Wiring Tips


Do you know just enough about wiring to get into trouble? Don't worry, it's not that hard... And with these tips, we'll help keep you out of trouble!

1. Grounds!

Everyone pays attention to the hot side of the electrical system. But for it to work right, it needs good grounds!

Bad grounds are the number one cause for problems.

If something stops working or just doesn't work right, it's usually a ground problem.

Example: Have you ever turned your headlights on and the turn signal indicators on the dash light up? If the headlights don't have a sufficient ground, they're going to try to ground through anything, even the turn signal lights.

Putting batteries in the back of the car can cause even more problems if not done right... The worst thing you can do is ground that battery on a body bolt. What you want to do is run a grommet through the floor and take the ground cable and stick it on the tailshaft on the transmission or run it to a bellhousing bolt, just like if the battery was up front.

Some professionals suggest grounding critical components like the headlights, taillights, and dashboard directly to the engine block or transmission, or to a common ground. Especially on older cars with older metal.

Many professional auto electricians will run ground wires from a unit to a common grounding point in an older car, just like in a fiberglass car. That way, there is no question if your ground is good, and it's just 1 extra wire...

You can pick a spot under the dash, stick a 1/4-20 bolt on there and tighten a nut on it. Then put all your grounds on there, including the one that goes to the engine block, and put it on that ground bolt. Then tighten a second nut on there and you're ready to go.

Once, a buddy and me were going to a show in his '26 Buick roadster. It was built much like a T-bucket and it had an electric fuel pump. It was wired in correctly, and grounded by screwing into the frame by the pump. We were about 50 miles out, and the pump quit...


What happened was the older metal of the frame simply wasn't carrying the current well enough. The pump overheated and shut down.

Fortunately, he had some extra wire and we screwed one end to the ground wire at the back and ran it to the front where we attached it to the negative side of the battery.

The pump started back up after it cooled down and we were trouble-free all the way there and back. When we got home he wired it in neatly and never had a problem after that...

Your system is only as good as it's grounds.

2. Protection is everything...

No matter how good a wire's insulation is, it doesn't stand a chance if it's installed badly.

Any time a wire goes through sheetmetal or anything like that, it should go through a grommet. The same goes for fiberglass. There are some sharp edges and little pieces of glass in it.

Fasten down wires so they can't flop around. A wire allowed to dangle on an exhaust manifold is going to short out. Dirt and vibration will wear through wires fanned out on a floorboard.

You want to protect your wires from:

-Heat sources

-Sharp edges


-Dirt and Corrosion

-Fuel, oil, etc.

Professionals go to great lengths to tie up wires and protect them from their environment. A few minutes of securing them can prevent hours of repairing a damaged system later on.

3. Tips to laying out an electrical harness.

-Install the fuse panel first.

-Install all electrical components second (switches, lights, gauges, etc.)

-Route and secure the wires third.

-Attach ends to the wiring and hook them up last.

Don't mount fuse panels in the trunk because it puts too much wire in the system and causes too much voltage loss by stretching the wires out way too long. Try to keep the electrical in the upper driver side under the dash.

4. Isolate your electical system.

Each fuse in a wiring harness may protect an individual circuit, but the wire that feeds the fuse panel (the heart of your electrical system) needs its own protection.

That wire needs something to protect it.

There are various things to protect that wire (fusible links, breakers, maxi fuses, etc.) There is lots of discussion on what's the best, but they all work fine.

But, whatever you use, it needs to go close to the power source to protect the system. It doesn't do anything if it's inside the car next to the fuse panel. A fuse or fusible link doesn't protect the fuse panel- it protects the wire that feeds it. That protects your electrical system...

Alternators can go out in dramatic and sometimes disastrous ways. It's very common for the diode bridge to go out, and when it does the alternator shorts out. If you don't have the power circuitry protected, your electrical system and everything else can go up in smoke.

5. Don't overload switches.

Switches do have their limits. Like the wires and fuses in a system, it can handle only so much current before it fails.

For example, a headlight switch may have enough capacity to handle normal sealed-beam headlights. However if you start getting some of those high intensity bulbs that pull 200 watts or more, then you're starting to overload their capacity. That also goes for the existing wiring.

The solution: Use a relay that pulls its power directly from the power source.

A relay is simply a very heavy-duty switch that does the job of a smaller switch. It lets the switch be just a trigger.

Though we used the headlight switch as an example, relays have lots of uses. There is a reason that all new cars today have quite a few relays. Electric fans, electric fuel pumps, horns, etc. pull more juice than most any switch can handle.

They're easy to install... Just look at the other articles in our 'Hot Rod Wiring' section for diagrams and how-to's.

6. Don't add to existing circuits in the fuse panel!

As the saying goes, you're never done with a car until you sell it or die. Whether changing or adding to them, people often want to add components to existing circuits.

While OEM and aftermarket wiring manufacturers combine several components on single circuits, they take things like current draw, wire gauge, and fuse capacity into consideration.

For example: adding accessories to an existing circuit may require a greater-capacity fuse, which may cause problems if that fuse's capacity exceeds the capacity of any single wire in that circuit.


The wires in a circuit are a certain gauge (size). The fuses are there to protect those wires. That's why there are different size fuses. They will blow before the wires will melt. Small gauge wire? 5 amp fuses. Bigger wire? 15 amp fuses. And so on.

So, if you try to add something else to a circuit and put in a bigger fuse, it now won't protect those smaller wires.

Now if a wire finds ground or its electrical unit goes out, the wire may catch fire without ever blowing its circuit's fuse.

Now you have a fire...

If a fuse panel is labeled to take a 10 or 15 amp fuse, don't over fuse that circuit because it's that way for a reason! It takes a 10 or a 15 amp fuse because that's what the wires in that circuit can handle.

When people add something to a curcuit, then put a bigger fuse in because the circuit keeps blowing, all they're doing is taking away the protection to that circuit. Now a fire can start...

Instead, consider an accessory fuse block. They're cheap, and easy to wire in... And, if you're thinking about a new wiring kit, go with a bigger panel than you need. It's just a few extra wires, and you'd rather have a few extra wires than not enough. In most cases people need a system bigger than they thought.

7. Buy good wire!

Like everything else in this world, there are varying degrees of wire quality.

There really isn't much difference in price for good wire and not so great wire. But people buy not so great wire because they don't know what good wire is or where to get it...

Well here you go...

For years the normal wire insulation was PVC. It was relatively tough, flexible, and inexpensive, and it does a so-so job. PVC however is a thermoplastic. Thermoplastic tends to oxidize and brittle with age.

We've all seen cheap wire that's cracked and brittle even when it's not that old...

You don't want that...

The new, good wire has cross-linked polyethylene insulation and is far more durable yet doesn't cost much more.

It's insulation is a thermo-setting plastic and remains pliable and resists oxidation and abrasion far more than cheap wire.

It also withstands far greater temperatures. The good wire can handle 257 degrees.

Cheap wire begins to fail at 185 degrees. (Normal engine temperature...)

Which would you rather have in your engine bay?

The good wire also has another benefit: more and finer strands. Electricity flows along the surface of a wire and not through a wire. So, a wire of equal size with more and finer strands can carry more current.

Ok, so where can I get it?

You may find it in quality automotive stores. Look for wire that says SXL, GXL, or TXL on it.

What you don't want is GPT wire (General Purpose Thermoplastic).

Any quality internet wire and terminal company will have good wire. (Plus quality terminals, connectors, heatshrink tubing, etc.) Try these suppliers:


If you are buying a wiring harness, most all of them use good wire. Just look for "cross link insulation" in the wire description... You can usually also buy spools of wire from them.

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