Explain cable overheating and shared-socket overloads
Add simultaneous appliance currents and compare them with a supplied shared-lead limit.
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Use P = IV.
- Add branch currents entering a shared conductor.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which relation gives current from supplied power and voltage?
What you will learn
- Add simultaneous appliance currents and compare them with a supplied shared-lead limit.
- Use the combined current and the applicable shared rating to assess a supplied overload scenario.
A shared cable carries the combined load
Overloading plugs, extension leads, single sockets or multiple sockets can make their shared parts carry excess current. Several appliances connected through a common extension lead draw their combined current through its shared conductors, plug and socket contacts. The number of empty outlets is not an electrical capacity rating. A single high-power load can overload a low-rated lead, and several modest loads can exceed a shared limit together.
Excess current causes excessive heating in resistive cables and contacts. Heat can damage insulation and ignite nearby material. A tightly wound cable reel can also retain heat: a written current comparison must use the stated rating for its actual conditions, not assume every coiled and unwound rating is the same.
Use supplied operating data
For the supplied ideal appliance examples, I = P/V gives each current and the currents add. At 230 V, simultaneous 1840 W and 1150 W loads draw 8 A and 5 A: 13 A in the shared lead. This exceeds a stated 10 A lead limit, even if there are spare outlets.
Rated power alone is insufficient to approve a real installation: operating conditions, manufacturer ratings, cable arrangements and protection matter. Do not daisy-chain extensions or raise a protective rating to accommodate overload. For suspected overheating or damage, take equipment out of use for competent assessment; this lesson does not prescribe installation work.
Pause and explain
Two supplied loads draw 8 A and 5 A through a lead rated at 10 A. What follows?
Worked example
A 230 V lead rated at 10 A supplies 1840 W and 1150 W simultaneously in the stated model. Assess the load.
Show the worked solution
- Total power is 2990 W.
- Current is 2990/230 = 13 A.
- 13 A exceeds the supplied 10 A limit.
Answer 13 A total; the stated shared lead is overloaded.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
At 230 V, add currents for 1840 W and 1150 W and compare with a supplied 10 A limit.
Give me a hint
Calculate each P/V before adding the shared current.
Compare my reasoning
- 1840/230 = 8 A.
- 1150/230 = 5 A.
- Total 13 A exceeds 10 A.
13 A; overloaded under the supplied conditions.
Look for these in your work
- Add simultaneous currents.
- State the compared limit.
A 230 V shared lead has a supplied 13 A limit. Three stated loads are 920 W, 1380 W and 460 W. Find their combined current.
Give me a hint
Add the stated powers or their individual currents.
Compare my reasoning
- Ptotal = 2760 W.
- Itotal = 2760/230 = 12 A.
- 12 A is below the supplied 13 A limit.
12 A, below the stated limit; this alone is not a real-installation approval.
Look for these in your work
- Use consistent power units.
- Keep the assessment within the supplied model.
A cable reel is labelled 5 A when wound and 13 A when fully unwound. A supplied load draws 8 A while it is wound. Explain why the 13 A label does not clear it.
Give me a hint
Select the rating for its current condition.
Compare my reasoning
- The present condition is wound.
- 8 A exceeds the wound limit of 5 A.
- Heat removal differs; the unwound label is not the applicable comparison.
The 8 A wound use exceeds its stated 5 A limit.
Look for these in your work
- Read the condition-specific rating.
- Do not infer suitability from another condition.
Common mistakes
- Count simultaneous current, not outlet count.
- Use the actual stated rating.
- A larger fuse does not increase cable capacity.
What can you explain now?
Why can spare extension outlets coexist with an overload?
Compare with the explanation
All connected loads share a current limit.
Unused openings do not raise the capacity of the shared cable, plug or contacts.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use the combined current and the applicable shared rating to assess a supplied overload scenario.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.