Technical Changing location of Bat to trunk/ Ga. of wire?

Same here! 4/0 is way too large for what you need if you use battery or welding cable that's fine strand. Don't compare amp ratings for normal electrical cable per NEC to the ratings put on fine strand welding or battery cables. The fine strand cable is double the amperage rating that household service cable is rated for. 2/0 welding or battery cable is rated to hold 400 amps at 50 ft. and 4/0 is 500 amps. Not sure why you'd ever need 500 amps on any car?

 

That's referred to in the industry as the 'skin effect' and is a real thing. But this really doesn't come into play until you get into really high currents and/or voltages as used in utility transmission lines. When you have a generator that can pump out megawatts and the voltage is boosted for transmission purposes into the 100KV range or beyond, then it becomes a factor that has to be dealt with. At anything below 600 volts it's totally ignored. If you want to see it manifested in use, at large generating plants the connection between the generator and the transmission booster transformers is buss duct. Rather than multiple cables or large cross-section bars, this is a round hollow tube made of generally 3/8" thick aluminum anywhere from 4 to 5 feet in diameter and the power 'travels' on both the inside and outside surfaces for maximum surface area. This is usually enclosed for protection, as defects in the surface can cause 'hot spots' and possible burn-through, or so I was told by an engineer (after my crew spent a week cleaning the inside of it of any dust/dirt). If we found a dodgy weld joint they had a fit...

If you want a fountain of the purest electrical snake oil, look no further than high-end audio cables. There was a reviewer at Stereo Review years ago (since passed) that when these first showed up put $10K into an escrow account and told these guys they could have the money if they could prove their claims in a double-blind listening test, and they could pick the listeners. He got only one taker (who failed), all the rest stayed far, far away...

Sorry Bones, that's not really true. It IS all about amps. Volts only enters the process to insure the insulation can contain/withstand the voltage used. You'll be hard-pressed to find a cable that isn't rated at least to 300V, most is good for 600. It's when you go above 600V that it starts being important. Most insulation is chosen for how well it protects the wire from damage, ease of installation, and resistance to environmental conditions.

Andy, how long are those truck cables? 6 feet? 8? 12?. At 12V and high current, voltage drop rears it's head quickly. Having a very deep current well helps with that, but at some point the drop will catch up with you.

BB427, comparing the NEC amp numbers and the welding amp numbers is apples and oranges; two different applications. Voltage drop is determined by the wire resistance in ohms and the current applied. Wire resistance is the material used, cable length, and the circular mil area. A 4/0 cable is a 4/0 cable, both will be identical in voltage drop at the same current, or so close as to not matter.

Each builder has to determine their needs, and decide how much 'emergency' capacity they need. In electrical, it seldom pays to go bare minimum although many do to 'save money'. Will a 2/0 be adequate on most cars? Probably, but it would be a good idea to at least check, not guess or take advice from someone who doesn't know your build or have the knowledge. It never ceases to amaze me how many guys will spare little expense on a build then cheap out on the wiring when in terms of reliability and safety it's a critical part of the vehicle.

 

My point on wire size is demonstrated with the six volt vs the twelve volt deal. Take in 1955 and 1956 you could get the same basic engine in the Ford, the Y-block. But 1955 had six volts, and a huge battery cable. Next year, 1956 Ford went to 12 volts and the battery cable was 1/2 the size to carry the exact same amount of energy to start the exact , or almost exact same engine. The main thing that changed was the voltage! Now that does have an effect on amperage. Also if you go to 120 volts, the wires on the say 2 or 3 horsepower motors drops down to around 12 gauge vs 4 or so gauge on 12 volts and 2/0 gauge on six volts. So my observations in this old world , is as the voltage goes down…. the wire size goes up to do the same job.


It also always amazed me that I can take my 200 amp battery charger out 100 feet with a 14 gauge extension cord and it will convert that power to 200 amps DC and has to have about 2 or 4 gauge cables to get that energy from the charger to the battery! 120 volts vs 12 volts….. again.

Also , many times here on the HAMB, it has been mentioned that if you have a voltage drop on an item, increase the size of the wire!




Bones

 

2 gauge
0 gauge
Either of those will work. As you know, make sure you have a good ground connection.

 

I realize this, but it's as you said, "all about amps", so using the NEC tables is still a good source for most, as whatever cable they choose they can still compare them on those tables.
The circular mills of a cable are a great way to determine ampacity, and that's why fine strand carries so much more current vs. lower strand counts. The lower the strand count, the more gaps between strands, and those gaps add up to less current carrying capacity, and less flexibility. Unless it's a single solid conductor, with zero flex, but great current capacity.

 

Both sides of the equation need to be clean and tight, not just the ground connections. I want to say that if one cable is larger the ground cable is where to use it.

I'm not smart enough to know the answer to this - the shop manuals say for example that the voltage drop from the POS battery post to battery starter terminal should not exceed 0.5 volts during cranking. Could it pass this test with under-size gauge cables? Or is it just testing for corroded connections?

