Technical Battery Cable Gauge Opinions Needed

Anyone have a clamp-on meter to actually measure for inrush current(amps) on a flathead or small block so we can compare?

 

Thanks for the dump truck tip. Old RV's and camper trailers usually have long extension cords for hooking up to the electricity at campsites.


I think aluminum wire is safe if you use OxGuard on the connections.

We live in one of those houses, built in 1969. After a second wall plug flared up for no apparent reason, I did a little research on the subject.

It seems aluminum forms a layer of oxidization on it's surface which causes resistance, which causes heat. A layer of OxGuard smeared on all the connections keeps them from oxidizing. We've lived here for 17 years without further incident.

In cars, aluminum's light weight may be beneficial over copper, if weight is one of your main concerns.


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Here in the Northeast, most panels are in our very damp basements, it's simply is quite risky here. Many areas have sump pumps in basements due to high watertables.

I do recall PEE WEE Herman going to the Alamo because someone told him his stolen bicycle was in the basement there...they all laughed and said there are no basements here...

 

A few things to add, make sure you measure all the twists and turns (it's never a straight shot) to get total length and the more strands in the cable (of the same gauge) the more flexible it is (and easier to break the strands with movement).

 

There's a phenomena known as 'current inrush' that exists in virtually all circuits. It's caused by the building up of the magnetic field that exists around any conductor that is carrying electricity. In most cases, the amount is very small and once the circuit is in operation and the conductor has reached magnetic 'saturation', this drops off to zero in most DC circuits and near zero in most AC circuits. The exception to this is inductive loads, such as an electric motor. This is not the same as 'locked rotor current' or 'full load amps'. You can have 'instantaneous' current of up to 1200% of FLC, and 'average' inrush of 300% is typical and is usually the figure used when sizing overcurrent protection and switching capacity (such as relays or switches). Failing to take this inrush into consideration will result in nuisance tripping and/or premature failure of switching contacts. With that said, you don't need to take inrush into consideration when figuring wire size; you use FLA (full load amps) for that.

Now, I have to say that I'm extremely skeptical that a 6V flathead starter only draws 65 amps when starting the motor. That's only slightly over 1/2 HP at 6 volts. If you were spinning the motor with all the plugs out, that I'd buy. 'Typical' full load amperage draws for most 12V automotive starters are in the 200-300 amp range, with most V8 starters reading about 250 amps (or about 4 HP). The starter on one of my 1100 CC 4-cylinder motorcycles draws 90 amps when in good repair, so it's extremely unlikely that a four-times-larger motor would take less. For a 6V starter, double these numbers, although a 6V starter operated on 12V will use less current (but still more than an actual 12V one).

So, what size wire do you need? It's all about the voltage drop, and bigger wire is always better.

Every circuit has voltage drop, and as current goes up, so does the drop. To reduce the drop, you increase the wire size. So what's an acceptable drop? Well, it depends.... LOL. The voltage drop calculator in post 25 is an excellent tool for figuring drop, but let's talk about how much is 'acceptable'. Generally speaking, the shorter the cable, the less drop you'll have and the effects of the drop won't show up as quickly. But there's a few things to keep in mind; one, battery voltage will drop under a large load. That's the battery's 'internal' voltage drop and that can vary due to battery design. A '12V' battery can drop to 10 volts under a heavy load. Two, reduced voltage will increase the current needed for the motor to do it's work. Current goes up, drop goes up, it can turn into a vicious circle. So keeping drop low is very important. If voltage gets too low, issues with the ignition system can surface.

So what's a good number? I like to keep drop at or below 3% total. In Dave's case, let's assume a 250 amp starter load, and he's going ten actual feet to the battery, and he's going to run both cables from the motor to the battery. It's VERY important you get the actual length correct, as a few feet of difference in length can make a difference.
#4 wire at 250 amps has a percentage drop of .5% per foot, so 20' of cable will result in a 10% loss.
#2 wire at 250 amps has a percentage drop of .33 per foot, so 20' of cable will lose 6.6%. 1/0 wire, you lose 4.1%, 2/0 it's now 3.3%. The last one I would consider acceptable. In reality, the #2 and 1/0 will probably work in most cases too. But consider this; what happens if you have a low battery, a hot-start issue, or a starter that's going bad? A 1 volt loss in battery voltage when using #2 wire will cause the current to climb to 277 amps, loss goes up to 7.3% and the additional loss will also increase current so now you're at nearly 300 amps. Same scenario with 2/0 wire? You'll still get the initial amp increase, but the increase due to voltage drop is now only 10 amps instead of nearly 25.

