Does everything run off of one 12ga wire?

[R.O.B.]

My mind is blown right now. From the way I see the wiring schematic, the 4ga battery cable only runs the starter. Everything else is running from a 12ga wire into the fuse box. I currently have no starter hooked up, nor the 4ga cable, but everything else is connected. Sure enough, I've got dash lights, headlights, etc.

Doesn’t it seem like a massive load for just the one 12ga wire? I mean, night, running headlights, brake lights, wipers if it's raining. That seems like a lot.

What am I not understanding?

[jabborabbo]

Originally Posted by R.O.B.

My mind is blown right now. From the way I see the wiring schematic, the 4ga battery cable only runs the starter. Everything else is running from a 12ga wire into the fuse box. I currently have no starter hooked up, nor the 4ga cable, but everything else is connected. Sure enough, I've got dash lights, headlights, etc.

Doesn’t it seem like a massive load for just the one 12ga wire? I mean, night, running headlights, brake lights, wipers if it's raining. That seems like a lot.

What am I not understanding?

12 gauge solid copper wire can handle up to 34 amps of current…12 gauge stranded (like 24 smaller wires bundled) copper wire can handle up to 14 amps…

[R.O.B.]

14 amps definitely doesn't sound like much at all. Especially when you have 20 amp fuses.

[HO455]

Keep in mind that most of these trucks came from the factory with 37 amp alternators. The only time that that one 12 guage wire from the battery was the sole source of power is when the engine wasn't running. The rest of the time the alternator supplies the power and that one wire is actually charging the battery.

Also back in the day when our trucks were on the drawing board (1965ish) stop and go traffic was not part of the design parameters. So drawing the full load from the battery you described wasn’t an issue since trucks didn't spend extend periods of time idling. If there was too much amperage drawn from the battery the fuseable link would fail and protect the 12 guage wire.

So now days if you use your truck as a daily driver, in Portland Oregon and your commute is like 10 miles. Expect that in the winter, with all the driving done in the dark, in the rain, with the defroster on high and lots of stop and go traffic, your battery will slowly go dead. The alternator won't keep up with the large electrical load with the majority of the commute done at an idle.

If your truck came equipped with a higher amperage alternator, then there was a additional wiring harness that doubled the wires between the alternator output terminal and the main power connection (splice) behind the drivers side headlight. (This additional harness doesn't show up on any of the common wiring diagrams available today) This kept the 62 amp alternator from damaging the 12 guage wire from the alternator to the main power connection.

The link below should help explain the system.

http://www.madelectrical.com/electri...evymain1.shtml

[PbFut]

Generally the the alternators of today on our trucks provide 60 to 90 amps but not continuous. They handle spikes to those levels. Same with the wires that service the various accessories. Most accessories will need a spike of energy to start but quickly drop to much lower levels. The spike needed is so short you do not heat a 12 guage wire. The big exception are the modern day audio amplifiers. Amps can often overwhelm older and even newer automotive electrical systems. If I were to have 2 spal fans and a a big Amp, then the wire to the positive block from battery may start to heat.
I would never wire a modern Amp or elec fan to the fuse block. It should be run from the positive block on the fender. Relay triggers should be powered from fuse block. Trailers and campers should have a run from the fender block. Each run from the fender block must be fused.

[Richard]

Unless something has changed, per NEC 14ga=15 amps, 12ga=20 amps, 10ga=30 amps, 8ga=40 amps (THHN=40/50 amps), 6ga=60 amps (THHN), goes on and easily found online. The sheath comes into play with the actual amperage rating of a wire. I have taken out and replaced 50 amp circuits that were on 12ga wire operating for many years. There are safety factors built into the standards, they have to be there. If you use the table then you will be pretty safe with any wire. When replacing wire it is always good practice to get the actual rated amperage of wire. The manufacturers know when these rules apply and what type of wire to use. To see an example look inside an old damaged appliance. Take an old non running appliance apart and look at the wire sizes inside the appliance, compare to wattage rating and most times they do not agree with the table. Note that NEC does not distinguish any difference between solid and stranded wires. The main reason stranded wire is used is for flexibility and ease of installation.

[R.O.B.]

So 20amps is what it can handle, equal to a 240 watt load.

Looks like I'm going to upgrade to a 10ga wire. 30 amps, 360 watt load.

If I need more, I'll relay the headlights. And of course the stereo will have it's own dedicated circuit completely by-passing the fuse box.

Thanks for everyone's help. (still blows my mind, lol)

[MikeB]

My 69 has been running for 54 years with a 12 gauge feed wire, and never even burned out the 16 gauge fusible link or melted the insulation. And that's been with a 60 or 70-something amp 12SI alternator for the past 20 years or so.

