sizing relays for accessories?
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New User
Joined: Apr 2002
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sizing relays for accessories?
Hello all, I've been reading up on all the issues involved in adding accessories (mainly fog and driving lights) to my Superduty. I've worked out the circuit diagrams for linking the aftermarket lights to the hi/lo beam circuits, using relays. I've found a whole bunch of web pages covering the proper wire gauge for various current draws and wire run lengths, and a few web pages giving advice on what fuse rating should be used with particular current loads (for example, one page recommends a 15A fuse for 10A draw, 20A fuse for 15A draw, etc). What I haven't found so far is any data or "thumb rules" for sizing relays for a given current load.
For example, lets say I have two 130W offroad lights being driven by a relay. 260W divided by 12.0v gives 21.7A, for which I might use a 25A fuse. What current rating should the relay have? Would 50% more than the fuse rating (i.e. 1.50x25.0 = 37.5, round up to 40A) be adequate?
:-X12
For example, lets say I have two 130W offroad lights being driven by a relay. 260W divided by 12.0v gives 21.7A, for which I might use a 25A fuse. What current rating should the relay have? Would 50% more than the fuse rating (i.e. 1.50x25.0 = 37.5, round up to 40A) be adequate?
:-X12
Posting Guru
Joined: Feb 2001
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From: Keweenaw Bay, MI
sizing relays for accessories?
The fuse is there to protect the circuit and electrical fires. The relay is just a device that is opened and closed electrically. Every relay that I have seen has a maximum contact amperage rating stamped on it somewhere. I would not exceed this rating but it will not hurt to be at or near the rated amps. Most of them are slightly under-rated anyway.
Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
sizing relays for accessories?
[updated:LAST EDITED ON 14-Dec-02 AT 06:33 PM (EST)]Use standard Bosch-type 30A relays. Ford used them on F-series from '92-up, and several other mfrs used them, too, so they're common as dirt. Even WM sells them from Blazer for driving lights. You can get sockets cheap, too (http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=330-070&DID=7), but they'll also take standard blade connectors without a socket. If your current draw is too high, just double up - it's cheaper than finding a higher-rated relay, and it's better to use the same relay everywhere so you don't need 5 kinds of spares.
Another good relay to use would be whatever is already used on the truck originally. It'll be harder to find sockets, but if for instance your fuel pump relay went out, you could pull the driving light relay & buy a new one when you got home.
The point is to get something that's common and cheap - don't look for a 15A relay or 22ga wire. Just use the 30A relay with 16 or 14ga wire for everything, and put in the proper fuse. It'll be cheaper & more reliable.
Another good relay to use would be whatever is already used on the truck originally. It'll be harder to find sockets, but if for instance your fuel pump relay went out, you could pull the driving light relay & buy a new one when you got home.
The point is to get something that's common and cheap - don't look for a 15A relay or 22ga wire. Just use the 30A relay with 16 or 14ga wire for everything, and put in the proper fuse. It'll be cheaper & more reliable.
Post Fiend
Joined: Jun 2002
Posts: 13,873
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From: Atlanta GA
sizing relays for accessories?
> Just use the 30A relay with 16 or 14ga wire for everything,
> and put in the proper fuse. It'll be cheaper & more reliable.
I agree 100%.
I use 30A relays and 12 gauge wire (.11 a foot at Home Depot) for stuff I want to add when rewiring my truck. West Marine has 100s of stores across the USA. Their catalog has an excellent chart for AMPS/Wire Length/Gauge that I use for sizing stuff. Plus, their stuff is top notch and worth the extra price.
> and put in the proper fuse. It'll be cheaper & more reliable.
I agree 100%.
I use 30A relays and 12 gauge wire (.11 a foot at Home Depot) for stuff I want to add when rewiring my truck. West Marine has 100s of stores across the USA. Their catalog has an excellent chart for AMPS/Wire Length/Gauge that I use for sizing stuff. Plus, their stuff is top notch and worth the extra price.
Elder User
Joined: Oct 2002
Posts: 513
Likes: 0
From: Lincoln NE
sizing relays for accessories?
A 30 amp relay will be just fine. We use em at work for about everything. And do like steve83 says, use a fuse, I'd use a 30 amp. Just make sure you put the fuse between the relay and the source, closer to the source the better.
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