 

That's not true. The given circular mil area for any cable is the aggregate of each conductor in the cable all added together, NOT including the 'gaps'. So while a 'conventional' conductor will have larger gaps, they're also far fewer. Welding cable has small gaps, but there's a hell of lot more of them. Actual conductor cross-section is the same in either case. They also make a 'compact conductor' cable where the individual strands are no longer round but are shaped to reduce any gap to near-zero. These have the same ampacity, strand count and circular mil area as the 'standard' cable, their primary advantage is smaller physical size allowing easier installation when pulling into a raceway and sometimes allowing a reduction in conduit size. You could use building wire as battery cables (I've done it) but it's a huge PITA to install because of the difficulty in forming it. It took me two days to get from the trunk to the engine compartment when I did it. It worked flawlessly for the two years I owned the car... but I won't do it again.

Welding cable is fine-strand for the simple reason that it's far more flexible and easier to handle. The higher current ratings are recognizing that due to the relatively short-duration of applied current due to duty cycles on the machine, you can apply more current without cable damage. But that's not the real reason the large sizes exist; it's the same story here, keeping voltage drop down. Get on a construction job, and stringing 200-300 feet of cable to get where you need to be can be common. As most welding machines operate at about 90V or less, voltage drop is a real issue. Most welders left on their own will just add another length rather than re-string the whole lead when they have to move location. We would have 'roll up day' every so often (pissing off the welders) to reduce these long strings and free up cables when we ran low. It's rarely used in permanent installations because it's tough to pull into raceways because of it's soft insulation and on long pulls where pulling tension can get into the thousands of pounds some wire strands can fail.

But fine-strand large cable does have one electrical characteristic different from the coarse-strand stuff. Going back to the water analogy, if you have a hose laying out with no or low water pressure, the water will dribble out and the hose just lays there. Increase the pressure/volume and the hose can thrash around like a snake with a broken back. 'Standard' electrical cables can do the same thing, but it's generally too small to see. You may see it in a too-small battery cable when power is applied as a 'twitch'. Apply high voltage/current and it can give a damned good imitation of a out-of-control fire hose, which is why they advise everyone to stay clear of downed power lines. In a fine-strand cable this is much less, and as it was explained to me, it's because the smaller wire strands can't develop the same 'leverage' as the larger strands can. Where I've seen it used the most is for grounding 'air terminals' (AKA lightning rods before technobabble got ahold of it...) as lightning strikes can cause the regular cable to 'thrash' and possibly cause other damage. There's actually specs for how well panelboard buss bars are mounted so in the case of a high fault current they won't break loose and make internal contact.

Both cables should be the same size. And if the builder decides to run both forward, they should increase the cable size as a properly-done ground to steel frame will be as good or better than a cable.

And a too-small cable will definitely contribute to excessive voltage drop. Every place there's a break in the conductor, whether it's crimp connector, solder joint, bolted/riveted connection or relay contacts, all contribute a small amount of loss. These add up, and where any one wouldn't be enough, all together they can be.

 

Right, but how quickly does this become a factor in starting or charging or actually bust the design limits? Let's say for example everything is all nice and new and shiny and crimped properly and tightened securely. Just the cable size is the only variable.

I'm not sure where the engineers came up with it, but a 0.5 volts drop is the accepted limit for the positive side under load, when energized. This number has been around in the automotive world probably for a hundred years or more. For the ground side of a starter circuit the limit is a bit less, 0.2 volts. Same thing, it's in all the ancient texts.

Why the difference, how come ground circuits are held to a "higher standard"? Or put another way, why are the positive cables always expected to drop more voltage, on average?

Right, but I've read if there was a choice or decision as it were, to use the larger size cable on the ground side. Seems like that would sort of be in keeping with the 0.2 vs. 0.5 voltage drop limit business, I dunno.

 

I'd hate to get into a betting game with you at the pub over electrical issues. I'd end up drunk and flat ass broke.

 

I suspect that it's recognition that ground connections are easier to make. Fewer connections and no switching contacts between the battery and ground, plus probably a shorter cable path.

You'd be safe. Get a few drinks in me and I won't remember half this stuff...

 

Hey, Bones;
Thanks for the reply. I mentioned starr washers, but made note of only internal starr washers - which I think work a bit better than outside starr washers due to where they actually cut into flat metal. However, using those starr washers was - & is - predicated on a thorough slathering of SuperLube on/in/between/& then recoated heavily over, the connection. Otherwise, I really don't like them.
Somebody posted on using No-Ox-ID, = good stuff. I haven't used it on daily-driver road use, so's I can't say how it'd hold up to salt/chemical spray/soakings/etc. CopperShield is another good one, but has a couple of quirks I don't like.
On the 4.0 cable. Overkill, true. Steve has covered the reasons I'd use it since I like it. Also use 2.0 & 0.0 if the cable length is fairly short. What I've seen & found, is over time, the cables get hardened from the heat/etc, both external & internal. Not to mention corrosion has its' effects, too. So sooner or later, the resistance goes up, creating more heat, & the cycle continues 'till failure - of some sort. The heavier cable buys you easier power transfer & more time before replacement. On the buses, cable lengths(not primary, for charging &/or jumping) can go 40'. But even the short ones, 2-5', from/to alt &/or starter, are very heavy, more than 4.0. Just a typical diesel engine, same as found in semis. The TK a/c motors use ~ 4 gauge, but of course, they're not as heavy loads as a starter - although they suck power like crazy when in use. Grounds are usually the same dia as power cables. Not much of this applies, I suppose, if it's a show car, drag car, or limited usage toy, since then you can get by/away w/much lighter materials, to whatever level you want to risk.
Just my opinion, no arguments.
Marcus...