I think the real world max current at 12 volts must be around 20-30 amps. In fact, the last time I measured current draw, it was 30-something amps (maybe close to 40) with just about everything turned on, including hi-speed blower and high beams. And I was still getting nearly 13 volts to the headlights with 14.2 at the battery.

That said, I am in the process of rewiring the truck, using 8 gauge wire between alternator and battery, and then 10 gauge wire to the fuse panel. If 12 gauge worked fine for all those years, my truck's gonna be very happy with the new wiring.

[RichardJ]

Below is as good of a 12Volt Automotive Wiring chart as you will find.

NEC wiring code and THHN (PCV insulation) is not Automotive wiring.

If you are talking about amperage load for automotive wiring, you must specify that you are talking about 12Volt wiring.
All amperage charts take into consideration the Length of the wire.

https://www.ogaled.com/how-to-select...lications.html

[Steeveedee]

There's obviously a large safety margin built into that chart. I remember writing safety specs for man-rated platforms, when I worked in defense. There was a safety factor of 5 in the OSHA specs, because sometimes people carry stupid amounts of weight on those platforms...like the time I dragged a main feed pump constant pressure pump governor up two ladders from the lower level of the engine room to the weather deck on the destroyer I was stationed on...and threw it overboard. Lots of heads turned at the noise. It was well over 100 pounds of cast iron and made quite the splash!

[71CHEVYSHORTBED402]

As I recall, even the wire to an HEI dist. is 12 Ga. Some insist on 10, but a good source for the board says 12 gets it done.

[Richard]

Wire is, well wire. The sheath or jacket on REAL automotive wire makes rating different than house wiring. Some reference to the rating in THHN vs standard in my post. Using the NEC chart is safe. I had said to look at actual amperage rating of wire. Many times on quality wire it is printed on sheath. Cheap wire is not always rated for automobile applications. This may be the case with wire harnesses from China VS USA vendors. Then again, may be the case if it is actually USA made or just sold USA. Hard to tell these days.

[A1971Blazer]

I always use GXL wire in the engine compartment and TXL everywhere else.
They have better insulation and are best for vehicle wiring....
anywhere there's a high amperage wire....I use a fusible link.

I bought the M&H upgrade kit for my '67 build, it came with a heavy duty 8 gauge additional feed wire with a fusible link that goes to the fuse block to increase the available current....
It plugs right into the bulkhead connector.....it also has several extra heavy duty fused feed wires in the block for accessories.

[MikeB]

Originally Posted by RichardJ

Below is as good of a 12Volt Automotive Wiring chart as you will find.

All amperage charts take into consideration the Length of the wire.

Well, that chart sure explains why our trucks' original wiring doesn't smoke with >30 amp loads. Looks like wire length is just as critical as wire size.

Something else: Although the 12 ga wire running across the radiator support from the battery to the factory "splice" (on the drivers side of the radiator) carries current to the fuse panel, some of that current is supplied by the alternator. And the splice-to-fuse panel wire is only 4.5 ft feet long.

Good reading here: http://www.madelectrical.com/electri...evymain1.shtml

[jumpsoffrock]

Originally Posted by 71CHEVYSHORTBED402

As I recall, even the wire to an HEI dist. is 12 Ga. Some insist on 10, but a good source for the board says 12 gets it done.

That topic has been beat to death on almost every classic auto forum google can find, and the common answers are: 12-16ga.
12 for people who want to be careful.
16ga for people who look to copy factory wiring from similar vehicles.


That in-and-of-itself shows how many different opinions there are on this stuff.

[MikeB]

Just found this on another forum.

"The National Electrical Code amperage flow for wire sizes doesn't apply to automotive use."

I'm just speculating here, but the 12 gauge/20 amp and 10 gauge/30 amp NEC specs may be due to having AC current running on the hot and neutral wires next to each other in the same jacketing. In other words, 20 amps current actually does 40 amps worth of heating inside Romex wire jacketing because it's running from the service panel to the load and back.

I guess we could say the same for automotive headlight wiring running in the same bundle as the fuse panel feed wire. But for standard high beams that would be 60 watts x 2 = 120 watts. Then 120 watts divided by 12-14 volts = 10.0-8.6 amps worth of additional heating. (Insert "exploding head" icon here.)

[72SB]

Relative to the headlight load on draw, that and all lights go "through" the headlight switch and associated wire length getting 12vdc power from switch to end point of light.

Regarding headlights, using a headlight relay harness, removes the "load" from headlight switch which results in much brighter headlights as well as all the other lights. The draw from headlights is then not running the wire length from switch but from AUX buss on PS core support.

I have installed them in my 67 car and 72 C10 and headlights are much brighter as are all the other lights as the load of the head lights is reduced running from light switch with the headlight relay harness.

