Main Power Feed (fuse)

[Barron]

question for electrical minded people on the forum.

Got the battery in the boot of the 172 track car. This also features an isolator switch with a fuse box built onto it. Until recently the car has been fine with a 125a megafuse (blow rating of 250a). Blown two of these fuses while starting the car in the past two days so replaced it with a 150a megafuse (blow rating of 300a)

what size fuse should i be using?

carl

 

[Barron]

cables gauge is 25mm²

 

[Dink]

i wouldn't put anything more than about 100A fuse on a 25mm2 cable tbh

what i don't get is if a fuse has a current rating but doesn't operate til twice that current

 

[Barron]

150a seems to be working fine

 

[Dink]

yeah it will work fine up until the point at which the cable melts and sets fire to the car

 

[Barron]

haha, its had a 125a in for the past year

 

[Jay Payne]

Maybe its your starter motor drawing to much current and blowing the fuse

 

[Barron]

Maybe its your starter motor drawing to much current and blowing the fuse

yeah i did think that, i honestly have no idea how to test it so going to get somebody i know to check it for me. Its a very possible idea as the starter motor has only been fitted for a couple of weeks.

carl

 

[agithegreat]

Starter motor drawing too much current for that length od cable. so you should uprate the cable to 50mm^2 then fuse it at 200A

 

[Barron]

so running a 150a on the current width wire is dangerous?

 

[agithegreat]

so running a 150a on the current width wire is dangerous?

Yes!

Ideally over 12' you shouldnt run any more than 80 amps off 25mm^2. BUT for a shorter length 9' then it's about 100a.
However 50mm^2 you're talking 250a at least over 12'.
I draw a nice 180a off 25mm^2 but the cable is only 2' long. And it gets warm. Will soon be upgraded to 50mm^2 though!

 

[geesus69]

this has me slightly worried, i'm going to check what rating my isolator is lol

 

[agithegreat]

Take a little read here... i know its for audio but the principle is exactly the same for current draw!
http://www.the12volt.com/info/recwirsz.asp

 

[Barron]

Yes!

Ideally over 12' you shouldnt run any more than 80 amps off 25mm^2. BUT for a shorter length 9' then it's about 100a.
However 50mm^2 you're talking 250a at least over 12'.
I draw a nice 180a off 25mm^2 but the cable is only 2' long. And it gets warm. Will soon be upgraded to 50mm^2 though!

but the starting of the car is not a constant supply. Its just a spike so surely it cant be that bad? (hopes) lol

 

[agithegreat]

Regardless of spikes or constant it's still current draw?
All i'm saying is that 25mm^2 cable over any length should not be fused greater than 100amp.
And that 50mm^2 can be fused at upto 250amp.
I see what you're saying about it's only a burst. BUT if you fuse said wire at 200amp (to allow the 150amp spike or so to pass). Then if a short were to occur the fuse would not necesserily protect the cable. THUS defeating the object of said fuse being on said cable.

 

[wallop]

Been a few threads about relocating batteries recently, also seems that no one has bothered to check out the safety side of things....

Which fuse was blowing???

The one protecting the cable from the battery, (in the boot), or the one protecting the cable from the alternator, (mounted in the engine bay).....

To use 4awg cable as a starter motor power supply over a short distance is fine, but to use 4awg over the length of a car is rather counter productive.....

4awg cable at 12' in length will drop around 0.5v at an 80amp draw, your starter is pulling way in excess of that even with warm oil and warm ambient temperatures.....Factor in colder winter temperatures, cold engine oil, higher cable resistance etc and the extra strain your placing on your starter circuit could lead to premature component failure.....

The points made in previous posts are quite valid, using a cable that isn't upto the job can produce problems, but to then simply exceed the cables fuse rating is simply stupid.....It may last 10 years, it may spontaneously combust next time you start the car....Either way it could affect any future ins claims in the event of fire or accident induced fire, Your choice though mate.....

Relevant thread

 

[Barron]

the fuse that keeps blowing is the fuse which is located approx 20 cm away from the battery.

this setup was done by angelworks tuning so im disapointed if they have used the incorrect cable :S

ta
carl

 

[Barron]

how it looks now, mega fuse is located in the black box which is connected to the isolator

 

[wallop]

I understand your problem mate, what is considered adequate by me may well be considered OTT to someone else....

Let me put it this way, if it was my car, I would use 0awg, if it was a customers car, I would use 0awg unless instructed to use 4awg to save the customer money, (in which case I would refuse the work on principle)....

The simple facts are, you've suffered two fuse failures, this would indicate your pulling upto 300amps through a cable rated at much less....

You could always pop over to TA and request advice from them....After all, I may be totally wrong....

 

[Dink]

i'm glad you actually realised that it isn't right.