Thread: Relays and Terminal blocks oh my

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Hey guys I am putting my fuse panel and battery with cut out switch and mega fuse stuff behind the seat. In the partition between the trunk and cab. My question is...
Should I put all the Relays in a line on this panel. And lable them acordingly to what they are. Then the next question is... Is it a good idea to run only one thing per relay.
Thanks for your help Guys. Your the best.

Yep, I'd put all of the relays on that panel, labeled for the service. You'll have one relay for each switched service. The purpose of the relay is to keep from burning up your switch contacts for high load devices, like one for the fuel pump, one for headlights, one for electric fan (if you have one), one for the starter. You'll also have one for your "Accessories" circuit, which will be switched "ON" by your ignition switch "ACC" contact (the only place you have multiple devices powered from one switch contact, right?), and will have a healthy output contact capacity, especially if you're running an amplified audio system.

General rule of thumb.. if you can label it, it makes diagnoses easier later on! 1 thing per relay?? If we're talking a electric fan or fuel pump, then yes. I wouldn't want a mixed load. And that's what you need to be watchful for. The total load of a circuit if you're going to apply multiple loads on a circuit. The wire has a rating and the relays will also be rated. You need to know the loading that you want to apply and don't overload / exceed the rating.

Most of your relays can mount at your panel with the possible exception of higher current devises (equipment) which can sometimes benefit by having their relays mounted near the devises they control.

Ideally, you would mount your high current relays nearest their high current devises, such as headlights, radiator fans, horns, exc.. Using larger wire sizes and fuses (rated for the current) on the load side of the relay, while operating (switching) the relay with smaller wires on the control side of the relay via another switch such as the ignition switch, exc.. On high current devices, mounting the relay near the devise will take full advantage of the relay’s ability to operate the large current loads while minimizing the length of the larger more expensive wires, translating into an economical control circuit that can handle a high current devise without applying the high current demands through the switch. Just remember to protect your relays if they are exposed.

Normally, one relay is used for each device. However, multiple devices can be controlled from one relay so long as the total load of the combined devices do not exceed the relay's maximum current capacity.

In our car’s “DC” circuits the load capacity can be determined by dividing the wattage of the devise by its voltage, ie., a 60 watt bulb at 12 volts will draw 5 amps (60 / 12 = 5). With the exception of inductive devises such as starter motors and fan motors, these can momentarily draw 25% higher currents at initial start up (a 10 amp motor can momentarily draw 12.5 amps at start). The fuse size should take into consideration the safe limit for protecting the wires, relays and devises, with the fuse size rated slightly higher than the devise's total load, but not more than the wire size and relay's maximum rated capacity.

Also, you can switch multiple relays from one switch or multiple switches (control sources) without them interfering with each other by using diode isolation on the control side of the relays. Diodes can prevent the control of one circuit from activating another circuit when it is not desired, such as using the A/C compressor circuit to operate the radiator fans, while not allowing the fans to operate the A/C.

Hopefully, I have not confused you.

Originally Posted by 36 sedan

.....Hopefully, I have not confused you.

All good stuff, but I had to read through it a couple of times....

I fully understand your approach, but there's no way I would put saving ten feet of heavy gauge wire over the convenience of having all of the relays in one, centralized location. And if mounting them close to the load meant exposing them to splash, spray and weather that would be an immediate pass for me. Especially considering that this is a '34 Coupe with no place to locate stuff out of sight, I'd mount them all together, near the fuse block. Other than the ACC circuit, I cannot think of any group of loads that are served from a common switch, which is the only thing that would support one relay feeding two or more devices but I may be overlooking something. Just my $0.02.

Originally Posted by rspears

And if mounting them close to the load meant exposing them to splash, spray and weather that would be an immediate pass for me.

Water proof relay;
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I have a Rebel Wire 21 cicuit harness. I will need to run a relay for the 2 fans, the electric fuel pump, the AC unit, And any other big items. But do I need to put them on anything else coming out of the regular fuse box. I see guys running the headlights, tailights, right from the fuse block out to the headlights, and tail lights. Without putting anything else in line.

OH OK I found some info through Watson. That you run a 70 amp relay between the ignition switch and the fuse box. On the ACC. side. That makes sense. Rather than putting a relay on every item coming out of your fuse box.

Originally Posted by Creep

OH OK I found some info through Watson. That you run a 70 amp relay between the ignition switch and the fuse box. On the ACC. side. That makes sense. Rather than putting a relay on every item coming out of your fuse box.

Could be wrong, but I think that 70 amp relay is only to keep you from frying your ACC contact in the ignition switch in the event that multiple ACC loads are "ON" when you select the "ACC" position on the ignition switch. Any of your high load devices, like fans, fuel pump, horns and headlights need individual relays to protect the switching contacts on those circuits, too. Did your Rebel Wire 21 come with a set of instructions that spell out the basic needs, like running either a fusible link or mega-fuse on your main feed to your fuse block to protect against a dead short at the block?

Oh I see what you mean. All the other loads are on a separate switch. Like Fuel pump switch, Fan switch, and such. Rather than have your ignition switch run them also. I get it now. Yes I do have a 65 amp mega fuse. My Rebel harness did not come with it but i picked one up.

Unfortunately, I believe I have confused you.

Your “Rebel 21” circuit wiring kit is a very thorough kit with great instructions and tech support. From my understanding they provide a complete wiring kit labeled, fused (even the fusible link) and ready for installation with control leads for any relays you’ll need, along with recommendations for relays, wiring the relays and placement.

To avoid any further confusion, may I suggest that you follow their instructions and tech advise.

This was my wiring in progress. All connection were soldered and shrink tubed throughout the truck so that there are no crimps to fail. The fuse box is an auxilliary box from a crown vic police car and has 3 additional relays.

Nice wiring job!

Also, I find it helpful to keep a simple sketch of my car with a key listing and showing all equipment locations that cannot be easily seen or found (especially good for the next owner).

Car silhouette.jpg

Yes TomJ i see you put your relays all in one spot. That is what I wanted to do. I have not called the Rebel wire company yet. But I must say I'm not to impressed with the instructions.