I'm finally getting around to "safetyizing" the three unfused/hot wires at the battery on my 914.

First, a disclaimer: this is on a 1972 car. I think power distribution on earlier cars is similar--but I know there are major wiring changes to the later cars. Always check the appropriate wiring diagrams for your car before any starting work. Disconnecting battery "-"/"ground" is a good idea too.

The wires I'm working with are shown here:
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There are two smaller red wires crimped into the single ring terminal on the left, and a larger single wire/ring terminal on the right. As designed, all three are hot and unfused.

If I'm reading the wiring diagrams correctly, the two smaller wires go to terminals 12 and 14 on the regulator plate and provide power to heater fan, fuel pump, rear window defogger, etc. Not sure of the total load/power on this circuit...

The larger wire on the right goes forward to the power distribution/junction near the fuse box (it's the bake-lite block shown in this picture [ripped off from advman89's earlier thread]):
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I think the power distribution/junction is the tiny "32" circled in this diagram:
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From the power distribution/junction, electrical power is distributed through additional unfused/hot wires to the column/ignition switch("26"), headlight switch ("28"), fuses 10, 11, and 12, and other components. Not sure of the total load on this circuit either...

As others here have done, I'm planning to install a supplemental fuse box between the battery "+" and these three wires to give myself additional margin of safety/peace of mind...

The box I'm planning to use is a Volvo part with a 8ga cable from the battery "+" to a common bus. The box can support a total of four circuits (I'm only going to use two, one for each ring terminal). The box currently has 40 amp fuses.
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I don't think any drilling or cutting will be needed, so I will be able to put the car back into original configuration in a few minutes.

Does anyone know the total load on these circuits? Given that alternator output is only 50 amps, I figure that new 40 amp replacement fuses for the ones shown in the box should be fine...but thought I should ask the folks here.

Thanks in advance for any advice, insights, and BTDTs!

Nice. I haven't done the math so take what I say as interwebz junk backed at this moment only by a gut feeling but no data.

40A fuses may be too much for the majority of the circuits, and too little if you're going to run one lead to the starter (which i wouldn't recommend putting a fuse on). The starter alone can easily pull 100A or more when trying to crank a freezing cold motor.

Here's what's driving my opinion. Take a 14 AWG wire. A pretty safe load is up to about 9A for a section of wiring that is about 10' long. But, if the 14 AWG wire is only 3' long, you could pull about 30 Amps though it with only a 2% voltage drop. The caveat here is that you can easily withstand more than a 2% voltage drop and not melt wire but where that line is drawn is fuzzy and also depends on the temperature of the wire (hot engine compartment will increase wire resistance).

So if you have three of these 10' long 14 awg wires fed from a singular 40A fuse, it is possible that you would be able to melt one of the 14 AWG wires without blowing a 40A fuse, thereby defeating the purpose of the 40A fuse. However, if you only had each of the 3 circuits at 3' in length the 40A fuse could be an appropriate choice.

You'll have to do some rough math to figure out what current loads are and then size appropriately. Here's a website that you might find helpful to understand wire gauge vs. wire length vs. current draw with 2% of less voltage drop. I'll gladly defer to one of the EE's on the site if they have a better way to guesstimate the loads vs. fuse sizing to prevent catastrophic failure.

https://wiringproducts.com/collections/14-g...automotive-wire

Here is an online tool that you may find helpful to do the math.
https://www.wirebarn.com/Wire-Calculator-_ep_41.html

Sounds like a great idea, but just a little hesitation when I think of the quote that seems to pop up around here numerous times; "If Porsche didn't do it, then there was probably a good reason since those German engineers are more intelligent than the rest of us normal humans". OK, maybe not verbatim, but still maybe dig a bit to see if there may be any drawbacks, and no offense intended to any German engineers or anyone else.

Earlier in life, I'd have tried to measure resistive loads for all the components and come up with a max expected current draw, but now, a clamp on current meter seems to make a lot more sense. Maybe try to energize everything on the circuits in question, or as much as possible anyway, and see how much current is actually being drawn. Clamp ons are fairly cheap. Definitely want to look for peaks since you don't want fuses blowing all the time. Again, it sounds like a great idea and I'm interested in seeing how you implement it.

Dan

Rather than go through all the trouble of mounting an extra fuse box, why not just add fusible link wire at the battery? Many cars have this from the factory and it's readily available.

Sure do it the easy way!

Hey, it's laziness, not wisdom.

Dan

I've also heard it said that Porsche was crazy to use un-fused circuits directly off the battery.

I have a 73 that had four unfused circuits diectly off the battery. What a major risk. I think the red OEM wires are #10 but not certain. Now each is fused with a 10 amp fuse via a fuse block for initial testing purposes. (Being very conservative). I believe a stranded #10 can handle closer to 30 amps at shorter distances. I will bump them up if needed as I dive further into the build/testing. If I were you I would consider separating the two reds that are combined on one circuit via the post connector. Give each one it's own fuse. My 2 cents.

Curious. Does a clamp on amp meter work on DC. I had thought they only worked on AC. Thought DC needed an on line meter. Never have tried my clamp on a DC source.

Everything old is new again

http://www.914world.com/bbs2/index.php?s=&...t&p=1528715

Not every clamp meter can measure DC current. All can do AC, some can do both. The clamp meter needs to use a hall effect sensor to measure the magnetic flux of a DC current. If you are buying a new clamp meter and want it to measure DC be sure to read the specifications. There are cheap DC clamp meters on Amazon. They are good for a quick check but don't expect decimal accuracy unless you have them calibrated.

Good question. Many do only measure AC. You have to look for one that also measures DC, but they are out there.

Dan

The purpose of a fuse is to limit the current to protect the wire. No fuse turns the wire itself into a fuse if it shorts, melting the insulation and possibly causing a fire. The additional wires coming off the battery should have a fuse somewhere. Either inline or in a block. The OP should have a separate fuse for each wire and not use a single 40 amp fuse. I have the same thing going on in my car and I still have to trace where they go and if they are fused.

That grounding block was removed...wires were then connected back to the battery--where I installed a small Fuse block. I can check tonight, but I think I'm running a 20, 15 and a 10 on that. I mounted mine with VHB tape and a couple tiny screws to the firewall near the battery.

So far. So good. I feel more confident in the wiring now. The crappy bullet connectors are all gone, in favor of soldered connections and heatshrink.

...and since I have been using various diagrams, I popped for the Prospero's Garage color chart off ebay. That should be here after I have all my wiring woes sorted.

If you still have the original door switches in the car, powered by Fuse 11--do yourself a favor and eliminate them!! Guess there was a reason I couldn't find the old school switches anywhere.

Best of luck. This has been one of the "easier" projects, but it's not without a LOT of previous owner WTFs!

Shocking this car didn't burn down their barn at some point.

I've done the same thing as my original harness had some issues, so when I installed another harness i didn't want ANY un-fuzed circuits.
Installed a 1 in 4 out fuze block with 2 -25 & 2 -10 amp fuzes. hewer she is:

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Thanks for all of the replies—lots to think about as I get back to the car this weekend! I especially appreciate the pictures…