruments
Central Heating Control Sequence of Operation
Diagnosing Boiler Thermostats & Fans
Diagnosing Air Pressure Switches
Diagnosing PCBs & Ignition Components
Diagnosing Thermocouples & Thermistors
Alternative & Emerging Heat Sources
Testing Immersion Heaters
Isolating & Correcting Appliance Faults
Lesson Overview
This lesson covers the fundamental principles of Ohm's law and electrical power calculations, enabling correct sizing of circuit protection and controls for domestic heating and plumbing appliances.
Learning Objectives
Apply Ohm's law (V = I × R) to calculate circuit voltage, operating current, and resistance.
Calculate electrical power and appliance running current (P = V × I) for plumbing equipment.
Select appropriately rated switches, control devices, and circuit protective fuses based on calculated loads.
Electrical Units & Ohm's Law
Electricity operates via a flow of electrons around a closed circuit from a source, through a load, and back again. In the UK, the standard single-phase mains supply voltage is nominally 230 V AC at 50 Hz, with an acceptable statutory tolerance between 217 V and 253 V.
| Electrical Term | Unit & Symbol | Basic Formula | Practical Plumbing Context |
|---|---|---|---|
| Voltage | Volts (V) | V = I × R | The electrical pressure pushing current through the circuit. |
| Current | Amperes (A) | I = V ÷ R | The rate of electron flow determining conductor and fuse sizing. |
| Resistance | Ohms (Ω) | R = V ÷ I | The opposition to current flow within heating elements or cables. |
| Power | Watts (W) | P = V × I | The rate of electrical energy consumption by motors or heaters. |
The mathematical relationship defining that voltage across a circuit equals the current multiplied by the resistance (V = I × R).
The difference in electrical potential between two conductive points in an electrical circuit, measured in volts.
British Standard / Regulation
Both line (brown) and neutral (blue) conductors are legally classified as live conductors because both carry electrical current under normal operational conditions.
Calculating Electrical Power & Running Current
Every electrical appliance has a power rating in watts (W) or kilowatts (kW). To determine the current drawn by an appliance from a nominal 230 V supply, the power formula is transposed to I = P ÷ V.
Exam Tip
A standard domestic 3 kW immersion heater element draws 13.04 A on a 230 V supply (3,000 W ÷ 230 V = 13.04 A). Any control switch or time clock installed must be rated above 13.04 A; fitting an accessory rated at only 10 A will cause rapid terminal burnout.
Conductor Resistance & Circuit Protection
All cables exhibit internal conductor resistance. Using Ohm's law (V = I × R), any resistance in the supply cable produces a voltage drop, reducing the actual potential difference reaching the appliance terminals and generating heat.
Important Note
When calculating fuse ratings for appliances like boilers, divide the rated wattage by the supply voltage. For example, a 600 W boiler requires a running current of 2.61 A (600 ÷ 230), requiring a 3 A fuse in the switched fused spur.
Key Takeaways
Ohm's law defines the relationship between potential difference, current, and resistance using the formula V = I × R.
Appliance running current is calculated using I = P ÷ V, which dictates minimum cable, switch, and fuse ratings.
A 3 kW, 230 V immersion heater draws 13.04 A, requiring a dedicated 16 A protective device and appropriately rated switches.
The UK single-phase mains supply operates at 230 V AC and a frequency of 50 Hz, cycling on and off 100 times per second.
Next Steps
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