wiring relays, auxillary fuse box

So my next wiring en devour is adding an auxiliary fuse box for my project car. Below is the spec sheets and my wiring diagram for review.What do you guys recommend i do, or how to wire this correctly??
Thanks for the help!

What a great post with good data! (Thanks!)

The fusebox blurb reckons "not more than 125A" protection for its supply circuit. You have 140A (2x70A).

And why 2 parallel relays and fuses - blow one, blow both (most likely).
For the benefit of others, we discussed that via PMs.


But I told someone 2 hours ago that I was going to bed...
I guess now they'll never believe what I say, or write!   

Oldspark, haha you told me to post this as a topic, so i did, LOL!

So i figure change the two relays from 70 amps to 60 amperes each.

One reason for the two relays, is that i have had a hard time finding a single relay large enough that is for automotive use..

Also i have double checked the amp requirements for the items in the aux fuse panel and they are correct

There, I told you uncle Peter would help, now try this useing an individual 40 amp relay for each fused circuit, probably a lot more efficient and cheaper, the fuses on the fuse block protect the relays and circuits, you won't need a master fuse if this fuse box is within a few inches of the battery:-fuse_layout.bmp

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Actually just looking at the fuse box again, can I assume either a fibreglass vehicle or boat? Sorry I just noticed the negative returns.
If this is for a standard car, it's totally unnecessary and expensive.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

it is for my old car, the existing fuse box is maxed out with the factory power accessories and i don't like trying to add more circuits to something to the factory one. i was just looking for a cleaner install for additional circuits. the fuse box is going in behind the dash to hide it some and i didn't want a whole mess of relays and and the bulk of the extra wiring needed behind there. the fuse box is for marine use and it was actually only $20 from an online dealer. the closest pre-built unit like this is around $80 and it is only rated for individual circuits of 20 amps

It's equivalent to $30 here from Raw Components the UK supplier and it's a complete waste of money. Buy a normal 12 way from the same supplier, complete with three x 4way relay boxes will cost you the same and more than adequate for your purposes.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Nice find there, so im thinking about these two componants. what do you think??
https://www.rawcomponents.co.uk/fuseholders/16w-fusebox-mta.html
https://www.rawcomponents.co.uk/relays-holders/holders/4-way-relay-box.html

78mc-bbc wrote:

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Oldspark, haha you told me to post this as a topic, so i did, LOL!

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Of course! I will never PM suggestions etc when it's a good "public" design project! (Never - absolutely never... LOL!) But I rarely "tell" someone to post as a new thread, I merely suggest (ha ha). (Talented people understand the merits of a public post anyway.)


But now I know why the parallel relays. I thought it may have been for redundancy - even if there was an unseen switch for the spare. (As some old fart recently wrote: "But if one fuse blows, why not the other?" etc etc. But that's ANOTHER design topic!)

I generally recommend against parallel relays & circuits to supply the total needs of a (power source) supply. That can require measures to share the current (eg, series resistance).
Besides, if one leg fails, the other(s) then blow if they can't handle the load. (Viz many paralleled relay circuits I have seen, and my former UPS - fun watching each individual unfused relays or power transistors/FETs take their turn to burn (ie - smoke, but that didn't rhyme with turn). Even if fused, that's "all" fuses blown (ok if two cheapies, but when more, or bigger costly fuses...)   
Instead IMO it's well worthwhile getting properly rated components.

I use a FRC6BC-1 which is 150A (~AUD$15; SY4073 - Automotive Relay 150A SPDT) for my battery isolator. (Its NC 100A 87a is not used.)
But there should be similar elsewhere, and with mounting options missing from the FRC6BC-1.


But two other issues:

Topology: I'm with father Howie. Split into smaller legs etc.
Or rather, IMO, certain things warrant their own fuses (if not relays) - eg, ejection seats, nitrous, other biggish loads.
Certain things suit a dedicated fuse panel - eg, gauges, warning lights, etc, though they often share a common fuse.    
Some fuseboxes have twin feeds so you can split IGN & ACC or battery +12V, or split from different master fuses for independence (eg, seats & nitrous separated gauges etc).

