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Relay Wiring For Fusebox

Discussion in 'Early CJ-5 and CJ-6 Tech' started by FinoCJ, Dec 7, 2018.

  1. Dec 7, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member Sponsor

    Denver, CO
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    I am looking to add a fusebox that is wired to keyed power. I currently have a small fusebox that is always hot - its wired directly to 12V source (battery). I have some new electrical add-ons, but don't want them to have continuous access to 12V - basically, they could easily get left on when exiting the jeep and drain out the battery - so figure it should be possible to add in a second fuse box that requires key activated 12V. I have never really messed with the key ignition (its not original but from PO) or wired in a relay. I assume I would rather have the 12V run through a relay to the fusebox rather than directly through the ignition switch?

    Any suggestions or tips on how to make this work is greatly appreciated....thanks!
     
  2. Dec 8, 2018
    Focker

    Focker Ran when parked... Runs while moving. Staff Member Sponsor

    Tri-Cities WA
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    You can run 12V directly from the battery through the relay to your fuse block. To activate the relay, run a wire from the ignition switch. The block won't be active until the key is turned.
     
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  3. Dec 8, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
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    You understand that anything that is only on when the key is on is switched somewhere. You need to consider the capacity of the switch as well as the capacity of the devices that are powered.

    A relay is just an electrically powered switch. It has a switch coil and the coil is a load, though typically a small one. Your usual Bosch-style relay is a rated at 30 or 40 amps. You need to add up all the switched loads that you want to power. If it's more than say 25 or 30 amps, you'll need a larger relay. Hi-Amp Automotive Relay - 12V DC, 200 Amp Note that this should be a worst-case demand, with everything running at full power, and you should leave some safety margin. Del-City is a good source for your fuse panel

    You also need to protect the wire that feeds your switched devices through the relay. Your starter solenoid is a good place to get power from, rather than directly from the battery. An example may be helpful. Here is the power feed for the lights on my J20:

    relays1.jpg

    Here I have a short unprotected wire to a 30A thermally-tripped circuit breaker with a waterproof cover. The circuit breaker protects the 10 ga wire that goes across the firewall to the opposite fender. On the other fender I have relays for the lights:

    relays2.jpg

    Here the relay coils are powered by the headlight switch, but you could power the coil from the ignition just as well. The relay coil needs something like 20 mA of power; not nothing, but roughly 1000x less than the relay contacts can supply. The relays and sockets were sourced from Parts Express: 12V Relays & Sockets in the Car Audio Department at Parts Express | 399
     
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  4. Dec 8, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member Sponsor

    Denver, CO
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    Thanks Tim for the parts link....I expect a max of 30 amps. I think the standard Bosch style 30/40 would be fine but could step up to this 50amp relay? "NTE R51-5D40-12F 50A 12 VDC SPDT Automotive Relay" from www.parts-express.com!
    ....Now I just have to figure out which terminal on the ignition switch goes to the relay and where to mount the relay. Thinking to mount the relay on the engine side of the firewall with the connecting high current wire going from 12V source through the relay and on to the fuse box on the inside firewall. Is it 10awg for 30amps? The wire from the ignition to the relay can be smaler.

    This project is tied to the new seat heaters going in and will also move a couple electrical loads from the 'hot' fuse box to the keyed fuse box.

    If this goes well then will maybe do the headlight relay...probably do that when I add some fog or extra lights on the front.
     
  5. Dec 8, 2018
    Focker

    Focker Ran when parked... Runs while moving. Staff Member Sponsor

    Tri-Cities WA
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    There are some pretty good DIY YouTube videos... It just depends on how extensive you want to get.

    You might want to add a circuit breaker too. Some have switches so you can kill the circuit when you want.
     
  6. Dec 8, 2018
    Focker

    Focker Ran when parked... Runs while moving. Staff Member Sponsor

    Tri-Cities WA
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    Here's a good one... FF to 8:30 for the explanation of his fuse block.
     
  7. Dec 8, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
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    I bought these "12 VDC Waterproof 5-Pin Bosch Style Relay SPDT 30/40A with Metal Bracket" from www.parts-express.com! because they are waterproof and inexpensive. I'd think more about location and weatherproofing with the NTE part. I expect the only difference is in the size of the relay contacts. It would be interesting to measure the contact resistance for both parts, but the result would be in tiny fractions of an ohm, and such measurements are difficult (impossible?) with inexpensive instruments.

