Big 3 cable upgrade crashes into ohm's law

DSinOR

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Hello, I found this board while searching for e-fan ideas for my 1996 yukon 5.7L Vortec. Great board!

I saw a few threads about the Big 3 cable upgrade: where you replace factory cables for battery+ to alternator and starter, and battery- to engine, frame, and chassis.

The idea is that fatter cables are less resistive and therefore more efficient, so fatter cables are better.

Consider this:
Copper wire is efficient.
Fatter wire (solid, stranded, or fine stranded) is more efficient than thinner wire.
BUT - fatter wire only delivers real benefits when the conductor length is long and/or the load is large.

When the runs are short and the loads are small, the incremental benefit of fatter wire is small.

This concept is best demonstrated by a simple chart:

You must be registered for see images attach


I made this chart using the free calculator at:
https://photovoltaic-software.com/solar-tools/dc-ac-drop-voltage-calculator

You can confirm the voltage drops using different calculators:
https://www.calculator.net/voltage-drop-calculator.html
or
https://grealpha.com/resources/dc-load-wiring-calculator/calc/voltage-drop/
etc.

I made the chart for 14vdc at 150°F, for 1 meter (39 inches) of wire.
The relationships are all predominantly linear.
When temperature decreases, conductive efficiency increases slightly, and vice versa.
When conductor length decreases or increases, conductive efficiency is linearly affected. Example: the power loss in a 13" wire will be 1/3 of the loss in the same wire 39" long, and the power loss in a 117" wire will be triple the loss in the same wire 39" long, all else the same.

Importantly, 1 meter of any size copper wire is not resistive. Because it is short.
A single meter of skinny 20g wire only has three hundredths of 1 ohm of resistance, and 1 ohm is nothing.

But, load matters. The higher the load, the more the resistance of the conductor matters in terms of power loss caused by resistance to current.

I made the chart more expansive than it needs to be, in order to show the big picture.
When your wire is too small for your load, a small increase in wire size makes a big improvement in power loss.
However, when your wire is suitable for your load, a small increase in wire size makes almost no improvement in power loss.

Using the '96 Yukon as an example:

Example 1:
The factory cable from battery+ to starter motor is 2awg copper cable.
Assuming this cable is 1 meter long, and assuming a 50 amp load, the power loss caused by the 2awg cable size is 2.9 watts, or less than 1% of the total wattage, which means that the cable is more than 99% efficient for that application.
If you change that 1 meter cable from 2awg to 2/0awg, and run the same 50amp load, the power loss associated with the new fatter 2/0 cable will be 1.4 watts, an even smaller fraction of 1% of the total load, still greater than 99% efficient.
If you pay $40 to change the factory 2awg cable to a fatter 2/0awg cable, then you have spent $40 to change a cable that is more than 99% efficient, to a cable that is more than 99% efficient.

Assuming a 100amp load and everything else the same, the power loss on the 2awg cable is 11.5 watts or 1% of the load, and the power loss on a 2/0 cable would be 5.7 watts and 0.5% of the load.
So the fatter 2/0 cable, at 100 amps steady draw, would change your system from 99% efficient to 99.5% efficient.

Example 2:
The factory cable from battery+ to the driver's side fuse block is 6awg copper cable.
Assuming this cable is 1 meter long, and assuming a 50 amp load, the power loss caused by the 6awg cable size is 6.3 watts, or 1% of the total wattage, which means that the cable is 99% efficient.
If you change that 1 meter cable to 2awg, and run the same 50amp load, the power loss associated with the new fatter 2awg cable will be 2.9 watts, or about a half of a percent.
So, by changing to the fatter 2awg cable, you have gone from 99% efficient to 99.5% efficient.

This cable never sees 100 amps, but you can do the same comparison at 100amps if you want to, and you will see that changing to a fatter wire makes very little real difference within a reasonable range of operating specifications.

