Circuit Breaker Advice

[shays4me]

I've been working on a dual battery conversion for my JKU for the last week and I'm wondering if I installed too big of a circuit breaker under the hood. The battery system uses a NW power products smart solenoid and I've got a 4 gauge cable running off of the house side of the solenoid to the breaker. I was initailly going to use a maxi-fuse, but there wasn't much real estate left under the hood after the battery install and once the fuse holder arrived I knew it wouldn't be up to the task for the wire size I was using. I went to my local O'reilly auto parts and they had some breakers in stock. I was allowed to take one outside and it fit perfectly on the battery box bracket so I purchased the smallest they had which was 100 amp. On the unprotected side of the breaker I have an eight guage and a twelve gauge wire that go to seperate circuit breakers for trailer brakes and a trailer battery charge circuit. On the protected side, I have three 14 gauge circuits that are all fused for off road lights and a 12vdc receptacle, a heavy six gauge wire going into the Jeep which will feed an eight circuit blue sea fuse panel, future fridge and diesel furnace. I have the wire protected in split loom from the circuit breaker to the fuse panel under the dash. I will also run a fused 30 amp relay inside for ignition switch circuits like lighting controls, cb radio, dash cam, etc... So the 100 amp fuse was installed next to the battery to protect it in case the six gauge wire goes to ground. Every circuit downstream from the six gauge cable has its own fuse. My question is if 100 amps is too much breaker? I have looked around on line and found all different sizes so I'm just not sure. I work with electruicity for a living but I work on existing circuits and I do not design them. If I add up all the downstream fuses with room for the future 100 amps doesn't seem like too much and the six gauge will handle 150-200 amps in it's four foot run. Anyone have any experience/insight into this?

 

[jonyjoe101]

Myself I would test them to make sure they will operate at the rated amps. I see you have some of the shortstop circuit breakers, I had one similar rated for 30 amps, but they were tripping at about 22 amps. The 100a littlefuse breaker is namebrand so it should trip at the appropriate rating.
On my system I never use more then 30 amps, thats the biggest fuse or breaker I would use. My roadpro cooker maxes out at about 12 amps , my solar panel maxes out at 27 amps, thats why I settle on 30 amps being the largest rating I use.

 

[Peter_n_Margaret]

Fuses and circuit breakers need to be sized to protect the cable, not the appliances.
Cheers,
Peter
OKA196 motorhome

 

[llamalander]

BlueSea publishes some good resources that are pretty well organized to help you sort through the calculations required.
https://www.bluesea.com/resources/1437
(this is the first of 2, the other links are at the bottom)

It is worth remembering that fuses or breakers exist to protect the downstream wire from overheating as well as disconnecting in case of a short.
Generally, you want to locate the fuse/breaker within 12" of the power source, or right at it--anything in between is unprotected.
Loom protects your wires from being damage by the truck, fuses protect your truck from being damaged by the wires.

The longer of two of the same size wires has greater resistance, the thinner of two equal lengths of wire has more resistance. You can see this in the first chart
Resistance produces heat and drops the voltage. In "12-volt" systems, voltage drop is a real concern, so the left side has a choice of the drop you are willing to accept
Oversizing wires allows the required current to run cooler and maintain voltage over longer runs.

The downstream loads do not determine the fuse rating, it is the overall resistance of the wire that the insulation can handle before it fails (melts or catches on fire)
The charts above reference wire rated to withstand 105º C, many common wires have lower ratings like 90º, 75º or 60º C--so don't assume or think your wire is top quality.
The hotter the ambient temp, the less room you have to overheat your wire, hence ratings for grouped wires or those in hot areas--engine bays, which the second chart illustrates.

Work backwards from your load: voltage drop + distance & amperage required will give you a wire size, then put a breaker on it at or near the battery that will protect the wire.

 

[shays4me]

BlueSea publishes some good resources that are pretty well organized to help you sort through the calculations required.
https://www.bluesea.com/resources/1437
(this is the first of 2, the other links are at the bottom)

It is worth remembering that fuses or breakers exist to protect the downstream wire from overheating as well as disconnecting in case of a short.
Generally, you want to locate the fuse/breaker within 12" of the power source, or right at it--anything in between is unprotected.
Loom protects your wires from being damage by the truck, fuses protect your truck from being damaged by the wires.

The longer of two of the same size wires has greater resistance, the thinner of two equal lengths of wire has more resistance. You can see this in the first chart
Resistance produces heat and drops the voltage. In "12-volt" systems, voltage drop is a real concern, so the left side has a choice of the drop you are willing to accept
Oversizing wires allows the required current to run cooler and maintain voltage over longer runs.

The downstream loads do not determine the fuse rating, it is the overall resistance of the wire that the insulation can handle before it fails (melts or catches on fire)
The charts above reference wire rated to withstand 105º C, many common wires have lower ratings like 90º, 75º or 60º C--so don't assume or think your wire is top quality.
The hotter the ambient temp, the less room you have to overheat your wire, hence ratings for grouped wires or those in hot areas--engine bays, which the second chart illustrates.

Work backwards from your load: voltage drop + distance & amperage required will give you a wire size, then put a breaker on it at or near the battery that will protect the wire.

Thanks for the link, that is the resource I've been needing. I don't know why I didn't think to check out blue sea, I've used a lot of their stuff!

 

[86scotty]

Thanks for the link, that is the resource I've been needing. I don't know why I didn't think to check out blue sea, I've used a lot of their stuff!

