This question is about figuring out the amps for a length of fusible link.
I am basing it on a 4" piece of dark blue fusible link that is connected to the starter relay and the wire runs to the ammeter.
This is what I have so far -
from Powerstream.com.

16g wire : 4.016 ohms (per 1000')
22 max amps for chassis wiring
3.7 max amps for power transmission

Ohm's law E/R = I - 12 volts/4.016 ohms = 2.98805 amps -or - 12 volts/.00134 ohms = 8955.2385 amps
( 4.016/1000'=.004016 - .04016/12"= .000335
.000335x4" = .00134 )


Ohms law E/I = R - 12 vots/22amps = .54545 ohms

Ohm's law IxR = E - 22 amps x 4.016 ohms = 88.352 volts -or- 22 amps x .00134 ohms = .02948 volts

I am definitly over thinkimg this. What am I missing ?

Thanks Jeff

The 16g wire has a resistance of 4.016milliohms/foot; at 4' it would be a resistance of about 16milliohms.

At zero current draw, there is no voltage drop in the fusible link. At 10A draw, you would loose 160millivolts, or 0.16V. Car off, battery would be around 12V, so you'd have 11.84V to run "stuff" -- odds are, the other wire voltage drop would also have to be factored in.

At your full tilt load of 26A, you're only looking at 0.4V drop across the fusible link.

Edit:

In operation, the fusible link should have little voltage drop across it. It does not have 12V across it. The loads in the car (outside of the starter; different wire) should measure 12V across them instead.

V=I*R, that is true. But your math is showing if you applied 12V across the fusible link, which would result in a melted wire.

OK, I still don't get the question. Are you asking how much current it will take to "blow" the fusible link? Or are you asking how much voltage drop you'll get across the fusible link? Are you thinking about replacing the link with a real breaker or fuse?

Calculating how much current it will take to blow a fuse link is very imprecise at best. Fuse links are super, super crude approximations to fuses or circuit breakers. They'll blow if there's a short circuit downstream for sure, but they won't blow at any particular amperage due to overloading the circuit. Depending on how hot it is under the hood, the current that will "blow" one can probably vary by 30% or more, and they will also burn up if they're heavily loaded for a long time so that there's to heat them up to the melting point slowly.

If you're wanting to replace a fuse link- use wire that is 4 gauge sizes smaller than the wire that you wish to protect. Use fuse link wire- DO NOT use ordinary wire with PVC or vinyl insulation unless you want toxic fumes and an underhood fire!

If you are thinking about sizing a fuse or circuit breaker to use instead of a fusible link, then its a bit trickier. In that case, I'd add up all the loads you plan to put on the protected circuit (in amps) and then size the breaker or fuse about 20% larger. But I'd also look up the rating for the wire that has to carry that load, and make sure that the "max load +20%" number is within its capability also.

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Doesn't the "drop 4 sizes down" for a fusible link assume the original wire just meets the needs of the circuit?

What do you do if wiring was replaced with heavier gauge to avoid voltage drops (headlights are an example). But you do know the amp draw.....

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

If you know the draw, I'd size a fuse for between 1.5 and 2 times the calculated maximum draw. I'd size a self-resetting breaker for 1.25 to 1.5 times the max anticipated draw.

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Thanks for the replies,
What I would like to do is replace the fusible link with a cartridge style fusible link (PAL /FLF style). The cicuit I am working on starts with the fusible link on the B terminal of the starter relay and the wire runs directly to the ammeter.
I have a fuse block which has a few PAL type fuses in it. (I am replacing the original fuse block).

The two quetions I'm trying to resolve right now are 1) what size fuse can I use to replace the fusible link? The exsisting fusible link is a 4" blue one : And 2) can I move the location of this fuse to the new fuse block ? I.E. - 8g wire from B terminal on starter relay to new PAL fuse in fuse block and from this fuse run an 8g wire to the ammeter .

Also if you look at my post - 2 alternator circuits- it may help you understand some of my confusion.

Thanks Jeff