Dimensioning: If the loads are known, it is easier determining total power (current) requirements.
That typically has 2 outcomes - eg, assume total peak current or fusing of individual loads totals (say) 150A. But that is not a realistic scenario, hence we use might use a 100A relay and "master" fuse & wire (or I'd probably use my 150A relay with 150A - 200A wire and a 100A fuse).
That meets the "max 125A" spec on the fusebox, and uses a more common (smaller) master fuse. (And if the relay & wire handle it, the fuse could be upgraded later.)

The 2nd dimensioning outcome is over-designing the supply anyhow. That's the same case as not knowing the total (future?) loads, and covered above - ie, despite only needing 125A (the max for the fusebox) or less for the loads, use an overrated supply distribution (especially cable, but also relay) but with suitable fusebox or load fuse (125A or 100A etc).

I usually over-design (over-size) the distribution so the fuses (or relays) can be upgraded later.
[ My battery isolator uses hefty cable - maybe 100A - 300A (one day I'll actually check!) despite its paltry 2.5A "normal load" requirement. But add the battery recharge current - typically well under 10A but can peak at ~45A - and my former 60A relay was fine. I'm now using the 150A relay for other reasons (testing etc). And I use 50A self-resetting circuit breakers at each end, 50A being the largest of it's kind - namely weatherproof & cheap ($7 each); a plastic version of Jaycar's Chassis Mount 50A Autoreset Circuit Breaker. ]

Remember - a fuse must protect its downstream components (wires, relays, etc) UNTIL the next fuse takes over. But that does not mean that fuse cannot be smaller - it just has to be big enough to handle the load. [But never exceed its downstream distribution which in turn may be much bigger than its required Amperage. {yes - a paraphrased repeat of the above.}]

BTW - the alternative to an IGN switched relay is for all power to be off the battery, but the individual loads are relay or switch controlled. EG, IGN +12V supplies the switch for the gauges, or (the switch to) the relay's coil that turns on gauges, seats, nitrous. That's usually my topology, but it is a case of minimal relays versus circuit independence etc.   


I also think of other outcomes, like:
- so much for another short comment or reply,
- see, I told you others would help!
- ... but I'm surprised there aren't more (responses from others),
- and no comments yet on the relay's 85 & 86 polarity!

Alas, breakfast coffee down, I'm out to adjust my toe-in. Can that defer my call to my GF to tell her when I'll be around...?    

POST EDIT: I just expanded a view of that 16-way modular fusebox. It does look good (I didn't see that it had relays, plus spare fuses. That's bluddy great!!)
But I'll keep the blurb below FYI. It may serve as my obituary anyhow.... /end post-edit.


Dang - I missed your reply....

I thought that 16 Way Modular Interlocking Fuse Box was fairly expensive, but maybe not... (eg - NA Auto (Australia))


But one thing - IMO a lot of those fuseboxes seem "cheap" as in brittle plastic, and covers that are likely to come off, or warp & break etc. (Post edit applies - not applicable to your linked fusebox(?).)   
It wouldn't surprise me of the plastics melted with normal fuse loadings. And in engine bays...?
(It's a bit like buying cheap garden solar lights and growing pots - within 1-3 months the solar windows are opaque and the pots or covers crack up! And by cheap I mean quality - not price - you can get great cheap stuff, and cheap cheap stuff (aka poo)!)   

Not that I have used those fuseboxes (I have a sample or 2 though) - I use boxes from vehicles (wreckers) which I know to be quality and up to the stresses of vehicle environments.

A good example is a 2-relay with ~6-8 blade fuses found on many early Isuzu Rodeos (1980s) though they use the JECS relay footprint (which I now use - IMO better than Hella/Bosch types and despite initially not liking the various pin locations depending on type, I now understand them (easy!) and it prevents the incorrect relay being used, though their black and (newer) blue SPDT & SPST relays are interchangeable).

But there are lots of others. Our Fords typically use Hella type relays.

And there are boxes with relay and blade-fuse and flink (fuselinks; the plastic type) combinations. (Flinks being typically 20A to 100A, blades up to 40A.)

