Cable size and appropriate fuse size to protect it

[Pagetslady]

Hi what size should i use to protect 40mm2 cable I will have 120 amp alternator on my new engine.
Mike

 

[sailorman]

what are you proposing to protect

Hi what size should i use to protect 40mm2 cable I will have 120 amp alternator on my new engine.
Mike

 

[pvb]

Hi what size should i use to protect 40mm2 cable I will have 120 amp alternator on my new engine.

You'd normally fuse cable next to the battery. If there's a short circuit, batteries can deliver very high currents (hundreds of amps), leading to cable overheat and ultimately fire. Your 120A alternator isn't going to overload 40mm2 cable, and you wouldn't normally fuse the output from an alternator, just connect it to the load side of the battery fuse.

 

[Chalker]

I'd put in a 150A fuse. The 40mm2 cable will easily handle that and the alternator does not need any more.

 

[pvb]

I'd put in a 150A fuse. The 40mm2 cable will easily handle that and the alternator does not need any more.

Where would you put that fuse?

 

[myquest]

what are you proposing to protect

The cable. So in this instance the fuse would be fitted close to the battery. Then at each reduction in cable current carrying capacity a smaller fuse is incorporated to supply that cable.
The cable. So in this instance the fuse would be fitted close to the battery. Then at each reduction in cable current carrying capacity a smaller fuse is incorporated to supply that cable.
It is unlikely that a battery would deliver more that the capacity of 40mm2 cables so if they shorted out the battery would just discharge very rapidly.

 

[Chalker]

Myquest +1

 

[pvb]

The cable. So in this instance the fuse would be fitted close to the battery. Then at each reduction in cable current carrying capacity a smaller fuse is incorporated to supply that cable.
It is unlikely that a battery would deliver more that the capacity of 40mm2 cables so if they shorted out the battery would just discharge very rapidly.

It's not clear whether you're saying the cable should be fused or not - your first statement says it should, your second statement says it needn't be!

 

[David2452]

Hi what size should i use to protect 40mm2 cable I will have 120 amp alternator on my new engine.
Mike

Assuming you mean battery + out then 40mm2 cable should have a 250a M.R.B.F. 40mm2 cable is nominaly rated at 300a so that gives a good margin, using an M.R.B.F. protects every mm of the cable including the terminal. The preceding advice assumes it is the cable you wish to protect, if there is a device of a lower rating at the other end of the cable then the manufacturers fuse rating should be adhered to also. If there is a device of higher rating then you need to change the cable to suit.

 

[David2452]

It is unlikely that a battery would deliver more that the capacity of 40mm2 cables so if they shorted out the battery would just discharge very rapidly.

In a dead short situation, (the prime reason for cable protection) even a pretty average battery will be capable of producing well in exess of the 300a rating of 40mm2 conductor. Even a relatively low grade battery in a decent condition can deliver over 500a for 30 seconds (its CCA rating is always published)

 

[PCUK]

Alternator output shouldn't be fused.