Relays vs bigger wire?

[KYJ10]

I am getting ready to finish my dual fan setup, and am have rewired everything using very thick gage wire. I read in some post when running thier fans, they ran relays to keep the wire from heating up. Well, I can be sure that won't be a problem, as I have run thick wire. So is there any other reason to run relays? Dennis

[ December 04, 2003, 03:18 PM: Message edited by: KYJ10 ]

[89grand]

Running relays keeps the current away from the trigger source such as a switch or whatever triggers the fans. You will want to run decent sized wire from the battery to the relay and back to the fan then you will only need a small wire from your trigger source such as a switch to the relay.

I have my electric fan triggered by the juction block wire that is only energized when the ignition switch is in the on position and that is connected to the relay for the trigger, then I have 12 gauge wire from the battery to the relay and from the relay to the fan. I also have a thermostat controler that only turns the fan on at a certain radiator temp so it doesn't run all of the time.

I suggest running a relay for anything that draws some current if you will be turning it on from inside the truck with a switch. Relays are cheap and are great insurance against wire fire. Another great thing about relays is you can use just about anything that powers up as a trigger for some other device without using the first thing to power it.

[ December 04, 2003, 03:47 PM: Message edited by: 89grand ]

[mdill]

You may also want to put a set of reversed biased
diodes across the relay, to reduce the arcing across
the relay contacts (Some relays have the diodes
built in). The arcing if left unchecked will greatly
reduce relay contact life. Also what have you used
to protect the wiring in the case of shorts ? Add
in either fuses or a fuseable link in the supply
line.

Mike D.

[89grand]

Yeah, good point about a fuse, I forgot about that. Definetely run a fuse inline from the battery to the relay.

[KYJ10]

Where can I get a relay, and what size do I get to run a set of dual fans that draw 30 amps? And Im not really sure how they hook up? Whats wrong with just running a 40 amp toggle with fuse?

[mdill]

Switches/relays/points ... do not like switching
large inductive loads. The turn on is fine, no
current, but when the contacts open, the current
in the circuit will keep flowing due to the
inductance of the circuit, so what will happen
is the current will start an arc across the
contacts this arc once esablished will burn untill
the inductive energy in the curcuit is dissipated.
The standard engineering method to handle this i
s to put a
reverse bias diode across the contacts, this
allows the current to keep flowing though the
diode when the contacts are opened. Now as for
using a toggle ?? are you planning to manualy
play thermostat by cycleing the fans yourself ?

[Joe Guilbeau]

Look at the Starter system in the Jeep, would you want to run 10 AWG wires to the Ignition Switch and back down to the starter from the battery...probably not.

Most any Auto Parts stores have 30 and 40 Amp relays (either SPST or SPDT).

You will place the relay in closest proximity between the battery positive cable and the fans themselves, running heavy duty 12 AWG cable from the battery to the relay switched contacts and then on to the fans.

A switch on the dashboard with 16 AWG wiring from the battery to the pos terminals on the switch and then to ground will be the relay energizing power.

Place a fuse in both power leads from the battery, one fuse of 30-40 amps (whatever the rating of the fan amperage draw) and another fuse rated for the dashboard switch (much lower amperage rated fuse in this case).

Now when you operate the switch, it the 16 AWG wires carry power to the relay (somewhere under the hood) and allow it's contacts to send the higher amperage needed by the fans to operate continuously.

Pretty much all there is to it.

[KYJ10]

Thanks, I ran 10-8 gage wire when I rewired my rig. But I went ahead and bought a termostat sender that will turn my fans on. It cames with the 40 amp relay, fuse and such. Dennis

[RottenDog]

I have been reading this post and I am a little confused. I am sure everyone knows what they are talking about, but I have a couple of questions.

I have installed a couple of auxillary fuse blocks to clean up the stock fuse block (missing connectors for fuses, etc.)and basically know exactly where I am with my wiring and spread the load.

The auxillary fuse blocks are directly connected to the battery with 10 gauge wire. Each fuse in the block is 20 amp. From each fuse connection I am running 14 gauge wire to a dash toggle switch. From the each dash toggle switch I am running a 14 gauge wire to each fan. I am running 14 gauge wire for each ground.

Do I need to run a relay in the engine compartment between each fuse and toggle switch?

Do I need to run a relay or an in line fuse (or both?) between the toggle switch and the fan? I will operate these fans manually.

Are these applications required for any additional electrical accessories, like an extra interior light?

Yes, I am electrically challenged.

Thanks.

[KYJ10]

Just out of curiosity, if you had the relay connected to the battery, but my sensing wire of my Alt is later down the line, wouldn't the fans be pulling more load off the battery, then the alt is sensing?