Example 3:
The factory cable from battery+ to the alternator is 6awg copper cable.
Assuming this cable is 0.66 meter long, and assuming a 50 amp load, the power loss caused by the 6awg cable size is 4.2 watts (0.66 of 6.3 watts from the chart), which is less than 1% of the total wattage, which means that the cable is more than 99% efficient.
If you change that 0.66 meter 6awg cable to a 2/0awg huge cable, and run the same 50amp load, the power loss associated with the new fatter 2/0awg cable will be 0.9 watts (0.66 of 1.4 watts), or way less than 1% of the total wattage, meaning that the new fatter cable is more than 99% efficient.

So, by changing the battery+ to alternator cable from 6awg to 2/0awg, you change the cable efficiency from greater than 99% to greater than 99%.

By now, you know how to use the table to compare 6awg to 2/0awg batt+toAlt cable at 100 amps.
6awg = 25.1watt loss x 0.66 = 16.6watt loss = 1.3% loss = 98.7% efficient at 100 amps.
2/0awg = 5.7watt loss x 0.66 = 3.8watt loss = 0.4% loss = 99.6% efficient at 100 amps,
which seems like Hey, maybe I want that 1% increase in efficiency, but you have to remember that you only get that 1% improvement when you make a massive jump in cable size AND your automobile is so screwed up that your alternator is constantly pushing 100 amps at your battery, in which case you will soon have a fire and you don't have to sweat that 1% anymore because your car burned down.

*******

The same basic concepts apply to ground cables (negative side cables).
Your ground cables need to be matched to your load for each and all circuits, but after that, small changes in ground cable size produce the same small changes in power transmission efficiency.

*******

Sooooooo,
If you buy a $200 Big-3-4-5-6 cable kit that has fatter wires for everything, you are changing the factory wiring from 99+% efficient to 99+% efficient.

That's it.

Additional comments:
1 - The chart above displays linear relationships that are inescapable. If you think you don't understand electronics, study the relationships for a while and you will see that physics correlates with common sense: nothing is free. There is no magic.

2 - The 1990's engineering capability at general motors company was not "poor". They did not design and build and sell trucks that had puny inadequate wiring.

3 - Temperature, and fine vs. coarse strands, and alloys, and lug connections, all matter.
A little.
Only a little.
The linear relationships of ohms law applied to conductive materials do not change just because you want them to.
If you have a really crappy dirty loose greasy crimp on a lug, it will cause problems. Clean up the connection, and the problems will go away. You don't need a 2/0awg wire for a 2awg job. Period.

4 - The OBS family is susceptible to problems caused by deteriorating ground connections. Find every ground connection and take it apart and clean all the surfaces. If the cable strap is cracked or corroded, replace it with a similar gauge of wire. A good ground is a good ground. You don't have to triple the wire size to maintain a good ground. A huge wire connected to a rusty greasy ground will be a crappy ground. Oil and dirt are not conductive, at all.

5 - There is such a thing as a ground loop. If you mod your harness like a madman, and then discover that you have voodoo electrical issues, do some research on a ground loop. Maybe you installed too many ground connections and made a weird loop. It happens.

6 - If your cables are old and the lug crimps are dirty and bent over and stressed out, then they will be more resistive than when new. Fix and clean them, or replace them, but don't think that tripling the cable size will create magic. Your problem was dirty bent-over stressed connections, not too-skinny wire.

7 - If you install a 500 amp alternator and a 2000 watt music amplifier in a car, you will need to use fatter wires everywhere that matters. Common sense.
But if you change a total nominal 50 amp load to a total nominal 60 amp load and you have relays and fuses that can handle startup spikes, you don't have to change any wires.

2 cents.

:)

Thx.

DS
 
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GoToGuy

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I did offer to post the charts and graphs we for Aircraft. It was fun reading about these "kits". It would seem a little knowledge was more dangerous to the wallet. Well done. Although i can see alot of glazed over eyeballs trying to comprehend the sacred "Ohms Law". Good job!
 

R422b

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But but but.......
[emoji1787] I like factory cables. I replaced the starter power cable on my 2002 tahoe with AC Delco.