Not to mention the Hyundai (or Daewoo) battery-post mounted 3-flink box or the Toyota equivalent (each suited to different applications; I use the former).


But my girlfriend is going to kill me. Slowly!

ok, thanks for the help again. So after taking some thought about what you said about using fuse boxes from other cars i thought of my daily driver which is an old chevy tahoe, and has this fuse box in the engine compartment. Take a look and let me know what you think on using something like this?? Also the two studs on the fuse box are constant 12 volt

Hospitals are great - they now have bedside internet access!
Jest kidding - I've done my toe-in, and the GF is cool - provided I get there within 50 minutes...

And noting my pseudo-withdrawal of my comments re your linked fusebox....


But yeah - that box looks great. Maybe (too?) big, but if space is okay.
In fact I'm intending(!) to do similar with my Chevy Wasp. (Well, it's not Chevy, but its next incarnation was as the Chevy LUV which became the KB & Rodeo (Isuzu).)

I'm mounting a 1991-1995 Rodeo -cum- Fontera fuse box. Very similar to yours pictured, but uses JIDECO (or JECS?) relays, ATS blades, and 2 L-type flinks. Two relay bases can be changed (for the 3 main types - 2xSPST, 2-opposing-SPST, and SPDT), and I've extracted the fuse connectors to reinsert or rewire as desired.


It's a matter of taste, and a desire for masochism.
It's more a case of what you think, and can do, and then if <whatever> is suitable.
But I like "standard" vehicle stuff in vehicles - even if from a 30-year younger vehicle. After all, it's what the original maker would have done. (Did I mention cheaper?)

Also probably more possibilities. You get to understand and can reconfigure later.
In my case, the excess of fuses solved another dilemna - wanting to be able to easily swap between outer permanent low-beams with traditional hi/low beams without complex switching and reconfiguration. (I have 4 headlights.) That will now be done via the fuses: In one case, the 2 fuses go there, for the other, I just swap their position. Simple!

It's a bit of work, but IMO cheaper (from a wreckers etc) and better. (My original firewall mounted fuse box consists of four 3AG glass fuses, and its 30A light circuit burnt out years ago. But added lighting, cooling fan, and other relays now reside beside the battery. You may be familiar with that scenario LOL?)

As for grabbing other fuseboxes, cut the wires as long as possible. I prefer to be able to terminate original wires than have to extend them.


Oh oh - 40 minutes, and I have yet to get cleaned up!

This thing about Peter and myself being mentally linked is becoming surreal at 12,000 miles apart or is it because we are both giving you the correct answers?
Look at the diagram I last posted, my suggestion on distributed power. Now this is the strangest part, I kept thinking about the fuse boxes from VW Golf/Rabbit/Jetta Mk 2 and 3. Also late 90s Ford Fiesta, nice and compact, except that model was never sold in the US.
They used a small gauge ignition feed from the ignition switch, 3 separate lines, permanent, ignition and lighting, and an internal bus bar and room for about 8 standard (Bosch/Tyco etc.) relays.
Yes those raw component fuse and relay boxes were exactly what I was thinking of. I've had a 6 way in various cars for at least 10 years with no problems. Lastly gauges drawing 4 amps? That's a bit excessive isn't it?


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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

well i double checked the amp on the gauges directly from Autometer's website and that's what they advise.
Also the size of the box i am showing above from the Tahoe is actually pretty small around 6"x4" or 15.24cm X 10.16cm for you metric type peoples.

I added LED lights to my 91 Buick Riviera and it came with fuse adapters to plug right into fuses in box....said to use radio or power windows....I plugged into 10 amp radio fuse and the lights would not shut off even when ignition was off....I thought radio was switched instead of constant....Then I ran it to ISLIGN Fuse thinking that was ignition so it would have 12 volts and be switched.....That caused the car to need a jump off with cables and a computer message with electrical problem......The lights now don't come on and I took and put old fuses back in to make it normal....What happened and where do I tap in to run these LED lights....
?????Thanks

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chief peters

Ign. + and side (parking/running) light + auto operation and legal.
MODS, can this post be relocated.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

EDIT: Never mind. This a thread-necro. I didn't see it was 4 years old brought back to life.