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I just want to run this past the forum to make sure I'm not undersizing or going overboard with my wiring. I'm putting a LM7 into a 1971 Monte Carlo, it has the 145a alternator. The battery is on the passenger's side, as is the alternator. The computer is on the driver's side firewall behind the fender, and the fusebox (BP Automotive DIY kit, no fan/starter/AC relays) for the computer is on the driver's side core support next to the horn relay (original junction point for the charging system). No AC at this time and running a mechanical fan. I'm using the stock LM7 battery cable from the battery to starter.
I'm planning on running 4awg from the alternator to a junction block on the firewall, probably near the brake booster. From there, I'll run the following:
8awg to the starter
8awg to the horn relay or another junction block which will power the standalone fusebox and any later accessories like headlight or fan relays
12awg to the stock fusebox (originally connected to the horn relay). I may upgrade this to 10awg in anticipation of running a newer head unit and gauge cluster.
I may also run a relay for the starter solenoid from here rather than using the 12awg wire running through the stock NSS.
I'm not running a large cable from the alternator to the battery due to the articles about the charging system from MADElectrical. I did try calling Mark to ask about wire sizing for alternators and systems with a higher demand than what he has listed. He felt that the computer was too far away from its fusebox , then went off on a lengthy tangent about the power requirements for older starter solenoids and I couldn't get the conversation back on track. I'll run a fusible link right off the alternator and between the junction block and original fusebox.
Am I on the right track here? Most of the posts that I've seen are for trunk-mounted batteries or large audio systems. I would certainly prefer to run 8-10awg for everything, but not if it's going to be inadequate.
Last edited by blackhawkMC; 08-14-2018 at 12:40 PM.
Reason: Added battery to starter cable information
Alternator goes to battery, not a fuse box. It should be around 4ga. The starter wire also needs to be AT LEAST 4ga. You know how batteries are rated in cold cranking amps? That means exactly what it says. Now do you really think an 8ga wire can handle 700amps+? I think you should look up some wiring diagrams before you do anything, you clearly have a lot to learn before you do any of this.
I should clarify, I have 4awg minimum running from the battery to the starter using the LM7 battery cable. I'm aware of the large power requirements when starting the vehicle.
There’s not a one-size-fits-all (pun intended) when it comes to wire sizing. Location of components and use of the vehicle can alter design choices. If the vehicle is used regularly for long highway runs - overcharging the battery can be a concern. However, if it will see a lot of short stops and starts - you may wish you’d picked a larger charging wire size. Use the available load/distance charts to make choices. I use the 145A alt as well (A/C, electric fans) - system outputs in the 14.4-14.8V range. Everything seems to work better. My battery is 16’-18’ away from starter and 20’ from alt. I made the big wire choice - working perfectly.
So if I'm reading this right, you have have a new fuse/relay box as well as you stock fusebox. If so, what's going to each fusebox and what wire are you feeding the new relay box with?
Aside from that, I would go 4ga to the starter as you already noted.
I think 4ga to the alternator is overkill, but it won't hurt.
The real key is where you place the voltage sense wire to the alternator/computer (not sure which one is regulating in your case). Don't connect the sense wire near the battery. Put is near the computer.
My battery is 16’-18’ away from starter and 20’ from alt. I made the big wire choice - working perfectly.
Looks like an E30 in you photo. LOL I'm guessing you're using the original wire that supported the BMW rear battery install. That is a huge gauge wire and is exactly why I use one in my rear battery Turbo Vette project.
So if I'm reading this right, you have have a new fuse/relay box as well as you stock fusebox. If so, what's going to each fusebox and what wire are you feeding the new relay box with?
Aside from that, I would go 4ga to the starter as you already noted.
I think 4ga to the alternator is overkill, but it won't hurt.
The real key is where you place the voltage sense wire to the alternator/computer (not sure which one is regulating in your case). Don't connect the sense wire near the battery. Put is near the computer.
The stock fusebox is powering interior/exterior lights, dash cluster, starter solenoid, radio, wipers, and fan. This is fed by a 12awg wire that was previously running to the core support, but will now terminate somewhere on the firewall.
The new fuse/relay box is powering the LM7 computer, coils/injectors, fuel pump, basically everything needed for a standalone engine. The new box would be fed by the second junction point or horn relay (the stock horn relay has a buss bar attached to it).
The computer is taking care of voltage sensing, I haven't modified the alternator connections other than removing the SBA connection.
Last edited by blackhawkMC; 08-14-2018 at 01:52 PM.
Never hook the alternator to the battery. You have read the MADD electrical tech articles, hook it to the a junction point and branch out from there. Using the starter lug works as a junction point, I just think it is better to use a point that isn't so hard to get to.
Also the computer doesn't really do anything for the alternator. Having the sense wire coming out of that is good, but it could just as easily be to an idiot light in the dash.
I say your wire sizes are bigger than needed. Won't hurt anything other than weight, money and making things harder to route.