Talking about GMT400s using Tapatalk
 

Erik the Awful

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I, too, believe that the Big 3 mod is overkill on wire size, but when you account for the fact that most people want a plug-and-play kit to put on their truck and forget about for the next twenty years, it's probably appropriately sized to account for deterioration of the wiring over time.

The 1990's engineering capability at general motors company was not "poor". They did not design and build and sell trucks that had puny inadequate wiring.
They sold trucks that had adequate wiring when new. After a couple years of service the wiring degrades to inadequacy. The Big 3 gives you future-proofing that considers aging.

There are probably bigger fish to fry out there. You might be right, but I'd be a bit more careful about throwing dirt on my first post, especially if this is someone's side gig. We don't need a flame war.
 

thegawd

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I see the above post and think, "holy **** bud your over thinking it." if were talking 2 and 3 foot wires that have been home rolled to superior standards, theres no doubt in my mind that they are MUCH better than what GM installed. the idea is not to make it work at the bare minimum but for it to last forever, that's my goal anyways. my positive leads are OEM, I have even replaced that entire lead with an ac delco on my sierra, from the battery to alternator n starter.

BUT explain gms use of the Piece Of **** braided ground wires? because they are NOT adequate and DO NOT match the battery cables like you describe. I agree gm made these electrical systems work, for a while. My grounds are 4 gauge insulated wire.

what do you think about gm's use of soy based insulated wires that rodents cant get enough of? people are guna be wiring up there vehicles twice because of this if it dosent burn to the ground. LMAO.

Al
 
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sewlow

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The factory wiring in these trucks is now 23 to 33 years old.
...in an environment that is not exactly conducive to longevity.
GM designed & installed the basic minimum wiring to run the factory installed equipment & accessories. Nothing more.
It's about the bottom line. Pennies per unit saved = hundreds of thousands of $$$ in the long run.
Start adding some aftermarket accessories to an aged electrical system & the gremlins will rear their heads.
These trucks do funky-weird **** with compromised grounds/power cables, many of which can appear to be unrelated to such.
 

Schurkey

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Hello, I found this board while searching for e-fan ideas for my 1996 yukon 5.7L Vortec. Great board!...
That's a first-post for the ages. Detailed, visual aids, descriptive, and with outside references to allow independent confirmation. Congrats!

Using the '96 Yukon as an example:

Example 1:
The factory cable from battery+ to starter motor is 2awg copper cable.
Sure about that?

I dicked with the battery cables on my '97 K2500 last summer. I'm thinking they were more like 4-gauge than 2-gauge.

But I've got no service history on this truck--maybe someone replaced the OEM cables already.

This is timely, 'cause just a day or two ago I shot my mouth off about 4-gauge cables being fine, with 2-gauge being more than adequate. But if the OEMs are using 2-gauge, I'd better look into the load they're supporting. Maybe it's higher than I'm expecting.

2 - The 1990's engineering capability at general motors company was not "poor". They did not design and build and sell trucks that had puny inadequate wiring.
I was pretty unimpressed with GM wire-harness gauge size when I was measuring voltage-drop in the fuel pump circuit of my two Luminas. They measured the same at every test-point.

I'd measure a tenth-of-a-volt drop everywhere I tested. The total drop from alternator + to the last access point before the wires went up and over the tank, was a full two volts on the supply side, with another volt on the ground side.

The alternator was producing 14.2 volts, the fuel pump was running on 12.2 on the supply side, with another volt lost on the ground side--total fuel pump voltage was down to 11. I did some digging into gauge size vs. power draw. GM was using "16 gauge" (Metric equivalent) wire on a circuit that industry standards would call for 12-gauge wire.

Rat Bastages!

OTOH, either car has enough fuel to hit the 112 mph speed limiter programmed into the computer.

BUT explain gms use of the Piece Of **** braided ground wires? because they are NOT adequate and DO NOT match the battery cables like you describe. I agree gm made these electrical systems work, for a while. My grounds are 4 gauge insulated wire.
You talking about those ground straps from cowl to engine, or maybe from body to frame? Looks like about 1/4" wide?
GM uses piece-of-**** braided ground wires because the electrical load is so tiny, AND those wires are larger than they appear.

