View single post by Mark Rosenbaum
 Posted: 06-13-2005 07:08 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
There are ways to calculate all that stuff, but usually one just looks up the data in a suitable reference source.  If you examine pages 2 and 3 of the British Wiring catalog at
you'll see good reliable guidelines for the wiring size(s) you'll need.  Note that this is a .pdf file and you'll need Acrobat Reader to open it.

For your fans, each relay's contacts must be rated for the full current they will carry, never less.  Only a few solid-state 'relays' allow doubling up to increase the current rating, and this is permissible only if the manufacturer explicitly says you can do it.  Note that relay contacts are rated for a much lower DC (automotive) current than AC (house) current -- and when dealing with DC current, relay contacts quickly either weld themselves shut, or burn away, when you exceed their rating.

Finally, using two relays in parallel, each able to carry the current required by a fan, results in only a slight increase in reliability.  This is because both relays will wear at about the same rate, and when the first one fails, the second can be expected to fail shortly thereafter.  It's generally best to have the second relay sitting in the car's glove box as a spare.