Depends on which circuit you want to protect. As an example you will draw around 600 amps from the battery whilst cranking the engine. Perhaps a bit more than this, due to the resistance in the battery cable as a result of the increased length.
Remember that the fuse is protecting the wire, not the bits the wire is attached too - its pointless having a 200amp circuit breaker if the wire will burn out at 150amp
OK, so this circuit breaker is going to be between the positive terminal of the battery & everything else?
Yeah thats right, i want to use that instead of a fuse so if something goes wrong i can just reset it rather than go looking for a new fuse.
OK, well going back to my first response. If the battery supplies 600 amps to start the engine [that's the peak amperage to start my TT, so I would imagine yours would be similar, unless your compression is lower] you will require a circuit breaker LARGER than 600 amps. It won't protect much will it? If you use a lower capacity circuit breaker, it will trip when you attempt to start the engine.
I see. So when people talk about putting an inline fuse on the positive cable, how big a fuse are they talking here?
I'm not so sure of this.... The typical Z32 battery rates somewhere between 500 and 600 cold cranking amps. Now this is a measure of current supplied over thirty seconds, which is a little different to saying that the starter motor draws 600 amps. I haven't measured a Z32 but I would think a 200amp breaker would be sufficient.
Well there is one way to find out. I'm basing the 600 amp figure on what my Fluke Scopemeter tells me, but your guess is probably more accurate.
Not sure what a scopemeter is? I would normally check with an inductive amp metre. Maybe it is drawing a total 600 amps before disengaging? But there's no way it is peaking at 600. I did a quick google search and while I couldn't find anything Nissan specific I did find some Toyota published literature that quotes 175a for their 6 cylinder starter motors. Based on casual observations I'd say your typical 6cyl motor pulls 180(ish) but again I haven't checked a Z32 before so don't take that as gospel. Assuming 175-180 you still want some head room to deal with a spike so 200 or maybe 220 would be ideal.
A scopemeter is just an oscilliscope that permits the recording of electrical waveforms amongst other things. You would have seen primary & secondary ignition waveforms on a screen in mechanical workshops, a scopemeter will capture these. It will also capture glitches in electrical circuits in millisecond increments. The peak amperage reading on my Z32 was 600 amps, with a fully charged battery.
i usually just use a tong tester. a scope is not really going to give you a proper reading i would think. 600amp seems a bit high
Another idea would be to not use a automatic breaker, but rather a manual one like what I'm using http://www.supercheapauto.com.au/on...ch.aspx?pid=214456&menuFrom=70704#Description
i use a 200amp fuse. i would reccomend testing your circuit breaker before installation and try make it trip as alot are known not to trip, i purchased a stinger 200amp one and it never tripped when tested so i use a anl fuse now.
"a scope is not really going to give you a proper reading i would think" Perhaps some research is required as to what a scope is capable of.
