Meltdown (sorry it repeated)

[IamAlan]

A longer wire run has a higher resistance. So less current, so less heat. So less stress. The only real exception to this is if the electricity is turning an electric motor. Less voltage at the terminals (because voltage was lost along the supply line) means the motor turns slower which actually increases the amp draw. I don't remember the exact details from college physics, but inductive loads can be counterintuitive.

Short runs actually have the potential to create more heat than long runs.
So the practical reality is that if your electrical system is properly protected with fuses, this won't happen.

That's not how this works. That's not how this works at all. Think of voltage as a pipe of a certain size, and current as flow within that pipeline. The resistance will increase because the voltage (size of the pipe) will decrease with distance. The current will demand to remain the same as the load is demanding a set of amps, building pressure (resistance and heat). The lower voltage will not allow the amount of current to flow and the wiring will overheat because the pipe is too small. The fuse will not trip because it is current sensitive, not voltage sensitive. The practical reality is that if you don't increase wire size in accordance with the distance from the source to the load, you're not doing it right. And you risk burning up your wiring, your dash, and your truck. And with a degree in Electrical engineering and 32 years as a product safety specialist, I guess you could say that I am an expert.

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[Dqban]

That's not how this works. That's not how this works at all. Think of voltage as a pipe of a certain size, and current as flow within that pipeline. The resistance will increase because the voltage (size of the pipe) will decrease with distance. The current will demand to remain the same as the load is demanding a set of amps, building pressure (resistance and heat). The lower voltage will not allow the amount of current to flow and the wiring will overheat because the pipe is too small. The fuse will not trip because it is current sensitive, not voltage sensitive. The practical reality is that if you don't increase wire size in accordance with the distance from the source to the load, you're not doing it right. And you risk burning up your wiring, your dash, and your truck. And with a degree in Electrical engineering and 32 years as a product safety specialist, I guess you could say that I am an expert.

I was thinking the same thing....Ive seen a few fires secondary to car stereo installs ( big systems with 1000+ watts requiring additional batteries and capacitors ect.). The cause was frequently something in the wiring....cheap and poorly insulated or just to thin. I used to see cheap "4 gauge" that was the right width on the outside but the actual copper on the inside was more like 6-8.

 

[dcmdon]

I was thinking the same thing....Ive seen a few fires secondary to car stereo installs ( big systems with 1000+ watts requiring additional batteries and capacitors ect.). The cause was frequently something in the wiring....cheap and poorly insulated or just to thin. I used to see cheap "4 gauge" that was the right width on the outside but the actual copper on the inside was more like 6-8.

What you are talking about is voltage loss along the wire. And you are absolutely right. The thicker the wire the better. But that is not what we are talking about.

We are talking about the amount of HEAT generated by wiring. And not just heat. Heat per unit length of wire. Resistance causes heat.

More current means more energy is lost to the wiring.

Ohm's law. P=IR has the solution. I'll get to it as soon as I get the kids out the door for school.

 

[sharpsicle]

A longer wire run has a higher resistance. So less current, so less heat. So less stress.

We are talking about the amount of HEAT generated by wiring. And not just heat. Heat per unit length of wire. Resistance causes heat.

Here's your problem. You've contradicted yourself pretty badly and everyone's been trying to correct it for you.

Your second post made more sense, but the original post you made was kinda bonkers.

 

[dcmdon]

Here's your problem. You've contradicted yourself pretty badly and everyone's been trying to correct it for you.

Your second post made more sense, but the original post you made was kinda bonkers.

Both are consistent with Ohms law and correct. With the one substantial caveat around inductive loads. If low voltage (caused by a voltage drop from a long wire run) is applied to an electric motor, its resistance can plummet which will result in more current which creates a lot of heat.

But that doesn't apply to lights, electronics, etc. It does apply to compressors, refrigerators.

 

[sharpsicle]

Both are consistent with Ohms law and correct. With the one substantial caveat around inductive loads. If low voltage (caused by a voltage drop from a long wire run) is applied to an electric motor, its resistance can plummet which will result in more current which creates a lot of heat.

But that doesn't apply to lights, electronics, etc. It does apply to compressors, refrigerators.

Nobody here is talking about compressors or refrigerators or AC current or anything. We're talking about a 12v DC vehicle system and the electrical accessories that are added on. Don't confuse the situation with irrelevant information.

 

[dcmdon]

Nobody here is talking about compressors or refrigerators or AC current or anything. We're talking about a 12v DC vehicle system and the electrical accessories that are added on. Don't confuse the situation with irrelevant information.

So now you have revealed your ignorance. I am not talking about anything that is AC. A lot of people install air compressors in their rig. That's an inductive load.

A refrigerator that runs on 12v is a pure inductive load.

Accessories like stereos and lights are resistive loads and would go along with what I said.

 

[sharpsicle]

So now you have revealed your ignorance. I am not talking about anything that is AC. A lot of people install air compressors in their rig. That's an inductive load.

A refrigerator that runs on 12v is a pure inductive load.

Accessories like stereos and lights are resistive loads and would go along with what I said.

Good! I didn't assume you were, but I wanted to clarify.

 

[dcmdon]

This is why its always good to do the math when you can.

It showed that I was lazy in how I said what I said. We were talking about heat and fire.

A longer wire WILL draw more power from the system. But it will have a LOWER wattage draw per foot of wire.

Wattage loss per foot is what determines the heat.

