I'll try to keep this concise. I want to install an auxiliary fuse box underhood for various accessories I'll be adding over time such as, but not limited to: light bars, light pods, radio (CB/Ham), etc.

Here's what I'm looking at so far:

Blue Sea 12-circuit covered fuse box with negative terminal - $40
https://www.amazon.com/Blue-Sea-Syst...use%2Bbox&th=1

Blue Sea 100-amp circuit breaker - $40
https://www.amazon.com/Blue-Sea-Syst...ircuit+breaker

WindyNation 2 gauge 4-foot red and 4-foot black pure copper cable with 8 cable lugs - $30
https://www.amazon.com/dp/B01N30NBNF...ding=UTF8&th=1


2 questions:

Is it bad or inadvisable to be running the setup this way where the fusebox is always hot since I don't want to run a 100-amp relay linked to the ignition/accessory?

Can I run a OEM-looking switch like what Rago Fabrication sells that manually controls a relay so I can make the fusebox hot at the push of a button or are the 18-gauge wires on the OEM-looking switch too thin to support the trip circuit to a 100-amp relay?

Any other help/suggestions are greatly appreciated. Thank you

Good choice on the fuse box... Personally I like having a ground buss built in to keep wiring cleaner.

There is no issue running the box always on / hot. You can just add relays to each individual circuit where applicable. You are going to need to do that to control individual loads anyways. Ham radio for example, at least my Yaesu, can be turned off at the head of the unit. That circuit is always live.

The breaker is a nice item for service and convenience, but its not a replacement for a fuse.

Let me repeat that one more time... A BREAKER IS NOT A REPLACEMENT FOR A FUSE.

You can weld a circuit breaker open with enough amperage... and watch entire thing burn to the ground. Where a breaker can fail, a fuse will not.

Size the fuse to protect the cable.

This is what I recommend.
https://www.amazon.com/Littelfuse-29...ords=fuse+mega

Example of the use you need...
https://www.amazon.com/Fuse-Mega-32V...ords=fuse+mega


Again... to stress the important of fusing... here is an image that will probably stick with you.


And I use what I recommended to you... its much better than the stuff the car audio guys generally use...

The point of a relay is to be able to control higher amperage loads using switches that are not cable of passing that current.

So a relay generally has an input and output for the load, ground connection, and a separate connection for the control which generally goes to a switch.

The control circuit passes little to no current so 18-20 AWG is a pretty standard wiring size for those applications.

Chances are you are going to end up with some bosch style relays. Here is a wiring diagram / example:

Thank you Bumbo.

What is the purpose of a 125amp fuse if the fusebox itself is only rated for 100amps? Would using a 100amp fuse be sufficient?

Also, if I'm going with the fuse then I assume I can ditch the breaker and then if I'm going with the fuse I'm perfectly safe leaving the entire circuit hot? I'd like this approach better as I'd like to minimize the number of components I need to have in this circuit.

The reason I wanted to go with the breaker wasn't so much that it would protect the circuit but also for the convenience of using it as a switch. If a breaker is unsuitable as a switch then do you have other recommendations for if I decide I'd just like to cut all the power to the entire circuit without removing the battery cable such as if I'm going on vacation or something?

Also, did I need to go with 2 gauge wiring or would 4 gauge be fine? I know larger gauge won't hurt but I'm just asking for curiosity's sake.

The fuse size I selected was only an example. As I mentioned above, the fuse is to protect the mains cabling. Size the fuse to protect the "big wire". Forget about everything else. You have load level fuses so fusing the fuse box sounds a bit silly. If you really want to protect the fuse panel, then that's a job for a breaker. Size that for the max amount of current you want to be able to feed into the panel.

You can really build this any way you like... some is personal preference, some is following some basic engineering best practices.

Safety has nothing to do with leaving it hot or not assuming it was built well...

If you have a 100 amp breaker and a 125 amp fuse for example, the breaker will go before the fuse if you were to have an over current condition during normal operation. This saves you $10 in fuse costs and a push of a button gets you back up and running.

