new alternator question....

[VICSVX]

i bought a 160A alternator from manicac electric motors and the site recomends a 100A fuse inline with the 12V power cable....

well i remember seeing the pix on their site here on the network and the pic is from the alternator mod and from that this thread it says use a 100A fuse for a 95A alternator....so what does that mean for me? get a biggr fuse or use a 100A one?

[LetItSnow]

With all these alternator upgrades and wiring mods, there is a serious lack of understanding of the fundamentals of DC circuits!

Before futzing with electrical stuffs, consider that electronics overrun with current CAN catch fire, which means that if you make mistakes with your car's wiring, your CAR can catch fire. This is a picture of a car that lit up due to an electrical problem:


Consider what the fuse's purpose is? If your max amperage from the alternator is 160A, and you use a fuse that's rated for 160A, what's the point? You may as well use a wire that can handle that current - it's the same thing!

You're protecting the devices you're powering from excessive output from the alternator, and protecting the wiring from damage that can be caused by a short. The fuses should be rated per the car's maximum demand, not the power source's capacity.

[BoxerFanatic]

That makes more sense, of course.

Fuse ratings based on circuit demand, rather than current supply. The current supply is not what needs protection, rather what needs to be protected from.

But then, what is the methodology for determining maximum safe demand, is it as simple as summing the current draw fuse ratings for all of the downstream circuits in the fusebox? Each of those circuit branches should be individually protected appropriately already, right?

Or is there some other method of determining what the main bus fuse should be? should it stay at the stock amp rating, (probably should remain close, anyway)

Should it be rated at or just below the total of the down-stream circuit fuse ratings... it also kind of goes back to the charging circuit modifications that others were talking about before, and how to protect the main bus to the fuse box, but also provide a good amount of current to keep the battery charged, and to prevent brown-outs under high demand from several of the car's circuits...

It is interesting stuff.

[kwren]

To make a long story short, use a 100 amp fuse for the alternator output circuit.

Keith

[VICSVX]

all that really makes a lot of sense!

thnx!

i wont get it in till monday but i'll b sure to get a 100A fuse. I really appreiciate all that info

[crazyhorse]

This intruiges me...What IS the max draw on a stock SVX?

If 100A is the maximum reccomendation, then why on earth did FHI install a paltry 95A alternator? I know that the likleyhood of every single circuit being in operation at once is extremely remote. However, on the off chance that it did happen, how much juice would it all pull?

I'm just a stickler to engineer for the worst case scenario, not the 95th percentile.

[LetItSnow]

Originally Posted by kwren

To make a long story short, use a 100 amp fuse for the alternator output circuit.

I'm sure Keith can make all clear as to why a 100A fuse is the perfect choice!

[Trevor]

A fuse is inserted within a circuit to protect the SUPPLY side of the circuit against short circuit or a dangerous overload. The fuse is rated so as to withstand any normal current within that SUPPLY circuit.

Within a distribution network, fuses are rated in accordance with the current capacity of the CONDUCTORS between the load and the point of SUPPLY, i.e. before/behind the fuse. The last point in protection being rated in accordance with the source of the supply. This last level of protection prevents the source being overloaded to the point that a dangerous level of heat could be produced and the source severely damaged.

In the situation under discussion, the object of alternator fuse, placed in the SUPPLY circuit from the alternator, is to prevent THE ALTERNATOR or connecting conductor, becoming overloaded beyond a safe maximum, at which point it would overheat to a dangerous level. It is the alternator maximum safe output which should fix the fuse rating, rather than the load, which is backed up via the battery. The distribution circuit fuse protection, protects the battery against overload.

A separate alternator fuse is required with the suggested “up grade”, due to the increased conductor size which is capable of imposing a greater load on the alternator, outside of the normal protection via the fusible link. In all respects this fuse is an improvement provided that the fuse is rated within the maximum output of the alternator.

Within the circuit here discussed, there is a second level of protection required, in the form of a possible back feed from the battery, which will become the point of SUPPLY in the event of a short circuit within the alternator or the connecting cable. This contingency would normally be taken care of via the fusible link, but the “upgrade” as has been specified, involves a connection direct to the battery, i.e. in this mode of protection, the source of SUPPLY.

In all of this, a conservative approach in respect of the fuse rating is warranted, but possible nuisance opening of the fuse, should be considered. In this respect, a fuse is unlikely to open unless in excess of 200% of the rated value is applied, for a time exceeding what could be a normal spike in the load.

[crazyhorse]

So, the fuse in the alt. wire is to protect the alt from an over draw situation. Say a short in the starting circuit. That's one of the few circuits that could hold the current needed (by itself)to fry the alt ,without melting down itself.

[Trevor]

Originally Posted by crazyhorse

So, the fuse in the alt. wire is to protect the alt from an over draw situation. Say a short in the starting circuit. That's one of the few circuits that could hold the current needed (by itself)to fry the alt ,without melting down itself.

NO. The alternator, as a point of SUPPLY, is protected against overload via the fuse board distribution fuses.

A short in the starting circuit will draw energy only direct from the battery. Importantly the battery is not protected against a short circuit within the starter, but this is under the control of the driver via the ignition switch and starter solenoid, and is therefore presumably is an accepted hazard.

If the battery was flat the car would not start and the alternator would not be in circuit. The charging circuit is completed only once the alternator produces energy, because the rectifier network blocks any reverse voltage. A cut out contactor was required within the old generator systems.

[VICSVX]

so i looked at the updated wireing for the alt. upgrade and....im confused as heck! alternator mod

has anyone in tucson area done it with success and wouldnt mind helping me out....Dayle?