LMC and other sources sell them or you can make your own if inclined. They are around $30.

The headlight switch then just triggers relays wired into the buss bar, PS. No cutting any stock wires. Plug & play.

pic where I installed mine

[kwmech]

It's just like your house. If you have a 200 amp panel and add up the circuit breakers you'll be way over 200. It goes off of usage. In the older trucks we have there wasn't a lot going on all at the same time when they were built. Now-a-days we have more goodies inside and outside the cab than what it was originally designed for. So yeah stepping it up is not a bad thing

[HO455]

Originally Posted by MikeB

Just found this on another forum.

"The National Electrical Code amperage flow for wire sizes doesn't apply to automotive use."

I'm just speculating here, but the 12 gauge/20 amp and 10 gauge/30 amp NEC specs may be due to having AC current running on the hot and neutral wires next to each other in the same jacketing. In other words, 20 amps current actually does 40 amps worth of heating inside Romex wire jacketing because it's running from the service panel to the load and back.

I guess we could say the same for automotive headlight wiring running in the same bundle as the fuse panel feed wire. But for standard high beams that would be 60 watts x 2 = 120 watts. Then 120 watts divided by 12-14 volts = 10.0-8.6 amps worth of additional heating. (Insert "exploding head" icon here.)

The differences in how AC and DC pass through conductors (The skin effect) is one of the reasons automotive wires are rated somewhat differently than AC conductors.

https://www.allaboutcircuits.com/tex...e-skin-effect/

[MikeB]

Originally Posted by HO455

The differences in how AC and DC pass through conductors (The skin effect) is one of the reasons automotive wires are rated somewhat differently than AC conductors.

https://www.allaboutcircuits.com/tex...e-skin-effect/

Thanks for bringing that up. It was something I learned back in Navy tech school, but I had forgotten about it.

[MikeB]

Originally Posted by 72SB

Regarding headlights, using a headlight relay harness, removes the "load" from headlight switch which results in much brighter headlights as well as all the other lights.

Very nice installation.

I have also found that a new wiring harness using 14 ga wire to the headlights (instead of the original 16 ga), along with a new headlight switch, brightens them up considerably.

On an 82 pickup with old OE wiring, I would see around 1.5 - 2.0 volt drop at the headlights. With the new harness, it was less than 0.5 volts. And just going from, let's say, 12 volts at the headlights to 13 volts makes a huge difference in brightness. I think it's related to the square of the current, maybe "Power = I squared R".

I actually had intended to install relays, but I still have them in a box!

[HO455]

Originally Posted by MikeB

Thanks for bringing that up. It was something I learned back in Navy tech school, but I had forgotten about it.

What do you know, that's the same place I learned about it.

[Wildkarde]

Originally Posted by jabborabbo

12 gauge solid copper wire can handle up to 34 amps of current…12 gauge stranded (like 24 smaller wires bundled) copper wire can handle up to 14 amps…

There is in general no difference in ampacity between solid and stranded. Wire guage is based on cross sectional area, which makes stranded wire slightly larger in overall diameter to make up for the air spaces between the strands. There can be small differences when x number of small stands adds up to a value thats smaller/larger than the equivalent solid wire since they don't make custom size strands.

You also can't really make blanket statements like 12g can handle 34A since wire length matters immensely for 12v systems.

The NEC charts are for 120v+ structure wiring and not really useful for us, partly due voltage, but also the AC vs DC, but mainly because the NEC values are set with lengths of like 200ft so that electricians don't need to do calculations for every circuit.

I recommend using a chart like https://mgispeedware.com/wire-size-chart/ and choosing 40C, and 3% voltage drop. With 12v systems voltage drop will generally become an issue long before melted wire will.

[MikeB]

Originally Posted by Wildkarde

There is in general no difference in ampacity between solid and stranded. Wire guage is based on cross sectional area, which makes stranded wire slightly larger in overall diameter to make up for the air spaces between the strands. There can be small differences when x number of small stands adds up to a value thats smaller/larger than the equivalent solid wire since they don't make custom size strands.

You also can't really make blanket statements like 12g can handle 34A since wire length matters immensely for 12v systems.

The NEC charts are for 120v+ structure wiring and not really useful for us, partly due voltage, but also the AC vs DC, but mainly because the NEC values are set with lengths of like 200ft so that electricians don't need to do calculations for every circuit.

Good information. Well said.

[franken]

Moving swiftly on.
Now we have a boxing match between those who babble about 12V DC and others who somehow equate that with 120/240VAC. Then we have gibberish about solid vs stranded wire.
Engineers an actual 80 or more years ago made it work with the wires-but today, we're arguing about it.
We are devolving.