If a wire comes loose and somewhere and you get a dead short causing the breaker to fail, then a correctly sized fuse will blow before the wiring catches on fire.

Asking if it is safe without any information about the rest of the system is just one of those questions... Is it safe to drink alcohol? What type of alcohol, how much alcohol? Are you operating a car when drinking alcohol? Are you at home? More to that question...


That's fine, you can use it as a switch too. That's why I said its nice for maintenance and convenience.

You can also buy a switch, to use as a switch... both can be the correct answer.
https://www.amazon.com/Blue-Sea-Syst...lue+sea+switch

That said, If you need to hit the switch to save the system from killing itself then you have a poor design...

1) You need better load control... I control all my loads at the individual item level. If everything is off and there is no parasitic drain then you dont benefit from any type of switch.

2) You can add a low voltage disconnect to feed the panel after the fuse and have a device protect you from yourself because I guarantee you will come back to a dead battery sooner or later without it, especially if you are running loads unattended.


Here is how I built mine...

Primary: Battery>Fuse>LVD>Fuse Panel
Secondary: Fuse Panel > Relay/Switch > Load



The LVD is a programmable low voltage disconnect... even if I forget to turn something off, it will turn it off for me to protect the battery and allow me to start the car even when the user fails.

Its a question about application and design...

If you don't know what you load requirements are how could you even start sizing a system correctly? Don't just arbitrarily pick a wire size. It's like buying a pair of shoes without measuring your feet first or knowing your size.

You can go bigger, but if you don't have a heavy load then you are just throwing money away. You end up with more expensive components across the entire system. It adds up fast, but at least you have the headroom.

If you go too small then you suffer from increased voltage drop as the heat and resistance will increase as the load increases. Resulting in poor system performance.

So without actual data, its impossible to answer that.

I started by buying or picking out what I wanted to install. Look at the spec sheets, look at the power requirements. Add it all up. Size it accordingly. There are online calculators that you can use to calc voltage trip and line size.

Start here, but I have other tools that I use too.
Circuit Wizard - Blue Sea Systems

I ended up measuring all my loads... since not everything I had came with a spec sheet, and you cant trust specs on Chinese light bars. Here is one that was advertised at 240 watts...



And then checking my work at full load conditions.

[QUOTE=Bumbo said...[/QUOTE]

@Bumbo - thanks for all this information. It's filling in a lot of gaps I didn't realize I had, and helping me rethink the electrical upgrade I'm in the design phase of right now.

kinda off-topic. is everything in a vehicle fused by default? for example our 4runners..?

For the most part... yes, it is. There are generally at least two factory fuse panels in most cars/trucks.

But don't expect to use those circuits for anything else other than what the factory has decided to use them for.

The reason most people add a second fuse panel is so we don't chop up the oem electrical and create issues.

No problem... I know some of my posts get long winded and to be honest I have no idea if people read this stuff sometimes because I can get a bit wordy.

I really appreciate the Likes.

If you want to run your design by me just let me know.

I used the amp chart on Blue Sea's site and it advised for 3% critical voltage drop at less than 6 feet (I don't really care about the 10% voltage drop vs. the 3% since the items I'm powering wouldn't qualify as critical) to use 4 gauge wire for a 100 amp circuit.

The fuse box is rated for a max of 100 amps so 4 gauge would be appropriate. That being said, I decided to up it to the next gauge because it was only a couple bucks more and because it just seemed to be a good idea to go a little higher than the minimum.

The circuit protection is meant for the big wire and the fuse box but I don't anticipate the big wire to be that long. Probably 2 feet max. That's why I originally opted to go for the breaker over the fuse but I see your point about the fuse being failsafe. Also, Blue Sea advises not to go higher than 125a on the fuse given that the box is rated for 100a.

I think I'll do the 125a fuse in conjunction with the 100a breaker like you said. Do I do the fuse first or the breaker? I assume the breaker.

My light bar is rated at 180 watts but I figured that was an optimistic number. Either way, I picked up a relay harness rated for 300 watts. Is there any reason not to trust a chinese relay harness if its at least rated 100+ watts more than the item it needs to power?

