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Fuse for trunk mounted battery

S

Shrinkdude

Member
Running things over in my head
I am mounting the battery and start relay in the trunk.
I am running 1ga cable to the starter from the relay
I plan to run 8 gauge from the battery side of the relay to a terminal post located where the battery was in the engine compartment.
Here is my question:
Why would I need to fuse so large (been reading posts on forums) in the trunk between battery side of relay and front terminal block/post?
Once the car starts then all power (ideally) is run from the alternator which has a 10 gauge at best, probably 12 ga.
I still have the old style not the 1 wire alternator
I was planning at a 30 amp fuse as I have a fairly stock system besides the electric fuel pump(3amp) and halogen headlights (6 amp).

Any insight would be helpful
 
AICS or Al I can say , is that you "must have " a fuse at the plus terminal of the battery.
 
This is what I did to minimize the risk:

I moved the battery and the solenoid to the trunk. I have a 1/0 wire from the battery to the solenoid then to the starter. That way its only live while I'm cranking the engine. If you end up going this way you do NOT want to fuse the starter cable!! From the solenoid, I have a 2 ga wire with a 175 amp fuse running to a distribution point under the dash. From there I power everything else (fuse panel, engine, lights, etc). I'm running extra relays and fuses for the fuel injection, fan, lights and driving lamps, but for most people it would go directly to the fuse panel.

I think this setup gives me as much safety as possible with all the high voltage wires going to the front either fused or only hot when needed.

-Shannon
 
Yes, that cable goes up to a distribution point under the dash where I connect the alternator (120 amp) and split off leads to the regular fuse box, the EFI ECU and fuel pump, the electric fan, A/C, headlights and driving lights.

This cable is hot all the time, so I thought it needed something.

-Shannon
 
Hey, do like me, I run'em straight and buy good insurance. :beer


:sm_NTA
 
"LastDeadLast" said:
This is what I did to minimize the risk:

I moved the battery and the solenoid to the trunk. I have a 1/0 wire from the battery to the solenoid then to the starter. That way its only live while I'm cranking the engine. If you end up going this way you do NOT want to fuse the starter cable!! From the solenoid, I have a 2 ga wire with a 175 amp fuse running to a distribution point under the dash. From there I power everything else (fuse panel, engine, lights, etc). I'm running extra relays and fuses for the fuel injection, fan, lights and driving lamps, but for most people it would go directly to the fuse panel.

I think this setup gives me as much safety as possible with all the high voltage wires going to the front either fused or only hot when needed.

-Shannon
Click to expand...
With that size fuse you may as well not even have one. That wire could melt and start a fire long before it blew. The current draw on that "feed" line will never see anything near that amperage draw, anyway. All your accessories will be getting their power from the alternator output not the trunk mounted battery (provided the engine is running). The alternator doesn't draw power so I am confused on why such a HUGE fuse. Where do you see the demand to try and accommodate so much amperage? Instead of being a safe-guard I think it might just be a hazard.

If I am missing something, please help me understand. It wouldn't be the first time I was out in left field!
 
"Horseplay" said:
With that size fuse you may as well not even have one. That wire could melt and start a fire long before it blew. The current draw on that "feed" line will never see anything near that amperage draw, anyway. All your accessories will be getting their power from the alternator output not the trunk mounted battery (provided the engine is running). The alternator doesn't draw power so I am confused on why such a HUGE fuse. Where do you see the demand to try and accommodate so much amperage? Instead of being a safe-guard I think it might just be a hazard.

If I am missing something, please help me understand. It wouldn't be the first time I was out in left field!
Click to expand...
This is my thinking as well and why I posted the question.
On a stock 65 amp alternator the wire going to the battery side of the relay is about 12 gauge which would handle up to 30 amps of load. The rating of the alternator is a max rating and not the continuous output. That is why I am questioning why a large fuse would be needed as long as I put one in slightly larger than my max amp load.
 
"LastDeadLast" said:
Yes, that cable goes up to a distribution point under the dash where I connect the alternator (120 amp) and split off leads to the regular fuse box, the EFI ECU and fuel pump, the electric fan, A/C, headlights and driving lights.

This cable is hot all the time, so I thought it needed something.

-Shannon
Click to expand...

That design only protects the system from a short in the cable, a short in the battery, or a short in the alternator.

I understand that with a long cable from the battery to the alternator, that you want to protect it against it shorting due to wire insulation wear due to the chassis, but a good design of the cable run should preclude that. Most fuses are from the battery to your internal distribution point. Unless you're running a high amp stereo, the typical max current should be about 40-75 amps. That can be protected with a fusible link or a fuse.
 
Midlife,

Thanks for the feed back.. Not to change the subject, but let me get your thoughts on this: I do have everything coming off of my distribution point with it's own fuse.. in other words the main fuse panel has is own fuse.. the driving lights, ECU, electric fan, etc... all have their own fuse off the distribution point under the dash. The only reason I put the 175 fuse was to protect against a catastrophic failure.. like an accident.

What would you change from this setup?

Thanks for any advice,

-Shannon

"Midlife" said:
That design only protects the system from a short in the cable, a short in the battery, or a short in the alternator.

I understand that with a long cable from the battery to the alternator, that you want to protect it against it shorting due to wire insulation wear due to the chassis, but a good design of the cable run should preclude that. Most fuses are from the battery to your internal distribution point. Unless you're running a high amp stereo, the typical max current should be about 40-75 amps. That can be protected with a fusible link or a fuse.
Click to expand...
 
That's OK, so long as EVERY line coming off of the main distribution line is fused.
 
Found this table for wire gauge, ratings and length for 12 vdc
No sense in putting a larger than wire rated fuse for power distribution wire. I am looking at 15-20 foot length with the 6 ga wire I plan to run I am looking at most a 100 amp rating. I still plan to fuse low, around 30-40 amp
 
Thought I would share what I have done so far on this.
I started from front and still need to finish my setup in the trunk.
I basically ran the starter cable, main power, relay and spare wires.
I made the terminal block at work out of a piece of UHMW. I threaded a piece of steel and bolted 2 3/8" bolts through the UHMW then I put a piece of masonite for mounting and extra insulation.
 

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