Your setup sounds good to me on one cup of coffee. Interesting that O'reilley (local to me too) has those breakers. I will check them out. Never having any luck getting this kind of stuff locally I usually go to Amazon. I love Blue Sea and have bought many of their isolators and other parts but don't think I've ever bought a breaker from simply because of the price. The Chinazon junk on Amazon is indeed junk so for future readers, don't do it! I've had great luck and gotten a good price from Bussman and used several of them in the last couple years.

 

[shays4me]

Your setup sounds good to me on one cup of coffee. Interesting that O'reilley (local to me too) has those breakers. I will check them out. Never having any luck getting this kind of stuff locally I usually go to Amazon. I love Blue Sea and have bought many of their isolators and other parts but don't think I've ever bought a breaker from simply because of the price. The Chinazon junk on Amazon is indeed junk so for future readers, don't do it! I've had great luck and gotten a good price from Bussman and used several of them in the last couple years.

I see the cheaper priced stuff on Amazon, but I’m not living in poverty and don’t want to buy something twice. The name brand stuff is generally a known quality so I buy it. Buy once, cry once is my motto and it has always payed dividends down the road. I don’t need to feel embarrassed about my setup either because it’s all first rate products and I do my best to install it correctly. The under the hood wiring and battery box have taken me three afternoons and two full days to complete. I had to fab a couple brackets to get things the way I liked them and there was a lot of trial and error running the cables and harnesses in such a tight space. Next up is the interior wiring. It was all previously wired but it was one circuit at a time. I’m going to go back and run harnesses because it’s become a bit of an ugly mess at the terminal board with all the in-line fuses and butt splices. It will all be continuous wire ran in split loom. I’ll build some extra circuits just in case but Im not anticipating many more mods now that I’m toward the end of buying my electronics and accessories.

 

[4000lbsOfGoat]

With the complexity of your intended setup you may want to consider something like an sPOD or similar. It'll save you a ton of wiring\fusing\switching hassle...

 

[shays4me]

With the complexity of your intended setup you may want to consider something like an sPOD or similar. It'll save you a ton of wiring\fusing\switching hassle...

The spod would take care of the under hood chores but I’d have to run wires back into the interior for the camper, fridge, brake controller and charge wire, etc. I don’t believe the spod is up to the task of running the gauge of wires that I have or the number of circuits. It’s a neat device that costs a lot of money and makes things simple for people who are unfamiliar with electricity and want to run a few lights and accessories. My needs are quite a bit more complex than that because my Jeep has a camper top and associated mods. I already have a switch bank with a bundled harness going out to run my relays which is what the spod essentially is and it was a lot cheaper not to mention it gave me more flexibility in the layout. The only thing the spod offers that I don’t have is fuses that are on the same board as the relay. That’s clean but it’s a sacrifice because every wire needs to go to the module even when it’s not in the direction of the accessories, which causes more voltage and current drop.

 

[VanWaLife]

As it seems you've got your initial question about circuit breaker size answered and we're now in the part of the post where everyone offers unsolicited advice / criticism I want to get my two cents in. I recently installed a high amp Speedway relay / fuse combo to get ignition switched power from my aux battery that may be relevant for your system. Pretty nice, not quite bluesea quality but close.

Speedway Single 70 Amp Relay Module

This single 70 amp relay board features one industrial 70 amp heavy-duty relay. Connect main battery power to the #10 battery input stud, and connect the component to be powered to the #10 output stud. Has three inputs that accept fork terminals for ground and switch power connection. This...

www.speedwaymotors.com

Also I wouldn't be shy about ditching the rubber cap your breaker comes with for one that covers the whole crimp like you're using elsewhere. Also with all the stuff you have planned you could consider some sort of low-voltage shutoff for the aux system. Lastly one thing I wish I'd done earlier in my wiring adventure was do a thorough drip test in the rain to see what areas get wet under the hood, including when I open the hood when it is wet. I've spent considerable time adding standoffs to keep stuff dry that I could have added during initial installation with better planning.

 

[shays4me]

As it seems you've got your initial question about circuit breaker size answered and we're now in the part of the post where everyone offers unsolicited advice / criticism I want to get my two cents in. I recently installed a high amp Speedway relay / fuse combo to get ignition switched power from my aux battery that may be relevant for your system. Pretty nice, not quite bluesea quality but close.

Speedway Single 70 Amp Relay Module

This single 70 amp relay board features one industrial 70 amp heavy-duty relay. Connect main battery power to the #10 battery input stud, and connect the component to be powered to the #10 output stud. Has three inputs that accept fork terminals for ground and switch power connection. This...

www.speedwaymotors.com

Also I wouldn't be shy about ditching the rubber cap your breaker comes with for one that covers the whole crimp like you're using elsewhere. Also with all the stuff you have planned you could consider some sort of low-voltage shutoff for the aux system. Lastly one thing I wish I'd done earlier in my wiring adventure was do a thorough drip test in the rain to see what areas get wet under the hood, including when I open the hood when it is wet. I've spent considerable time adding standoffs to keep stuff dry that I could have added during initial installation with better planning.

My ignition on circuits are all low draw like relay control and a dash camera so I’m going to use a 30 amp standard relay for them but thanks for the link. As for the aux power shutoff the breaker I have has that feature. If I wanted to store the Jeep for a time period I could just push the yellow button on the top and it trips the breaker. I really liked that feature when I seen it at the auto parts store. Less is more and one item that does two jobs makes a lot of sense. I really do appreciate all the comments and advice. I’ve learned a lot today in this thread! Like I said i work on electrical circuits for a living so I’m not a novice but i’m not a master in the automotive field for sure!

 

[jameslandry]

T Tocas works well in a variety of mobile environments, including buses, RVs, and boats. It can support large loads and is dependable and long-lasting.