Can I wire two 30/40 amp relays in parallel to simulate a bigger realy?? I am thinking of buying a 75amp relay for my fan, but have a butt load of 30/40 relays laying around. I figure if I can wire two 30/40 amps in parrellel , I can save $25..
In theory, yes. In practice, the quality of the crimp on connectors, wiring and connections always works so that one relay carries more current than the other. How much more, be it a fraction of an amp, or several amps is an unknown quantity. If you do a top quality job soldering & splicing, the difference can be small enough to not be a problem.
You can NOT double up on relays, switches, wire, etc to increase the current carrying ability.
The problem comes from the real life considerations. In real life, EVERY wire, relay and so on varies by some amount. So, one relay (wire, switch, resistor, etc) will carry more current than the other.
Also, to make things MUCH MUCH MUCH worse for a relay (or switch) is the real life consideration that EVERY switch and relay bounces for "x-amount" of time. So, when the first relay switches on, those contacts will be VERY overstressed. Then, when the last relay switches OFF those contacts will be VERY overstressed.
So, you'll get a LOT of pitting on the contacts. The more pitting, the more resistance. The more resistance, the more pitting. The more pitting, the more resistance. .... See a pattern yet? Well, eventually, Poof!
There are ways to use multiple wires/contacts to increase the current capability. But, you must know the details about the wires, contacts, etc. However, switches and relays NEVER work for any extended amount of time if they are used past their ratings. To figure out the failure rate requires a lot of information and analysis. Not all "20 amp" relays are made the same or rated the same. To add to the problem is the "start up current" requirement of what you're driving. Yea, it can get involved. That's why people like me get paid for doing what we do.
Joe -
BS/MS Computer Engineering, BS Electrical Engineering,
Principal Design Engineer for over a dozen years.
Commercial/ Defense/ Space designs.
its kinda like if you were to run 1000 14 gauge wires from your positive teminal to your starter motor.
its gonna fry every one of them. for some reason elctricity dosent split it self up very well.
im no expert, but i have fried alot of wires by trying to
"distribute" electicity through different wires. you need the correct relay.