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Old 01-15-2012, 10:24 AM
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Figuring out relay amp ratings?

Hey All, I'm looking to do some custom wiring work with some of GM's later-model fuse/relay block and relays, and am having some difficulty finding any good specification-type documentation on the different relays that are used. I don't really have anything that would push these relays to their max, but it would be nice to know if it's safe to use one of the smaller relays in place of a larger one.

You can see what I'm working with in the pic below - the fuse block (with terminals removed) can be found in just about any later-model GM car (don't know what year they changed to this style terminal, but probably 95+) that almost exclusively uses Metri-Pack 280 style terminals for the relays and mini fuses, and Metri-Pack 800 terminals for the maxi fuses (the post on the right connects to a buss bar for the maxi's). The relays shown are the large & small SPST (grey) and SPDT (black) that would be used in this kind of fuse block and what I'm trying to find the amp ratings for.

Now, one way to guess at the ratings is to go with the max rating for the terminals, and the Metri-Pack 280 has a max amp rating of 30 amps with the correct wire (per Delphi's web site and terminal supply sites). So then my next question is do both the large and small relays have the same 30 amp rating or do the smaller relays have a lower rating?

I'd like to figure this out since being able to use the small relays would increase the number of relays that can be used in this fuse block - At minimum it can hold 3 large and 2 small relays, but at max it can hold 8 small relays giving quite a bit of flexibility in a custom situation.

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Old 01-15-2012, 10:46 AM
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A pretty good indicator of the amp rating is the size of the fuse on the circuit the relay is controlling. The micro relays are probably 20 amp max rated,seeing most of the ones on GM cars and light trucks are fused with a 20 amp fuse or less, while the full size bosch style can be 30 amp or 40 amp, some of those have HUGE spades for terminal 30 and 87 87a, they could be rated at 50 or more.
As far as exact specs, IDK , deductive reasoning can be a good aproach to making a good educated guess.
Those full size relays in the picture with the puny little spade connectors may not be very heavy duty. I would be suprised if they are more than 20 amp rated, I could be wrong.
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Old 01-15-2012, 02:30 PM
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Here is a link to a Bosch catalog that has part numbers and ratings

Bosch relays
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Old 01-16-2012, 08:12 AM
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I was figuring a ballpark number for the small relays would be 15 amps (stupidly simple reason: they're half the size of the large ones so half max amp rating ), though I guess I can get a better look at the next ones I check out. As far as the large ones possibly being less than 30 amps I'm not sure, which is why I'm trying to figure it out. I know that my '00 Trans Am had these types of relays controlling everything including the fans (one fan could be run by one relay, but both fans on at the same time got their power from 3 relays in parallel), so they can probably handle anything I'd throw at them - but it's better to be sure depending on the situation.

T-Bucket - I had found that pdf earlier, but the problem is it seems to deal with the older style relays that used 1/4" blade terminals, not these newer style relays (at quick glance at least). Something like this would be excellent for the newer style, but I haven't found a manufacturer yet who supplies this info in an easily accessible location (Omron, Tyco, Siemens, Bosch, AC Delco, etc).
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Old 12-24-2012, 05:57 PM
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Bump, this question is bugging me too. I see new cars that run 30 amp micro fuse but then run a 30 amp maxi in the same module. I'm guessing it has something to do with reliability and failure due to duty cycle and heat. I want to use all micro relays and fuses (Cooper bussman power distribution module) which is rated for 30 amp per connector but then if so why would cars use anything other than micro for 30 amps or less.
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Old 12-25-2012, 07:10 PM
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Take a look at the detailed spec sheet like this link. Stay within the spec. and relay will live a long, dependable life. In theory, of course.

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