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Unread 04-20-2013, 02:59 PM   #1
Stinkbug
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Sizing ANL fuse for big 3 upgrade

I am just about finished with upgrading the wires from my alternator to PDC (part of my big 3 upgrade) but have a question about sizing the ANL fuse.

• I have a 1997 TJ with a 4.0 and 117amp factory alternator.
• I have replaced the alternator to PDC wire with 4 gauge welding wire.
• I have an ANL fuse holder wired between the alternator and PDC

From what I have read, the alternator can put out between 90% and 110% of its rating. That would leave me to believe I need 130amp fuse. Locally I have hit up all the auto parts, marine parts, and audio shops only to find 100 and 150 amp fuses.

I have read of many people replacing their alternators with “high output” models, upgrade the wiring accordingly, and adding higher rated fuses with no problems.

With that in mind, can I use the 150 amp fuse without running the risk of damaging my PDC or electrical system? Will the PDC be protected by the maxi-fuses in it and thus protect the rest of the system?

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Unread 04-20-2013, 03:19 PM   #2
Jerry Bransford
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The PDC will never see but a small percentage of the alternator's available amperage, it is only the battery that would see anywhere near that many amperes and then only when the battery is severely discharged. I'd be surprised if the PDC ever consumed more than probably 40 max amps even with all the lights on. The battery & starter have their own direct connections & don't draw current through the PDC.
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Unread 04-20-2013, 03:48 PM   #3
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Yep, rarely in stock applications is the alternator putting out it's full rated amperage. Some additional lights and an amp or two however can start pulling quite a few amps. A winch will also max out you alternators output. A 100 amp should be fine. If you've got a winch go with 150.
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Unread 04-20-2013, 03:54 PM   #4
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Thanks for chiming in Jerry and Freeskier.

I guess where I get a little tripped up is that both the battery feed wire to the PDC and the output wire from the alternator both go to the same post at the back of the PDC. It makes sense that those maxi fuses in the PDC must be controlling what is actually allowed to go in to the PDC no matter what the output of the alternator.

Is it safe to say my ANL fuse (or the stock fusible link) is only to protect the wire from the alternator itself?

If that is the case my 18" or so of 4 Gauge should be OK with a 150 amp fuse right (I do have a winch)?
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Unread 04-20-2013, 04:01 PM   #5
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Quote:
Originally Posted by freeskier93 View Post
Yep, rarely in stock applications is the alternator putting out it's full rated amperage. Some additional lights and an amp or two however can start pulling quite a few amps. A winch will also max out you alternators output. A 100 amp should be fine. If you've got a winch go with 150.
He's talking about fusing the PDC. The winch doesn't connect through the PDC and a winch can draw well over 400 amps.
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Unread 04-20-2013, 04:02 PM   #6
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Quote:
Originally Posted by Stinkbug View Post
Thanks for chiming in Jerry and Freeskier.

I guess where I get a little tripped up is that both the battery feed wire to the PDC and the output wire from the alternator both go to the same post at the back of the PDC. It makes sense that those maxi fuses in the PDC must be controlling what is actually allowed to go in to the PDC no matter what the output of the alternator.

Is it safe to say my ANL fuse (or the stock fusible link) is only to protect the wire from the alternator itself?

If that is the case my 18" or so of 4 Gauge should be OK with a 150 amp fuse right (I do have a winch)?
The alternator output is controlled by the computer. It monitors the voltage and adjusts accordingly. As devices turn on and draw more amperage, voltage drops. The computer sees this drop and increases the magnetic field within the alternator which increases the voltage again.

Yes, the point of the fuse is to protect the wire. Now that I think about it 4 gauge wire isn't rated to 150 amps, you should probably go with the 100 amp fuse.
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Unread 04-20-2013, 04:03 PM   #7
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Quote:
Originally Posted by Jerry Bransford View Post
He's talking about fusing the PDC. The winch doesn't connect through the PDC and a winch can draw well over 400 amps.
A winch is still going to cause a voltage drop within the system and cause the alternator to put out more current.
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Unread 04-20-2013, 04:06 PM   #8
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Quote:
Originally Posted by freeskier93 View Post
A winch is still going to cause a voltage drop within the system and cause the alternator to put out more current.
You need to go back & study basic electrical theory a little more. Again, the winch doesn't draw current through the PDC, it is connected direcly (!) to the battery.
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Unread 04-20-2013, 04:16 PM   #9
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Quote:
Originally Posted by Jerry Bransford View Post
You need to go back & study basic electrical theory a little more. Again, the winch doesn't draw current through the PDC, it is connected direcly (!) to the battery.
So Jerry, how is it the alternator re charges the battery every time you start your vehicle? Or after a winch pull?

