A wire will get hot if it takes too much current -or- if a contact point is dirty (often through sparking when not properly tightened) a high resistance joint acts as a localised heater. Sparks over time build up a layer of soot and the little heater becomes ever more efficient.
If a joint has a resistance of just 1 ohm the power dissipated at a dirty junction in an 8.5kW shower is a frightening 1.3kW (Power Watts = I² R)
Given that the breaker has not tripped I suspect the second reason - but I just guess.
To be fully compliant with safety rules and common sense a qualified electrician might be best.
Peccavi
September 2009
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