Hope this clarifies this somewhat....

 

Good point. If you don't maintain your car, then you might want to oversize the battery cables.

You can apply this principle to other areas of the car, too. If you don't maintain the brakes, you need a double master cylinder. If you don't maintain the cooling system, you need a larger radiator. If you don't check the oil, you need a deep sump oil pan.

 

I am stunned .....

not in a bad way either.

I have steadfastly held to that exact thought for decades. We can argue all year long about pros and cons of either system, and BOTH of these can cause serious incidents if there is no maintenance at all, and if the driver can't fathom what a slightly wet brake even acts like with a slight wheel cylinder leak, that may drain the system in several months of ignoring that "strange feel".

Aggressive driving, like waiting till the last second, approaching backed up traffic at a light, requires more pedal pressure than a normal stop. So, if one line pops on a rusty dual master car with that aggressive stop, your brain won't process the fact of "why ain't it stopping as good" and blam.

my biggest gripe about some inexperienced people who add a dual, have the foolish misconception that the car will stop OK in a circuit fail. Nothing is further from the truth....and worse yet, are the ones who don't bother to check fluid, because it's a dual, there is no need to worry.

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I used 00. Make sure you get a good ground from the block to the frame as well.

 

I always use welding cable on builds and my local welding supply sells it by the foot. They also will crimp the correct ends on free if you buy them there. The only problem is that all of the cables are black so you need to do something to distinguish the positive cable. I either use red electrical tape to wrap a band around it next to the fitting or slip a piece of red shrink tubeing on before they crimp it.

 

You can get the cable in red and black at an industrial battery shop. Think big electric forklift batterys. D

 

Welding cable is nice to use for jumper cables because it is flexible and easier to handle. But somebody above mentioned that because it is made up of a greater number of smaller wires that it will carry more electricity, and that is false. 00 copper cable carries the same current capacity whether it is made of fewer number of larger wires or a greater number of smaller wires. The current capacity remains the same, but flexibility is different. For a fixed installation on your car flexibility may be important for routing, or it may not be, it really depends on the car and the installation. But the idea that welding cable is always best is wrong. If you don't need the added flexibility, if you have access to 2/0 battery cable, and the cost is less, than standard 2/0 battery cable would be the better fit for the job.

 

It's more than just a maintenance issue, but stuff does fail on occasion. And going 'small' can build-in a handicap to your system. Example: with a front-mounted battery, #4 cables can be completely acceptable with only a 2% total drop at a length of 4' for both cables. This will add a bit over 5 amps to the load, assuming a 250 amp starter load, for a total of 255 amps. Move the battery to the trunk, and to be 'safe' you bump the wire size to #2. But now your drop is 6.6% and this adds another 12 amps load, now you're up to 267 amps. That 12 amps will also generate about 150 watts of heat, and heat is the enemy of electrical systems.

 

On a car that was already together I'd be looking at the voltage drop of the entire circuit to locate undersized or damaged cables and bad connections.
http://chevy.oldcarmanualproject.com/shop/1955/55csm06y24.html

This says a 12v 1955 Chevy starter could draw 75 amps running free on the bench.
http://chevy.oldcarmanualproject.com/shop/1955/55csm06y52.html

I'd expect the initial surge when starting a car to be considerably higher. Like hundreds of amps.
But if the car is in good tune it should not crank for very long, thus heating of the cable is not much of a concern.
Excessive circuit Voltage drop while cranking is a problem.

 

Hay F&j some of those hiline (power to the transformer) wires have aluminum ...
the wire is kinda stiff when ya get a bunch of strands held together...
but still usable
DISCLAIMER people DO NOT go steal or cut down hiline wires you may
get the SHOCK of your LIFE...

 

that makes sense....and a motor running free is not doing any work, you need to size the system for the motor working hard. The inrush current isn't the problem, as mentioned. It's the power it takes to turn the engine over fast enough for it to start running.

Most batteries these days provide plenty of power, and a #2 cable should be fine for a 12 foot run (10 feet positive cable, one foot at each end tied to the frame for the negative cable, but keep all the connections tight and clean!)

You'd be surprised what you can get away with starting a car. But if you go skimpy, maintenance is important. That means replacing the battery on a regular basis, too.

 

Just as a reference, my wife's 1989, Mercedes 560SL with power everything runs a #1 (50mm) cable from the trunk mounted battery (about 10ft.) to the starter and a #4 (25mm) ground strap to the uni-body close to the battery. Thats 50mm squared, can't find a way to post that.

 

50 sq mm maybe?

