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Discussion Starter #1
My new electric fan draws 18.5 amps, steady state. Should a 30/40 amp relay handle the start up load or do I need a 70/amp relay?
 

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Yeah, I agree. 6 feet of 12 gauge is rated for 30 amps. That would be more than enough. Happy wiring!


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As long as it is a quality relay then you should be fine. Try and go with a Bosch, Tyco, Potter/Brumfield, and such. The ones with no names or ones that you cannot find info on are the ones you need to pass on. Also too, maybe get a relay socket as this can make changing one easy if needed. Mount the relay too with the terminals facing downward and fuse the main power input off of the battery with the proper sized fuse.

Jim
 

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"Happy Wiring" is an oxymoron for a moron like me when it comes to wires.
"Happy Wiring" is an oxymoron for a moron like me when it comes to wires.
The more you do it the easier it gets. Reach out anytime if you’re stuck. Jim’s correct in the quality of the relays and wiring advice. There are a lot of smart dudes on here ready to give you a hand.


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I have gone through several autozone type relays. They stick on or wont come on at all after only a few months. Im waiting for a Bosch 50amp to come from amazon. I have a single 16 in Spal that pulls close to 25 amps. The cheap 40amp relays cant take that.
 

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Discussion Starter #9
As long as it is a quality relay then you should be fine. Try and go with a Bosch, Tyco, Potter/Brumfield, and such. The ones with no names or ones that you cannot find info on are the ones you need to pass on. Also too, maybe get a relay socket as this can make changing one easy if needed. Mount the relay too with the terminals facing downward and fuse the main power input off of the battery with the proper sized fuse.

Jim
I have waterproof relay assemblies from "Online LED store". I can't tell who makes them but they've been totally reliable so far on my headlights. Do you think they'll suffice? Even if they work flawlessly, my concern is will the 40/30 amp rating be enough to handle the fan and a/c compressor clutch? 18.5 amps for the fan and ~ 3 to 4 amps for the clutch.
 

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Discussion Starter #10
The more you do it the easier it gets. Reach out anytime if you’re stuck. Jim’s correct in the quality of the relays and wiring advice. There are a lot of smart dudes on here ready to give you a hand.


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You are definitely correct about Jim. He's the best and has helped me before. Best advice he's given me so far is, "Map/diagram it out before you start"
 

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I have waterproof relay assemblies from "Online LED store". I can't tell who makes them but they've been totally reliable so far on my headlights. Do you think they'll suffice? Even if they work flawlessly, my concern is will the 40/30 amp rating be enough to handle the fan and a/c compressor clutch? 18.5 amps for the fan and ~ 3 to 4 amps for the clutch.
If you have had good luck with the waterproof assemblies, then certainly try them for your fan and clutch. Who know who might have made them but I do know some manufacturer's will make things in a big enough order to a customers spec and leave off things like who made them and/or a part number. I do remember years ago when Precision Power had their own line of car audio amplifiers and when you took them apart they were full of electronics from end to end and then had another retailer down the road selling an amplifier under another name that they informed people was also made by Precision Power but was half the price. The amplifier's were the same size physically and luckily got one in to look at and took the bottom cover off and the circuitry in both were almost the same BUT the one for half price only had the internals filled halfway up. The reason it was half price is it was half the amp internal wise.

As far as ratings, the fans will draw a high inrush current to get them going but will then settle down and I really doubt a fan drawing 18.5A all the time while running will hurt a peak 40A relay. Just like a fuse rated at 30A, it will pass 60A for a time and not blow or be hurt.

Jim
 

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That's great info. I was pondering the fuse size as well. Being the compressor clutch and fan were going to start at the same time, I was concerned it might overload the fuse during what I call "spool up" or "start up" draw. Based on this info, I'm thinking a 30a fuse with the 40/30a relay should work fine and provide overload protection for the wiring. My El Camino has dual fans and its wiring came as a kit with a 30a fuse and a 40/30a relay for each fan. I remember reading somewhere that the fuse was to protect the wiring and not the power using device, in this case the fan.
 

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That's great info. I was pondering the fuse size as well. Being the compressor clutch and fan were going to start at the same time, I was concerned it might overload the fuse during what I call "spool up" or "start up" draw. Based on this info, I'm thinking a 30a fuse with the 40/30a relay should work fine and provide overload protection for the wiring. My El Camino has dual fans and its wiring came as a kit with a 30a fuse and a 40/30a relay for each fan. I remember reading somewhere that the fuse was to protect the wiring and not the power using device, in this case the fan.
Glad the info helps.

You will have what I and others call an inrush current that is a higher than normal current just to get things going and then the amperage demands go down to a more steady level.

Yes, the fuse is to protect the wiring and the variables will of course tha gauge of the wire but also taken into consideration would be the wires insulation type and the wire length (and probably some other things like in a conduit or free air) but from doing car audio since the late 70's I normally am only dealing with up to a 17'-18' length power wire that would go from a front mounted battery back to an amplifier in the trunk and if I ran a 10 gauge wire, I put in a maximum of a 30A fuse but if an 8 gauge was run I went up to a 60A max. On bigger systems with a 4 gauge,up to maybe a 150A max and then with a 1/0 wire a 300A max fuse.

Also on long runs you will have more voltage drop so an increase in wire gauge can reduce that drop. Some charts are figured on a 3% drop while others might be 5% or more.

Remember we are dealing with low voltage unlike house voltages. A 4 volt drop in a 12V system is a lot but 4 volts lost in a 120V system is not near as bad.

Jim
 
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