And I bought a thermostat with a relay, but was wondering about the wiring. There is a smaller wire that says hook uo to a trigger, to energize the relay. If I ran this off my starter toggle switch, would it only draw to energize the relay, or does it draw the whole time it's on to keep it energized? And Im guessing that if it is drawing all the time, it's just a tiny amount of amp draw? Dennis

[Joe Guilbeau]

Originally posted by RottenDog:
I have been reading this post and I am a little confused. I am sure everyone knows what they are talking about, but I have a couple of questions.

I have installed a couple of auxillary fuse blocks to clean up the stock fuse block (missing connectors for fuses, etc.)and basically know exactly where I am with my wiring and spread the load.

The auxillary fuse blocks are directly connected to the battery with 10 gauge wire. Each fuse in the block is 20 amp. From each fuse connection I am running 14 gauge wire to a dash toggle switch. From the each dash toggle switch I am running a 14 gauge wire to each fan. I am running 14 gauge wire for each ground.

Do I need to run a relay in the engine compartment between each fuse and toggle switch?

Do I need to run a relay or an in line fuse (or both?) between the toggle switch and the fan? I will operate these fans manually.

Are these applications required for any additional electrical accessories, like an extra interior light?

Yes, I am electrically challenged.

Thanks.This won't be technical, but pay attention...

A 14 American Wire Gauge wire has 2.525 Ohms per 1000-feet.

Lets say that you have a 12Vdc component and you run 10-feet of 14 AWG wire to a toggle switch and then 10-feet of 14 AWG wire back to the component to complete the path.

You now have 20 feet of 14 AWG wire. Doing the math (2.525 Ohms / 1000-Feet)= 0.002525 or 2.525 milliohms per foot.

We have 20 feet of wire...so (20-feet x 0.002525 ohms) = 0.0505 ohms

50.5 milliohms is not much resistance. However when we add the current into the equation, then we begin to see how the wiring begins to affect the issues at hand.

30 amps is about right for headlights or fans.

Voltage drop in wire is Voltage = 1 x R
(I = current in amps)
(R = resistance in ohms)

So, we have 30 amps x 0.0505 ohms...1.515 volts being lost to the wiring alone, and that is not considering the connections and terminations.

This means that the thing that is at the end of that wire (be it headlights, fans, or whatever) is not getting the 12Vdc that it should, it is getting 12Vdc minus 1.515Vdc or 10.485 Vdc.

This is the reason that relays are utilized, from the battery to the (lets say we are powering a fan) is at most 6 feet, we can also run heavier duty wire over a shorter distance and use the 14 AWG toggle switch to control the relay. Now, no loss of voltage in terms of IR drop to the toggle switch which operates the relay. The relay contacts are designed for larger current carrying capacity wiring, so we can run 8 AWG wire, the 6-feet that it will take to deliver the power to the relay from the battery, and then from the relay to the fans themselves.

Now we have 6-feet of 8 AWG which has 0.628 ohms per 1000 feet or 0.003768 ohms for an IR-drop of 0.11304 Vdc...so we now deliver 11.88696Vdc to the device.

Now, one might argue that we could run 8 AWG wire to and from the toggle switch, but that gets to be a bit sloppy.

This is the reason that relays are used.

The solenoid is a perfect example...imagine if there is a total resistance of 0.025 ohms from the battery terminal to the starter terminal, sounds great, huh? This is 25-thousandths of one single ohm!

No problem, until we factor in the 300-amps that the starter draws...this works out to an IR drop of 300 amps x 0.025 ohms!

This, my friend is 7.5Vdc lost in that connection, and the heat that developes is not pretty. Now we have 12Vdc - 7.5Vdc...which is 4.5Vdc to operate the starter...again, not pretty!

So, we use a solenoid to turn the starter and hope that the resistance of entire battery cable, both terminations to the battery cable, the solenoid contacts, and the starter cable and both terminations on the starter cable are much less than 0.0025 Ohms, which doesn't sound like much, until the amps get high.

That is why (the examples above are merely to illustrate the theory behind the logic) we like relays in vehicles.

[Lindel]

Bearing what Joe has said, and Joe is correct by the way, this is the primary reason that I always suggest to Upgrade existing wiring when possible.

Auto manufacturers are not going to provide anymore than the bare minimum when it comes to wiring. That means the smallest wire that will do the job adequately, which doesn't necessarily mean properly.

If you visit my jeep page, you'll see a link to an article that I wrote for FSJ magazine about Jeeps and wiring. Not as in depth as Joe provides, but good info IMHO.

My background comes from being an electronics tech for the last 28 years, including lots of "school of hard knocks" training. ;)