The primary ground wire is from the battery - to engine; and the only reason it's huge is because of the starter motor.

A 4-gauge ground from the firewall to the back of the head is exactly the sort of enormous that the original poster warned about. There's no harm to it except to your wallet. There's also no benefit to it. Same with the braided ground strap between body and frame, and any other place they used those thin braided straps. There's just not that much load on 'em.

what do you think about gm's use of soy based insulated wires that rodents cant get enough of? people are guna be wiring up there vehicles twice because of this if it dosent burn to the ground. LMAO.

Al
I have a very ratty '66 Toronado. Got tired of the windows not going up 'n' down. Pulled both door panels off, re-wired all the stuff going through the door hinge area. Had to go way up under the dash to get rid of all the green copper so I could solder onto copper-colored copper.

I crimped and soldered everything. Got rid of a small mountain of hard-insulation, green copper in favor of whatever brand new, supple wire was on the rack at my local NAPA. Windows would go up and down real nice. The lights in the doors were bright. I'm real pleased.

Come out to the car about six weeks later, start the engine...and I see sparks under the dash near the park-brake area. Friggin' squirrels had eaten all the nice supple insulation off the new wires, left the old wires alone.

It's not just GM's wire insulation.

inrush currents for starter motors can hit 300-400 Amps. Upsizing conductors never hurts in power system design.
Who cares about inrush current? The inrush current doesn't last one second. Doesn't last long enough to warm the cable or cause any issues.

By the time the ammeter can get a reading on it, it's ~200 amps or below. Some big-block Olds and Caddy might go to 225 amps. The newer mini-starters are lower than that--150-ish.

Worst-case would be a winter situation, where the oil is thick so the engine is real hard to crank. But there's still little concern about the amperage heating the wires because
1. Wire temperature starts off as cold as everything else, a cable at -30F can have considerable temperature rise before the insulation is in any danger.
2. The wire is somewhat more efficient when cold, and
3. The starter is likely to melt before there's any danger to the cable itself.



Short story:
1. There's no reason to go nuts with gauge sizing, but in select applications increasing the gauge size can be beneficial, especially if you're adding load the engineers didn't account for--crazy stereo amplifiers, more-powerful alternators, etc.
2. The voltage-drop test tells all. If your voltage drop across a loaded circuit is acceptable, upgrading the gauge size does nothing useful. The trick is to assure that the circuit being tested is appropriately loaded when the test is performed.
 

thegawd

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those ground straps turn to dust. that's my complaint. haha. so no harm no foul with using larger than necessary wires, I had these on hand from lawnmowers. I cant believe the grounding straps on lawnmowers are much higher quality than gm's. I also took these apart and soldered them up with double wall heat shrink wraps. I have no idea how long one of those braided wires will actually last but it goes against everything I know about wiring.... exposed to everything that it is and being copper bolted to steel its galvanic reaction is probably actually what destroys them. everyone of them that I tugged on in my burb split in 2 with little resistance. I replaced them before they left us stranded. it didn't cost me anything but some time, solder and shrink wraps.

Soy based wiring! is there any labels stateing such? its so far from green tech and rather a by product from another DIRTY industry. my house is surrounded by soy beans and we're I mean my family and I are organic growers. lol. can you picture the next gen electric vehicles wired up with it? I picture exploding cars just like the exploding lithium ion battery in cheap vapes.... lol.

I know GM just started using the soy wiring with the 2019? model year.... whenever they stopped providing a transmission dipstick.

I'm definitely not an expert, I'm just trying to get educated in what I like or need to know. and also educated in facts of why I wont buy a new vehicle. haha.

Cheers!

Al
 
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TheAutumnWind

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Great post!

No kill like overkill. I have a completely unnecessary big-3 on my truck. I'll likely do the starter wire soon too. TBH though for me its just about replacing 30 year old wiring with new, and I can make my own cables for cheap. 1/0 and 4 awg 100% OFC welding cable is less expensive than buying premade CCA cables.

That being said the wiring for the headlights leaves a lot to be desired. A relay setup is a must IMHO.
 
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