Technical Electrical - Using a master relay instead of multiple smaller ones?

Is there any reason, or cons to using one larger master relay feeding a fused panel for the "12v switched" power circuits rather than multiple little ones. I'd rather run larger wire from a central location than a bunch of relays.

Something like:

 

Might be helpful to know more about what all these little relays power, and why you need them? What kind of stuff are you adding to the car that will overload the normal ignition switch circuit?

But looking at the big picture, OEMs use a whole bunch of little relays on modern cars ("modern" meaning cars designed in the past 30 years or so), and they are expensive, and the wiring to them is complicated and expensive, so perhaps there's a good reason why they do it.

 

If you run everything off one relay, one short will shut the whole car down. Multiple relays let you get back home.

 

The reason it's not done is it's akin to hooking up equipment only capable of handling 10 amps to a 40 amp breaker , you could burn down the circuit without blowing the breaker , i.e. very limited / no circuit protection .

 

Not if it goes to a separate fuse panel...

 

Not if it is toggling power to a fuse panel.

 

I see very few devices on the average hotrod that require a relay, so not sure why I'd want one big honking relay? An electric fan or electric fuel pump are about it for one of my cars, and the other is only the fan. All the other circuits have survived on old cars for many decades without relays being needed, so I don't add relays just for the fun of it.

 

Relays are basically electrically controlled switches. A small current turns on and off a larger current.
If you want a big switch (relay) to handle key on feed, it can be done. This could also be handled by a properly sized key switch, though, because for the most part rods don't require a lot of current as mentioned.

I am actually considering such a thing, because I want to be able to unplug the dash harness from the chassis harness to remove the body and I don't want the 'key on' going to and from the switch thru the connector.
You will need to size everything properly and have circuit protection. It isn't a work around, just designing to requirements. It will not replace a bunch of little ones. All the 'little ones' are discrete switches for other high current functions, such as circuits for fans, fuel pump, headlights, or other accessories. It will add another layer to the simple ignition switch, from switch off/on to switch commands relay off/on.

 

On my '46 I use a relay activated by the column drop on-off switch. The relay carries the current load instead of the switch. The column drop switch is used to activate the relay only, because the switch isn't robust enough to handle my extra current load. I have small relays controlling the head lights, horns, heater fan, elect. choke, elect. trans. kickdown (T400), and elect. fuel pump.

 

The thing about relays is that they allow a component......say an ignition switch to activate a circuit that can carry a heavier load without it ever going thru the switch itself. That's very helpful because the switch and many other components don't get exposed to high loads. The wiring is smaller and the heavier wire used in the relay part of the circuit is often shorter because it only has to route around in the engine compartment.........not go inside the car and back out to the component under the hood.

So I think the consideration might be whether you have to run full size wiring for each circuit or are better served running smaller wire that activate relays and allow shorter runs of the heavier gage wiring......and are ALL of the components of each system capable of handling the higher amps ?

Aside from that, you might want to use a Circuit Breaker Maxi Fuse rather than an actual maxi-fuse or whatever if you are still in the planning stages.


If you really want to get great info and insight into this, check out " Automotive Wiring 101 by Crazy Steve" https:www.jalopyjournal.com/forum/threads/wiring-101.843579/

(You might initially scroll down to System Design Part 3 but its best if you wade thru some of the earlier stuff. I ended up printing a copy of this with deletions of many of the comments for clarity.....laminated it and put it in a binder. It's that good.)

 

So your trying to have a master kill switch? If so, how do you kill the switch that will energize the relay? Seems over thought to me.

 

BUT you can remove a bunch of the small ones if not all. For instance power locks, rated at 10 amps, just run them on a 10 amp circuit AS LONG AS the switches and actuators support 10 amps. Right?

Power windows is another, the switches are rated for 15 amps. No need to have a separate relay in the door....

 

But will the switches themselves hold up better with full current running thru them, or are they going to last longer just activating another circuit to do the heavier current load? When switches (of all types) do fail, its often difficult to find exact replacements.

 

Yes, you can remove small relays and just go through a switch and circuit that can handle the load. This is addressed by @ekimneirbo as to why they are used.
However, you asked about Using a master relay instead of multiple smaller ones?
There is no reason to use a big relay to feed many separate circuits to remove smaller relays.

Her is a basic article covering them. It doesn't go into computer stuff much so is more HAMB friendly than most.
https://www.fbeletronic.com/underst...ys-your-essential-guide-to-vehicle-electrics/

 

It just doesn't seem to make much sense to me, considering my battery is under my cab. You see a lot of builds using a "relay" panel, centrally located and still have to run heavier wire from it to the device.

 

^^^^^ Then your not understanding why relays are used.

 

Are solid-state relays (as long as they're rated) suitable for this purpose?

 

I used a stinger brand relay to power my main fuse block

 

Info here on, how to not overload ignition switch current.
https://watsons-streetworks.com/wp-content/uploads/2018/02/a-current-topic.pdf

 

No I understand perfectly, but as far as saving wire???? Do you really....

In regards to the switches, when the manufacturer specifically said product does not need a relay then why add one.

My main concern is drawing 10/15 amps oof of the ignition switch voltage which is what most aftermarket harness do.

Am I going to eliminate all relays....no. But quite a few can be. Fuel pump, and wipers are two others that come to mind. Fused at panel. Keyed on when relay kicks. Why add another relay in the system?

