Thread: 120A alternator upgrade

I would go as big as is practical for the size stud on the back of the alternator. I would imagine 16 mm would fit ok, 25 may be getting a bit big, but if it fits, then I would use it. If you really feel a need to fit a fuse, I would use a 120 amp fuse. If your 120 amp alternator is at full load, then you may well have a problem somewhere - that's a lot of power.

Originally Posted by bacicat2000

I would go as big as is practical for the size stud on the back of the alternator. I would imagine 16 mm would fit ok, 25 may be getting a bit big, but if it fits, then I would use it. If you really feel a need to fit a fuse, I would use a 120 amp fuse. If your 120 amp alternator is at full load, then you may well have a problem somewhere - that's a lot of power.

Thanks - I'll have a look at what will physically fit when it arrives. I'd prefer to go heavier gauge rather than too light.

To be honest, I'm not 100% sold on the fuse idea. From what I've read online, a fuse can protect against shorted diodes in the alternator itself - but I don't know how real that risk is.
I've got a winch fitted, so when that's in use I'd expect to be using the full output of the alternator.

Steve

The trouble with fuses under a load near their rating is that there is a considerable voltage loss across it (could be around 0.5-1V). I've had fuses fail with vibration at near their rated current as the fusible bit is heated and slightly soft. I wouldn't run a fuse under say 160A in that application as its prime duty is to protect the battery cables if the alternator shorts internally. With a 16mm2 or 25mm2 cable you could easily run a 200A fuse and it would protect the cable. Perhaps two 100A maxi fuses side by side, a wreckers should have dozens of them at hand. Or if you prefer run a circuit breaker, at least you can reset it.

I like the idea of 2 maxi fuses as it would also mean I could run 2 smaller diameter cables which would be easier to route than one large one.
I've seen a few melted maxi fuse holders on 12v compressors, and with much smaller fuses than 110A. Whats the best source for decent quality holders?

Steve

Originally Posted by steveG

I've finally ordered a decent 120A alternator to replace the original 40A one on my County.
Obviously there's a need to upgrade the output wire from the alternator, but I'm confused as to what size cable to use. I've tried 3 different online cable size calculators but if I put in 12v, 120A and 1m length they all return different recommendations - from ~8mm2 to ~25mm2.
What size should I use?

For a 2m run from alternator, 8mm2 cable is way too light - it'll vapourize at full load. Alternator that size, I'd be running #4 gauge cable - interwebs tells me that it is around 20mm2 in local currency. That'll happily take close to 200A without problem. You can of course go heavier, but as you're no doubt aware, you're going to hit the physical limitations of what size terminals or rings you can use to connect it to the alternator.

I'm thinking I should also install some sort of fuse at the alternator end. An ANL style fuse in a holder seems to be the best option I've found so far.
What size should the fuse be - 120A? Or does it need to be slightly above that so it doesn't blow when the alternator is at full output?

Go 120A or slightly lower - the fuse is there only to melt before anything else does if there's a huge draw on the alternator. Something high enough to not get fried by the alternator, low enough to not melt before the wire does. The ANL fuse holders are tidy, so why not.

Where'd you find a 120A alternator with a vacuum pump, by the way?


[edit]misread the initial values in the OP - updated accordingly[/edit]

Thanks Mike.
Alternator was through ebay, but its decent quality with 2 year local warranty rather than a $150 Chinese special.

Steve

Originally Posted by steveG

Thanks Mike.
Alternator was through ebay, but its decent quality with 2 year local warranty rather than a $150 Chinese special.

Steve

Any chance of a link for future reference???

Hi Steve and because you are running a winch, go for the thickest cable you can fit.

The thinest cable you can safely use with a 120 amp alternator is 4B&S ( 20mm2 )

The thicker you go, the less voltage drop you will have while winching.

As far as the size of the stud on the alternator goes, you can use any size cable eye terminal and if there is a restriction because of a limited clearance area on the back of the alternator, you can always file the edges of a larger terminal down till it fits.

The size of the fuse will be governed by the thickness of the cable, for example, 4B&S ( 20mm2 ) = 120 amp fuse, 3B&S ( 25mm2 ) 150 amp fuse, and 2B&S ( 32mm2 ) = 200 amp fuse.

Don't forget that you will probably need to up size your earth cables.