Look at it this way. If you lost 10 watts through 1 inch of wire, it would get very very hot. Its basically a light bulb.

If you lost 10 watts through 1000 ft of wire it would have all that length to disipate that 10 watts of heat.

In my example of a 250 ft and a 1000 ft run (big numbers used because otherwise the math ended up with very small numbers and the decimal places were difficult to keep track of)

The wire in the 250 ft run had to dissipate more than TWICE the heat per foot (.0273 watts per foot) compared to the 1000 ft run (.01181)

Bottom line. The wire in a shorter run will run hotter than the wire in a longer run.

So here's the math.

Lets assume we have a 48 watt resistive load. LED headlight for example.

Amps x Volts = Watts.

X amps x 12 volts = 48 watts. Solve for X and we get that that light draws 4 amps.

P=IR. Or Volts = Amps x Ohms

12volts = 4 Amps x X ohms. Solve for X and you get a resistance of 3 ohms for that LED headlight.

So now we know that our resistive load draws 48 watts. Its current draw is 4 amps and its resistance is 3 ohms.

Now lets do it with 250 ft of wire.

Resistance from the bulb is 3 ohms, resistance from the wire is .625 ohms. Total resistance is 3.625 ohms.

Volts = Current x Resistance. 12= X x 3.625. Solve for X and get 3.310 So with 12 volts driving it, the bulb and wire will draw 3.310 amps.

So now back to just the wire. P=IR. 3.310 amps x .625 ohms = 2.063 volts of drop across the wire.

Power lost to the wire - Power = Voltage x Current - 2.063 volts x 3.310 amps = 6.828 watts are lost across that piece of wire. For a power density of .0273 watts per foot.

Now lets do it with 1000 ft of wire.

Resistance from the bulb is 3 ohms, resistance from the wire is 2.5 ohms. Total resistance is 5.5 ohms.

Volts = Current x Resistance. 12= X x 5.5. Solve for X and get 2.18. So with 12 volts driving it, the bulb and wire will draw 2.18 amps.

So now back to just the wire. P=IR. .2.18 amps x 2.5 ohms = 5.45 volts of drop across the wire.

Power lost to the wire - Power = Voltage x Current - 5.45 volts x 2.18 amps = 11.81 watts are lost across that piece of wire. For a power density of .01181 watts per foot.

 

[dcmdon]

I encourage you to check my math. If I dropped a decimal somewhere it would completely change the outcome.

 

[jsalbre]

You’re ignoring the voltage drop over your wires. That light isn’t receiving 12v but it still wants its 48w, so the amp draw is now higher, causing more heat.

At 5.45v it’s now drawing 8.807a

 

[sharpsicle]

Look at it this way. If you lost 10 watts through 1 inch of wire, it would get very very hot. Its basically a light bulb.

If you lost 10 watts through 1000 ft of wire it would have all that length to disipate that 10 watts of heat.

This is not a good example. In order to see both these scenarios, you'd have to have two completely different pieces of wire. You can't compare the two like that. If the wires are equal and only vary in length, there's no way you're going to have the same loss over two different lengths. You'd need a second variable.

Electrical resistance increases proportionally to length, so longer wire will naturally have more resistance. To have a shorter wire have the same resistance as a longer wire, it needs to be fundamentally different in composition or size.

 

[dcmdon]

This is not a good example. In order to see both these scenarios, you'd have to have two completely different pieces of wire. You can't compare the two like that. If the wires are equal and only vary in length, there's no way you're going to have the same loss over two different lengths. You'd need a second variable.

Electrical resistance increases proportionally to length, so longer wire will naturally have more resistance. To have a shorter wire have the same resistance as a longer wire, it needs to be fundamentally different in composition or size.

This is great. You seem sincerely interested in all this.

The longer wire has a greater resistance. You can see that above. you are correct that resistance is proportional to length (and inversely proportional to cross sectional area, which is why wire charts give cross sectional area).
In my example above, the resistance of the 250 ft wire is .625. The 1000 ft wire is 2.5.

250/1000 = .25
.625/2.5 = .25

So in my calculations, the resistance is proportional to the length. Or said another way. The 1000 ft wire is 4 times as long as the 250 foot wire and its resistance is 4 times as much.

 

[dcmdon]

You’re ignoring the voltage drop over your wires. That light isn’t receiving 12v but it still wants its 48w, so the amp draw is now higher, causing more heat.

At 5.45v it’s now drawing 8.807a

It can't draw more amps because the resistance of the whole wire/load system is fixed.

If you have a fixed resistance (R) and a fixed voltage (V) then the current draw is fixed (I)

Of course what you have described is EXACTLY how an electrical motor that is driving something like a refrigerator or air compressor will behave. When they stall resistance goes to near zero drawing huge currents and wreaking havoc.

Which is why early on I said that what I was describing was how things worked with a normal resistive load representative of electronics, lights. But not an inductive load like a DC electric motor.

 

[Dryfly24]

Who knows what was OEM and what was modified… There are a lot of additions to that rig…My bet is that it wasn’t done correctly..

That was my thought as well. The other thing I noticed after watching the video - and I’m no expert so take that for what it’s worth - is that that doesn’t look like that fire is anywhere near the transmission. That looks like it’s up near the firewall high on the engine somewhere. If you look at the damaged dash and what was melted I’m betting it was an electrical issue. Transmission cutting a fuel line and that fire would be mostly below the truck no?

Either way, I’m gonna go out on limb and say that was caused by a bad mod.

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