These old switches we use, headlights. Wipers, etc, most of which still work, handled all the current.

 

I use them on higher draw devices, high beam, low beam, starter, and horn. Switches then have lower current and last longer. I run a large wire to the front of the firewall to feed the nearby relays. I don’t like the possibility of an electrical fir under the dash.

 

I wouldn't use one large master relays to replace several smaller one circuit only relays.
On the other hand I would if occasion called for it use a large master relay to power the switched side of the fuse panel to lessen the draw through the ignition switch accessory circuit. I'm thinking that one of my OT cars with a 3 letter name is set up that way. There is a high amp relay that powers several lower amp circuits that all are fused and have relays. That cuts down the amps going through the ignition switch.

Experience that hasn't been good says that the switches in most of our Hamb friendly vehicles aren't designed to handle high amp loads. Usually the headlight switch when we change to Halogen bulbs and in my case run H-4 bulbs that are 55/100 watt to light up the roads I travel on. My OT No extra options except a heater and AM radio 71 truck ate dimmer switches with those 55/100 bulbs until I installed relays. That equals, Have the need for a relay to protect the circuit between the relay and the battery use one, don't have the need, you may not need one or want one. The circuit being the wiring and switches.

 

1 large relay to input power to a fuse panel? or to take the place of several smaller relays?. If it's the latter, how would you control multiple components with one input. Relays go between a load and a switch (input), so the input doesn't carry the full load. I've used high current relays(solenoids) like shown above to activate a single or double circuit (activated by a single input) for a high current application.

I might be missing the point on this one as I skimmed the thread on my lunch break.

 

With proper design, there's no reason not to. This is done all the time in the wider electrical industry. I cover all of this in the Wiring 101 thread. I will note that you can eliminate relays by doing this in some cases, but this will vary on a case-by-case basis. As to the worry about 'losing' your panel feed, the same issue exists whether you use a relay or don't. If this bothers you, you could install a properly-size fuse to protect it (although I'd recommended a circuit breaker over a fuse) but because of the varying loads presented by the branch circuits is extremely hard to do. But as all you're really concerned about is short-circuit protection for the panel feeder, addressing that with careful routing to protect it from physical damage is all that's really required IMO. This is done that way in virtually ALL end-user electrical installations in the US; the utility DOES NOT FUSE the secondary side of their transformers. Generally, in those rare instances when the primary fuse blows you've melted something already.

If your individual branch circuits are fused correctly, there is NO overload exposure for your panel feeds.

It's critically important that you observe de-rate factors. Wire size, switches, relays, fuses ALL need to be de-rated to 80% of their maximum rating to mitigate voltage drop (although depending on circuit length, additional calcs should be done), maintain contact life in switching devices, and nuisance tripping. This is especially true for circuits with motors loads (including solenoids and horns) where inrush current can exceed 300% of running current.

There are literally endless ways these can be wired depending on what you want, but to my mind keeping component count down is the best path. If one relay can replace four, that's a good thing...

 

I install circuit breakers on everything I wire. Good protection against the unknown and unwelcome. They also make great kill switches and long term storage isolators. They are not relays.

 

I don't get what you are trying to do here either. All your small circuits relays still need to be wired from their own control circuits to their respective device loads. Your horns from its relay controlled by its horn button. Your headlights from its relay controlled by its headlight switch. A heater fan from its relay controlled by its heater control switch. and so on? All your small load circuits still need to be wired from their respective fuses or breakers to their respective relays controlled by their respective control switches. Using a single relay to control your fuse panel is not needed. You would still need a circuit to control the coil of that one relay by another switch.

 

Seriously @tlmartin84, I understand that you don't understand...........I'm an "electrically challenged" individual and there is no limit to how much I fail to grasp about electricity. The Wiring 101 will help you to understand not only the best way..........but why its best. Its lengthy, but if you take the time to read what Crazy Steve provided, it will help you understand.

 

Funny you mention Steve, he commented above.

 

Somehow relays have attained 'magic elixir' status in the aftermarket automotive electrical industry. Technically, there's only one condition where one should be used; where the load you're switching will exceed the current rating of the switch. Legitimate uses would be things like cooling fans, and possibly AC or air bag compressors to name a few. If you upgrade the feeder to the switched half for your 'unified' fuse panel and it draws more than what the typical ignition switch can control (likely), a relay is a great control choice. But if you're trying to 'protect' switches, keep in mind any multispeed motors (heater fans, wipers) will require relays for each speed.

But the aftermarket has given relays a second task; mitigating voltage drop. This should be addressed in the basic harness design but sadly isn't. Instead, they sell you those 'kits' (now fed separately, not from the fuse panel) to fix it. You no longer have a unified fuse panel.

Circuit breakers are a mixed bag. Fuses offer the fastest response times and accuracy for overloads or short circuits. Circuit breakers aren't as accurate and can tolerate overloads for much longer that fuses will. I've seen 15 amp breakers withstand 33% overloads for 7-8 minutes before tripping, long enough to do damage in some cases. They're also slower reacting on short circuits but not objectively so. One big advantage is they are usually resettable, either manually or automatically. Generally, if the circuit has a fixed maximum load (most branch circuits), I'll use a fuse as the best protection. But in circuits like panel feeds where the load can vary by quite a bit, a breaker can make sense. If the wire has been sized using diversity, size the breaker to the wire.