    I used 10 ga because that's what I had, much larger wire gets expensive, and I knew it would be ok from experience. I wanted the least resistance possible for the long run across the firewall. Here's one table: AWG Wire Gauges Current Ratings Pretty sure multi-core means separate insulated wires in a bundle, not stranded. It matters how the wire is routed. The main issue is the amount of heat produced by the resistance of the wires. Larger wire has less resistance and makes less heat per amp (P = I^2 R). If the wire is in conduit or a bundle with other wires, the rating is lower than if it were in free air. Note the table above says 52 amps for a single core, and 30 for 3 cores. I guess that means 3 10 ga wires in the same cable, each carrying maximum current of 30 amps. Hope we aren't in the weeds here ... 10 ga will be fine. Yes, the wire from the ignition to the relay coil can be much smaller, say 18 ga or even smaller, but you don't find primary wire much smaller than 18 typically (maybe it's a structural/strength/durability issue?)
     
    Last edited: Dec 8, 2018
  8. Dec 8, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member Sponsor

    Denver, CO
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    40A weatherproof relay looks about right. Was thinking of using 10 gauge solid wire (52A) as the current carrying wire - its got to go from the 12V source on the passenger side (battery, starter, alternator post?) across to the lower firewall on the driver to the relay and then into cab to the box. I see most people use the matching 5 blade female connectors with the relay, so will check that 10g solid wire will fit into that connector.
    I think I found the correct wire from the ignition switch to go to the relay to activate - but I am going to remove the ignition to ensure its all correct and clean up any connections. The PO had an electric fuel pump that would kick on with keyed power wired directly from the ignition switch - I capped off the wire when I removed the electric pump - guessing that will be the wire to go to activate the relay.

    Just to ask - what are the advantage and disadvantage to the different 12V sources (say battery terminal, starter solenoid post, alternator post etc)?

    thanks - I am starting to see how this is all going to fit together and hopefully get a clean install. My engine bay is bit sloppy as it is...don't need to make it any worse with more hodgepodge wiring.
     
  9. Dec 8, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
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    I was feeding 2 14 ga wires for the pair of relays, so I just spliced the 10ga into those two wires. Using 10 ga all the way to the panel will be optimal, but a couple of inches of smaller gauge wire in the path will make a negligible difference.

    You can put the wire on the battery terminal if you want. I don't like to connect to the battery post because there is always some corrosion there. The J20 has the fender mounted solenoid, and that seems like a better place to get power from, plus it makes my unprotected wire very short. The alternator could be a good location if the charge wire goes direct to the battery (ie voltmeter equipped, not ammeter equipped). Electrically they are all nearly identical.
     
    Last edited: Dec 8, 2018
  10. Dec 8, 2018
    sterlclan

    sterlclan Member Sponsor

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    use stranded wire, solid wire will work harden and fail.
     
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  11. Dec 8, 2018
    66cjlasvegas

    66cjlasvegas Sponsor Sponsor

    las vegas, nv
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    I agree with sterlclan, never use solid wire on anything that moves.
     
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  12. Dec 8, 2018
    Alan28

    Alan28 Sponsor Sponsor

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    I have prefered to divide what is under the key control. The fact to turn the key with an effort to make the starter work can make the device turning and wires can be damaged.

    The key gives electricity to ingnition but the starter is controled by a push button. I am not a good electrician but if I concentrate I can understand. So I added fuses to wome electric lines which are not well protected as on a new car.
    Of ourse it would be better to rewire all, use a fuse box or protection, line by line. And to put a protection around wires coming through the firewall.
    I do electricity in my house, but in a car I don't know well the parts to use. I paid a pro to rewire some and it is clean.
    There is a lot f good people to help you here.
    My first idea was to bring power through a keyboard device for safety, this appliance feeding all parts.
    But it works like it is, with a push start button which I am happy to push, not having to keep this key stupidly turned on.:steamed:
     
  13. Dec 8, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member Sponsor

    Denver, CO
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    sounds good - at this point its wait for some of the parts and pieces to come in.
     
  14. Dec 8, 2018
    Sierra Bum

    Sierra Bum Member

    The High Sierra
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    Just another idea...Same thing, beefier relay...safe and easy

    You can use a continuous duty solenoid to power your auxiliary circuit. A Cole Hersee solenoid is compact and comes in amp ratings from 80 to 200+ amps. The relay itself probably only pulls a couple amps. I usually go over kill for a main fusebox lead wire.
    1. Run a wire from the acc side of your ignition switch to the small + pole that engages the solenoid.
    2. Run the appropriate sized awg cable from your battery to one side of the solenoid (around 6 gauge for you load)
    3. Run another 6g cable from the other big solenoid pole to the fuse box.
    4. Run a ground wire for the other small solenoid pole.

    The circuit will be key off-on.

    Final suggestion: Don't use the ign pole on the ignition switch to power the relay. If you want to power your accessories (key-on) with the engine off, you don't want your ignition circuit powered up.

    [​IMG]
     
    Last edited: Dec 8, 2018
    Rich M. likes this.
  15. Dec 8, 2018
    Sierra Bum

    Sierra Bum Member

    The High Sierra
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    Everybody has probably seen this but it is a useful reference
    [​IMG]
     
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