The key is wire lengths and load distribution. You may have full amps coming out of the alternator at any point in time but it won't all be going to the same place. 8 ga from the alternator to a junction box a few feet away is plenty. You can buy 145 amp fuse holders that fit 8 ga wire. Fusible links work but they suck. From the junction box branch off to the various loads, I have my battery connected to the junction box with I think a 10 gauge wire and 40 amp fuse. The battery has never draw or passed more amps than that. I have relays for the electric fan at the junction box with a 40 fuse. I then have the two stock 10 gauge wires heading to the stock fuse box off a single fuse.
Last thing I bitch about is always run dedicated ground wires to each load and never use body panels for that purpose. Electrical gremlins are too hard to find at times.
Forgive the MS Paint drawing, but this is what I'm looking at. The upper junction point will be on the firewall, somewhere between the center and the brake booster. The lower junction point will be on the driver's side core support, next to the LM7 fusebox.
Look more closely. Don't know where my sig went in the post above. 1/0 cable between battery and starter and between under-hood power distribution block and battery - two separate connections. 1/0 cable for ground in trunk.
Whether you connect alt charge line directly to battery - or to a distribution point which is also connected to the battery, the alternator is connected to the battery. The electrons don't discriminate and they're WAY quicker than any of our LS powered vehicles.
Blackhawk - I'd go bigger from battery to starter; and I'd go 8 gauge from each junction to both fuseboxes.
As for grounds -- as long as the battery is properly grounded to the chassis/unibody and the engine is properly grounded to the chassis/unibody -- separate ground cables aren't necessary, and in the case of big loads (see video) a separate ground is usually worse than the using the chassis/unibody as a ground.
Per LT1Swap it looks like the battery to starter connection that I'm using is 2awg.
I believe that the largest wire that I can run to the stock fusebox is 10awg without heavier modification, I think that's the max for Packard 56. I'm not sure what the largest size is for the BP Automotive fuse box since it came with an input power wire, but I believe it's Metri Pack 280 so that's limited to 10awg as well.
Whether you connect alt charge line directly to battery - or to a distribution point which is also connected to the battery, the alternator is connected to the battery. The electrons don't discriminate and they're WAY quicker than any of our LS powered vehicles.
You do realize batteries are non-linear devices? A normal load you need to double the voltage to double the current flow. Batteries have an internal resistance that varies with state of charge,age, temperature and the chemical reaction going on inside. Increase the voltage a few ticks and the battery can turn into this giant current suck drawing huge amps.
Also you do realize all wires have resistance. Even though all points are connected together, current flow through the wires causes voltage drops putting them all at different voltages. All points are connected together, yet they are all at different voltages. Hence all this talk about wire size and lengths.
So you run welding wire from the alternator to the battery and from there to the fuse box and the rest of the car. Your sense wire down stream from fuse box says voltage is low, so the alternator cranks up the voltage. But you have this huge current drain going into first load it sees (the battery) absorbing whatever the alternator puts out. This prevents the downstream voltages from rising. Net result is battery getting overcharged, overheating, possible electrolysis and hydrogen gas and to top it off poor voltage regulation. Might as well run a one wire alternator.
As for running grounds through body panels that is one of the reasons American car makers lost market share they have never recovered. I'll bet there isn't a single other guy on this site who lost a day of his life chasing down an intermittent problem that turned out to be some corroded ground wire. Planned obsolescence. Save a couple of bucks on wiring and build a car that needs to be replaced after several year. Next thing you know all of your customers are buying Japanese because they ran ground wires. If you notice no modern car uses body panels to carry ground currents anymore, yet you tell people it is a good idea? Will it work? For a time, until something gets corroded.
Really, I know I'm just some loud mouth with a stupid screen name, but you probably should stop giving out electrical advice. You seem more ego than fact.
LOL - well, here we go again. Yeah - I know Ohm's Law "Pop". I = V/R. If you didn't have such a childish, visceral reaction to just about anything I post, you might have noticed I was the first one in the thread to tell the OP to use load/distance charts to select his wire sizes. Neither I nor the OP said anything about running the alternator charge line first to the battery and then on to other loads - mine's not set up that way. Seems like making it up that way was needed so you could continue to regale us with your counsel. As for grounds - when those connections are made properly, as I referred to multiple times, they work well. You have to be aware of the possibility of corrosion/debris/vibration possibly interfering with those connections over time and minimize the chances of a problem there. It's not rocket science on a 12V DC system. Running separate ground connections is a beautiful thing - knock yourself out if you want to do it that way - and the same concerns about corrosion/vibration/debris exist for every one of those connections. Is it strictly necessary? No, it's not. My current hotrod is in year 22 of the "swap beta test". According to you, my fouled up electricals are about to shut it down at any moment - thanks for the heads up.
Opinions vary. The OP gets to take all this in --- and then make his choices. I'm good with that.
Oh, and you could change your screen name if you wanted to.
08-14-2018, 10:19 AM
Charging System Sizing Clarification 
