It's not a big deal to put in a breaker or sort out the third wire but I would suggest if there's any doubt in what you are doing, probably best to call a electrician. 
 
As for laws, as a home owner you are able to do electrical work yourself with a permit. Also my understanding if all you are doing is replacing existing, a permit is not required. 
 
Still again, if you are unsure, get a electrician. 
 
As for tripping when not around, rather I come home and find the tank off than the wife coming home and finding me dead. 
		
	
		
		
		
		
		
	
		
		
	
	
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