Anyway, I won't ever be adding anything that has huge power requirements such as a refrigerator, stereo components, etc. The largest draw item will probably be the light bar and the rest will probably just be ditch lights and communication devices and maybe a standalone GPS unit.

Thanks again for the info and please keep the lesson going.

Fuse before the breaker...
Battery > Fuse > Breaker > Panel > Accessory

Voltage drop is a big deal... because the more you have, the hotter that wire gets. An engine bay is already hot, every little bit can help and your electronics will thank you by working more efficiently because of the higher input voltage.

Ham radio IMO is a critical load, they do appreciate and work better with higher input voltage. Too much voltage drop and the receive end starts hearing problems and or your output power is reduced. Its critical because that device along with GPS are personal saftey items.

Either way... 6' of run, 4 awg is fine. 125a fuse and 100a breaker are in agreement with my selection software. You don't have that much stuff, so you are looking really good even with 4 AWG. You have plenty of overhead. So far so good.

I would advise against the 2 AWG upgrade for your needs.

I'm sure your 300 watt wiring hardness is fine.... most I have seen in the past use 16 or 14 AWG wire, fine for the light bar.

Radio communication loads are intermittent anyways... only during transmit they use real power. Use the wire it came with, they supply good stuff. Which radio do you have, or plan on buying?

Plus you can have more than 100 amps of stuff connected to that panel if you wanted to... For example an air compressor. You can always turn something off to free up capacity for something else.

If you accidentally turned on too many things at the same time, the breaker gets to do its job.

If you are building your own cables make you get a crimper designed to do it. Don't use a screw driver and hammer, and don't solder them either.

The 2awg is already on the way, is it going to be a problem with that and should I send it back in favor of 4awg? I'll be using a hydraulic crimper with heat shrink tubing. So, flowing solder into the crimped end is a no-go?

I assume I had the circuit backwards after reading more into it. Now I understand why the breaker goes closest to the fuse box. Since the loads are drawing power upstream and the breaker will trip before the overload gets to the fuse?

I must have misunderstood what they define as a critical load. I figured things like the radiator fans, fuel pump, etc. are critical loads but accessories such as non-factory lights, ham/cb radio, etc. would not be critical. I should explain that I'll also have a handheld triband ham radio as well should I need to get out of the vehicle and still need to maintain communication with others.

I updated / edited that post as you were typing to correct and organize some thoughts so you might want to re-read it.

hydraulic crimper is good, that's what I use. I used adhesive lined shrink to seal the cable to help against corrosion and an extra layer of black for protection. If you look close at my pictures you can see it.

2 AWG is fine, it's just harder to work with. The fuse block terminals are small, and you need to make sure the lugs themselves are sized correctly for the posts as well. Same goes for the breaker. If the hole in the lug is too big, you have a reduced contact patch and you actually end up with performance loss/issues as you just created a high resistance area.

EDIT: Also... you might have fitment issues with 2 AWG... I used 4 AWG in my picture, and after the heat shrink it was a pretty tight fit after getting the clear cover on it.

DO NOT SOLDER... soldering those types of connections create a point of failure. If not correctly strain relieved they will work harden and eventually break. You want to crimp and without getting into detail, its just better.

I really like Temco for my stuff... American company, Good quality, Made in the USA...

Here is what I mean by lug and terminal size... Both are these are 4 AWG. One has a 1/4" hole, the other has a 3/8" hole. Blue Sea publishes all the terminal and stud sizes on the spec sheets... You can get all the right stuff without any question or confusion.

25 Lot 4 AWG Ring 1/4" Hole Terminal Lug Bare Copper Uninsulated Gauge | eBay

25 Lot 4 AWG Ring 3/8" Hole Terminal Lug Bare Copper Uninsulated Gauge

Just go on ebay and search "temco lug"

Also I like their cable... I buy in bulk, I stock this stuff because I work on other peoples cars too.

TEMCo 4 Gauge AWG Welding Lead & Car Battery Cable Copper Wire | MADE IN USA


I am a bit of a ham myself... VX-7R is my main radio, Kenwood in the emergency coms kit, Yaesu in the dash.