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Determining correct gauge of wiring


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There are some sections of my car (the electrical) that needs to be replaced. Of course, the correct thing to do would be to replace all the wiring, but because of finances, this is not an option at this time. I noticed that in the actual service manual (37 Pontiac) there is no mention of wire gauge. How does a person tell what would be the correct gauge of wire to use? I know that I could measure the original gauge, but I've read that the newer wiring might not have to be the same gauge as the original, but I do not know if this is true or not. What is the best way to determine to correct size of wire? Does it go by what the wires are being used for? Is there a standard rule of thumb on this?

I would appreciate any opinions or facts on this subject, and I thank you in advance for your time.

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There are some sections of my car (the electrical) that needs to be replaced. Of course, the correct thing to do would be to replace all the wiring, but because of finances, this is not an option at this time. I noticed that in the actual service manual (37 Pontiac) there is no mention of wire gauge. How does a person tell what would be the correct gauge of wire to use? I know that I could measure the original gauge, but I've read that the newer wiring might not have to be the same gauge as the original, but I do not know if this is true or not. What is the best way to determine to correct size of wire? Does it go by what the wires are being used for? Is there a standard rule of thumb on this?

I would appreciate any opinions or facts on this subject, and I thank you in advance for your time.

Rather than get into a big explanation of calculating current load on a given circuit plus accumulated resistance in wire over distance, what exactly do the wire(s) you need to replace serve?

If you are dealing with dash lights 18 ga. hookup wire should be ample in most cases. If talking about a power circuit to a heater fan and control its probably 14 ga. or use what is often labeled as "primary" wire, taillight/stoplights/parking lights probably 16 ga. All gauges of wire should be stranded and not solid. Use too small of a gauge for load and it will melt the jacket. Use too small of a gauge for lighting of any nature and if the jacket doesn't melt you might have dim lights.

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Thank you Jim. What I have in mind is this. Since I am restoring the 2 horns on my car, I see that there are 2 smaller wires going from the horn relay through its own grommit to under the dash. These 2 wires I would imagine would probably be 16 gauge. But, the actual wires going from the horn relay to the horn is much bigger, so I would assume that they would be either 12 or 14 gauge. My profession is in electronics, so wiring up circuits is no big deal to me, buy as with my other questions, if I do the job, I want to make sure that I do it correctly, so it will not have to be done again later. Are the replacement wires supposed to be the same color with/tracer color as the originals wiring? I am assuming that they should be. I also see the wiring going to the generator are also in bad shape, so I would have to assume that these would be 12 gauge?

What I would really like to do is to make my own harness over a period of time, but do not install it until it is finished. When a person is working on a shoe-string budget, any cash output has to be watched. Now the horn circuit is a different story. These wires are run seperately from the large harness, so this is something I could take care of before I tackle the big job.

I see that there is a lot of this new cloth-covered wiring being sold on Ebay, but I don't know if I will be able to find the exact color/tracer there.

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Thank you Jim. What I have in mind is this. Since I am restoring the 2 horns on my car, I see that there are 2 smaller wires going from the horn relay through its own grommit to under the dash. These 2 wires I would imagine would probably be 16 gauge. But, the actual wires going from the horn relay to the horn is much bigger, so I would assume that they would be either 12 or 14 gauge. My profession is in electronics, so wiring up circuits is no big deal to me, buy as with my other questions, if I do the job, I want to make sure that I do it correctly, so it will not have to be done again later. Are the replacement wires supposed to be the same color with/tracer color as the originals wiring? I am assuming that they should be. I also see the wiring going to the generator are also in bad shape, so I would have to assume that these would be 12 gauge?

What I would really like to do is to make my own harness over a period of time, but do not install it until it is finished. When a person is working on a shoe-string budget, any cash output has to be watched. Now the horn circuit is a different story. These wires are run seperately from the large harness, so this is something I could take care of before I tackle the big job.

I see that there is a lot of this new cloth-covered wiring being sold on Ebay, but I don't know if I will be able to find the exact color/tracer there.

Jim the chances of finding color coded wire of the correct gauge is probably going to be impossible unless you live near some electronics manufacturing company with a surplus store. And even then it would probably be pure luck given larger gauge wire is not frequently used unless power conversion is involved. I've often thought of making some sort of Mickey Mouse device to stripe wire that I could stick various color Marks a Lot pens or similar into but I've never gotten around to doing it. I absolutely hate changing wire color coding in a circuit.

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Jim,

I have had very good success with Rhode Island Wiring. They have the correct gauge, color, and tracers for the wire you need. I believe they work directly from factory wiring diagrams. You can buy complete harnesses or individual supplies for your projects. Good phone communication.

Take a look- www.riwire.com

Good luck- Dave.

Edited by straight8pontiac
mistake on original url (see edit history)
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Jim ,Rhode Island wiring can supply original type wire by the foot. They will also help you in determining the correct gauge. In most of the wiring harnesses the colors of the wire denote the gauge. Personally,I would recommend purchasing a complete ready made harness. It would eliminate any bad wiring you may have missed,it would be much easier to install and they usually have the correct terminals on them. You would probably not be able to create a really satisfactory(original type)on your own. A new unit would be color coded as per original and they come with diagrams. I know that they are a little pricy,but a new harness will help eliminate chances of fire and future electrical problems. I've seen so many "restorers" complain about electrical problems simply because they tried to rig their own wiring or did not want to make the investment. Two steps in restoration that you don't want to cheat on,if you want to really enjoy your car for a long time,are mechanicals and electrical. Judging by your concern about quality and correctness I think you already know what you shoud do and I know you'll be happy you did!

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You still have the original wiring so you know what gauge wire to use.

Something to understand.

The fuses protect the wire and are sized for the amount of current the particular gauge and insulation of wire can handle without smoking. The fuses are not sized for the current draw of the devices on the wire. The wire is sized to the needs of the devices and the fuse sized to the wire.

So what I am getting at, you need to be sure you replace the existing wire with the same gauge or thicker wire for proper protection.

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Thank you guys for the feed-back! As a hobby, I collect and restore the old TVs from the 40's, and I have had to re-wire many of them over the years. As far as the horn circuit goes, this will be a simple job. It is completely seperate from the large wiring harness, so doing this would not be piecing it together. I forgot to mention, I also have an ace up my sleeve. There is a guy out west who rewired his 37 Pontiac, and he is going to send me his original harness he removed. With this old harness, this will make it much easier to make my own before the instillation. I have both the black and white cloth tape, and I believe that I can make it look good as a finished product. Yes, I know what would be best (to buy the complete harness) but that is easier said than done when it comes to paying for it. If I can make my harness over a period of time, and just pay for the wire as I get it, I can manage that.

A by the sea, thank you for that insight. That didn't even occur to me.

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