 

It has been posted in this Thread to run your negative Cable from the Starter/Block back to the Battery (and not to the Frame)-in reality, the Frame is really a large Buss Bar, and has far less resistance than Cable-(don't believe me? Use your DOM and measure it-it's true)-
As far as Cable itself, I work in the Heavy equipment Industry, and all we use is Welding Cable (which has it's merits), but I go one further and use a higher wire count, tinned Cable (it's Coast Guard approved, due to the fact that it is tinned)-
https://ceautoelectricsupply.com/battery-cable/

Now, I totally agree with Squirrel (it's nice to ave money!)-I am just one of those idiots that can't decide what's good enough, so I try to use the best I can afford-

 

That's not entirely true. Ultra-fine strand cable such as welding cable can withstand high currents for periods of time without damage better than 'conventional' cable, but the same voltage drop calculations per size still apply. This has as much to do with the insulation used on the cable as the number of wire strands. If you have a high-vibration environment, it's better there too.

And if comparing factory rear-mounted-battery installations, the Jaguar XJS uses a 1/0 sized cable about 12' long from the trunk to the engine compartment for the positive. This is a nice cable, with a very heavy-duty factory-molded-on battery terminal and a 5/16 eye at the engine end. They are a PITA to remove however, running under the car and covered with metal skid panels.... On the battery negative side, they use a short cable of the same size to the body at both ends.

Using aluminum cable as a weight-saving measure or for lower cost is fine, but it's a poorer conductor so you need to bump wire size up a couple of sizes to get the same voltage drop. It's also critical to have properly-done crimp connections for terminations; bolted mechanical connections are extremely susceptible to vibration and will quickly fail if they get even the slightest bit loose. It's also much less corrosion resistant, so protection of the wire stands from the elements is also critical. Aluminum cable (in sizes larger than #8) is still commonly used in electrical installations (generally because of cost); as long as very careful attention is paid to terminations, it's just as reliable as copper. Make sure you use crimps rated for aluminum cable (dual-rated ones will say 'AL - CU' on them).

 

Aluminum wire is not really designed for installation on stuff that moves...I can't bring myself to use it on a car. It has rather large strands, too.

 

I was referring to aluminum welding cable; I agree, aluminum building cable would not be a good choice. Truthfully, I wouldn't use any building wire larger than #6 or maybe #4 in an automotive application, copper or aluminum. Above that, use welding cable if the wire isn't specifically for automotive use.

 

35Window beat me to the draw on using thr frame as a ground cable. I've built many cars using that method over the years and never had a problem with it.
I think that the key is making sure that your connections to the frame are clean, corrosion free, numerous, and TIGHT.
If it's a frame off, ground up build, a good idea is to plan ahead on battery location and points forward where you to bring the ground back to body or components such as the engine and starter. Braze the head of a 3/8" SS bolt to the frame at those points, tape up the bolt threads while painting then when you build and wire, peel off the masking and you have good, corrosion resistant ground points.

 

where do you find aluminum welding cable?

 

I'd never use aluminum cable on a vehicle exposed to moisture and movement.
http://www.weldingcable.biz/welder_leads_conductor.html

I'd prefer 00 cable. Heck your only running one line back there. The other can be grounded right to the frame.
Ive run my battery at a six foot length for 10 years now grounded to the frame.

 

I just got a NEW Posie's wiring system for my 1940 Ford SBC powered car Posie specified to use "one gage + & - battery cable to front or REAR mounted Batteries, Remember to run the Negative cable directly to the starter mounting bolts" I have always run my ground to the frame as my Naval Avaiation experience all ways used the skin of the aircraft as a ground path way (hence the nick name for all AE's "one wires")
I am going to follw Ron Francis instructions this time when I rewire the coupe this winter. And I will crimp on the cable ends (copper)
Just saying...

 

geoford41,

I, too, wired my Car recently with a Ron Francis kit-while I think their products are first rate, there are a few things that they recommend that I don't agree with, and, obviously, the Battery Cable thing is one of them-
They also tried to push their grounding system (meant for Fiberglass Cars) on me (my Car is Steel, so I didn't appreciate that)-
It's great that you understand Electrical, however I would be a bit cautious of everything they try to tell you-

 

I would never make it in Sales...

 

It actually looks like this mm2, with a small 2 after the mm.

 

yeah, it's an exponent.

mm ²

 

Y'know, I have used it and seen it for sale but it's been more than a few years since then (maybe 15 or so), so it may have been an idea that didn't pan out. I used it on a large industrial job one year when I took a welder call (loved it; it was a LOT easier to drag around!) but that was the only time I personally used it. So it may not exist anymore....