Your battery is also connected to the PDC. The battery and alternator are nothing more than parallel circuits and anything you add to the battery is also in parallel with the alternator, and will therefore pull power from the alternator as well. Kirchoff's laws prove this.

When the vehicle is off all power is pulled from the battery. When the vehicle is running power is pulled from the alternator, because the alternator voltage is higher than the battery current flows in the other direction to the battery recharging it.
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Unread 04-20-2013, 05:19 PM   #10
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Just to sure we are talking about the same thing, I took a quick picture... The wire coming in from the left is from the alternator. It is about 18 inches long and fused roughly in the middle with an ANL fuse holder. I terminates (as it did from the factory) on the same post as the wire from the battery that fees the PDC.

I now understand the purpose of the original fusible link that has been replaced by the ANL fuse is to protect the alternator wire, not the PDC.

I have searched the web for wire sizing charts and it seems that on such a short run (<2 feet) of 4 gauge, I am well within the limits. However, since fuses are cheaper than Jeeps, please let me know if I have misread the charts.
photo-1-.jpg  

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Unread 04-20-2013, 05:32 PM   #11
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Quote:
Originally Posted by freeskier93 View Post
Now that I think about it 4 gauge wire isn't rated to 150 amps, you should probably go with the 100 amp fuse.
An 18 inch length of 4 gauge wire (stranded) using 13.5 vdc is rated to over 300 amps. http://www.gtsparkplugs.com/WireSizeCalc.html
Quote:
Originally Posted by freeskier93 View Post

When the vehicle is off all power is pulled from the battery. When the vehicle is running power is pulled from the alternator, because the alternator voltage is higher than the battery current flows in the other direction to the battery recharging it.
It sounds like your confusing voltage with amperage.

"Amperage, or current, is a measure of the amount of electrons moving in a circuit.
Voltage is a measure of how much force those electrons are under.
In a circuit, say a light and switch in your home when the light is on there is a voltage across the filament of the bulb that is pushing amperage through the circuit.
When the switch is off there is voltage across the switch but there is no current flowing because it is "blocked" by the switch.
An analogy that normally helps to illustrate the difference between voltage and amperage:
you have a garden hose, the nozzle is closed. you've got pressure but no flow-voltage but no current (amperage). Open the nozzle and the pressure in the hose causes the water to flow - turn on the light and the voltage causes the current to flow (amperage)"

The battery supplies the bulk of the current (amperage) that the winch is going to use under load...
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Unread 04-20-2013, 05:34 PM   #12
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Quote:
Originally Posted by Knuckelhead View Post
A 5 foot length of 4 gauge wire (stranded) using 13.5 vdc is rated to around 200 amps.


It sounds like your confusing voltage with amperage.

"Amperage, or current, is a measure of the amount of electrons moving in a circuit.
Voltage is a measure of how much force those electrons are under.
In a circuit, say a light and switch in your home when the light is on there is a voltage across the filament of the bulb that is pushing amperage through the circuit.
When the switch is off there is voltage across the switch but there is no current flowing because it is "blocked" by the switch.
An analogy that normally helps to illustrate the difference between voltage and amperage:
you have a garden hose, the nozzle is closed. you've got pressure but no flow-voltage but no current (amperage). Open the nozzle and the pressure in the hose causes the water to flow - turn on the light and the voltage causes the current to flow (amperage)"

The battery supplies the bulk of the current (amperage) that the winch is going to use under load...
I'm not confusing anything. I understand voltage and current very well.
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Unread 04-20-2013, 05:46 PM   #13
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Pretty good explanation with graphics here. http://www.bcae1.com/charging.htm
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Unread 04-20-2013, 05:55 PM   #14
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So it sounds like I am good with my 150 amp fuse. Thanks guys.
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Unread 04-20-2013, 06:35 PM   #15
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Quote:
Originally Posted by freeskier93 View Post
I'm not confusing anything. I understand voltage and current very well.
Quote:
Originally Posted by freeskier93 View Post
Pretty good explanation with graphics here. http://www.bcae1.com/charging.htm
"As long as the engine is running, all of the power for the accessories is delivered by the alternator. The battery is actually a load on the charging system. The only time that the battery would supply power with the engine running is when the current capacity of the alternator is exceeded or when engine is at a very low idle." http://www.bcae1.com/charging.htm

Thanks for verifying my point...
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