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		What type of soldering wire? Tin/lead ok?
Flux paste as well?
Will a 60W electric soldering iron be up to the job or is gas / blow torch required?
Cable thickness is 25mm² and 75mm².
	
	
	
	
		
	
 
 
	
	
	
		
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		You'll never get them sufficiently hot enough to do a decent job of soldering them without melting about 8 inches of insulation off the cable in either direction in the process.
Why not just weld them together?
	
	
	
	
		
	
 
 
	
	
	
		
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		plumbing solder and a blowtorch
	
	
	
	
		
	
 
 
	
	
	
		
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		crimp and adhesive lined heatshrink, or a longer cable.
	
	
	
	
		
	
 
 
	
	
	
		
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		What are you using it for 75mm2 is about an inch thick. There is no way you are going to get a decent solder of that.
	
	
	
	
		
	
 
 
	
	
	
		
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		Crimp lugs on either cable and bolt them together instead, you'd need a gas torch to solder them otherwise, which will ruin a good portion of the insulation as Dum-Dum wrote above. 
Also, a soldered joint is prone to cracking because of the vibrations in any vehicle.
	
	
	
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		To give you an idea, 75mm sq is about what is used on a 3P supply to a small factory!
	
	
	
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		Just get someone with the right tools to crimp them.
And wtf is one 70mm^2 when the other is only 25?!
	
	
	
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		100amp joint box for mains tails will tale 25 mm. They are about £10 from an electrical wholesaler. But 75 mm? That's about the size of CD.
	
	
	
	
		
	
 
 
	
	
	
		
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		Lol @ some of these responses! 
25mm² is what is fitted to my Datsun relocating the battery to the boot currently. I've just crushed the wires into the crimps as a semi-permanent job and they need soldering.
The 75mm² is what's on order for the 306's relocation jobby. I've got the crimps in front of me, it'll be fine. That's rated to 500A.
	
	
	
	
		
	
 
 
	
	
	
		
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		as said plenty of flux and a blowtorch and feed both ends with a good amount of solder then join togther, ive soldered my earth wire on my mig in this fashion.