 

Gene when I put the battery behind the cab in my C-10 I used 00 weld cable. That was actually overkill ought cable would have worked just fine.

I have a battery relocation kit in the shop from Summit racing that used 0 cable for the hot and 00 cable for the ground. The ground is very important. Keep that in mind, run it as short as possible and make sure that it lands on clean metal on the chassis.

 

Again, the key thing to remember is motor loads don't react to low voltage the same as a resistive circuit. In a resistive circuit, low voltage simply means the device won't give full output; your lights will be dim or you'll have poor spark for your ignition. Because there's less voltage, current is also reduced.

In a motor circuit, low voltage will cause circuit current to increase, not drop. How much can vary; a heater fan won't go up that much because it's not direct-coupled to the load, in this case it's just moving air. But if the motor is direct-coupled via a belt drive or gear train, any changes in voltage or load can have a drastic effect on the amount of current drawn. This is reflected in the fact that most motors will have a HP rating on them if labeled for use, not the usual watts for non-motor devices. The HP rating defines how much work that motor can do. If the motor isn't labeled, you can determine HP by multiplying the full-load current by the volts to get watts, then divide that by 750. That may or may not indicate the actual HP rating, just what's needed for that application. So lets do an example...

As I said before, the 'average' V8 starter draws about 250 amps at (for this purpose) 12V. So 250 X 12 = 3000 watts at 'normal' operating conditions. Divide that by 750, the motor is developing 4 HP. Now drop voltage by 10%. It's still going to take that same 4 HP or 3000 watts to spin the motor, but now the formula is 3000 divided by 10.8 volts. Amps are now 278, a 28 amp increase. The exact same thing happens when you increase load; add 10%, watts go up to 3300, so now its 3300 divided by 12 = 275 amps, pretty much the same results. But if you're unlucky enough to have several faults, like a starter that's going away causing you to run the battery down, current can climb rapidly. As current goes up, so does the amount of voltage drop. I'll note that 4/0 cable offers a 40% reduction in cable resistance for the same length compared to 2/0. And adding just .01 Ohm in the circuit will double voltage drop here, add .1 ohm and it's ten times. This points out how critical all connections need to be good.

And for the guys that worry that the frame isn't a good enough conductor so run two cables forward, don't bother. The square inch cross-sectional area of a 2/0 wire is only a bit over .1 sq/in. A frame built with 2" X 3" .125 wall square tube and at least two crossmembers built with the same tubing has a cross section of 2.5 sq/in, or almost 25 time greater. Steel isn't as good a conductor, but it's not 25 time less, it's a bit under 10 times, giving plenty of headroom.

 

With everything that's already been said, how about a real-world cable need.

Engine is a mild 327 Chevy and the length of the cable would be around 7' depending on where the battery ends up in the trunk.

What is the optimal cable & size?

The most recent picture of the project:

 

We did ECM [Electro Chemical Machining] at work many moons ago so imagine a stack of really big cables to handle [some bigger, some smaller] 20volts from 10,000 to 30,000amps per machine. I used #1 welding cable with SBF.

 

Steve, you are right about the length, I would think in 90% of the trucks I see would have at the most, 12 feet of cable from the battery box to the starter. Usually the frame rail is used as the ground as well. Batteries will ground to the frame, then the ground lug on the end of the starter will go to the frame, the engine block ground will go to the frame, and finally the cab will ground to the frame.

 

& that's the way the grounds at a minimum should be run, if everything is in nice shape. No? Then you might want/need "floating" grounds. Done that before, but not my favorite way...
Marcus...

 

LINCOLN ls HAS A TRUNK MOUNTED BATTERY. WORKED FINE BUT REMEMBER THESE NEW style starters are gear drive and take less amps. I put the battery (walmart cheepy) and 2 guage welding cable. ran one to the engine too. Then i bought a powermaster started from Speedway. cranks the 10.5 :1 compression 430 lincoln motor very good. Make sure you solder all cable ends and terminals. do it right and it will work.

 

Do they actually draw less current, or do they have a mechanical advantage?

 

I've never looked into it, but they should draw less. With permanent magnets for the field, you only have the armature draw. I suspect the gear drive offers some mechanical advantage also.

The disadvantage is in a hard-start situation, the finite limit of the field strength limits power. A 'conventional' series-wound starter drawing more power under heavy load also increases the field strength up to saturation.

 

It looks to me from just a quick glance it's at least comparable. Gear reduction seems to be the main benefit, esp with high compression engines. Not very traditional. Jus' Sayin'.