Why a short circuit can burn your car to the ground

This week, we begin a new series of articles on how electricity works in a car and how to use a multimeter to troubleshoot it.

First, why say “in a car?” Does electricity work differently in a car than anywhere else? Of course not. But the fact that, in a vintage car, you’re dealing with low-voltage direct current (DC), and the battery is grounded to the car's body, makes it a unique electrical environment.

In this first installment, we want to give enough background so that, in the coming weeks, when we say “measure the current in the circuit,” you know what both current and a circuit are. We’re going to do this from the standpoint of teaching you just enough about electricity to be able to calculate the current that can flow, and the power that can be dissipated, in a short circuit, so you can understand why a short can cause an electrical fire.

What is Electricity? For this discussion, electricity is the flow of charge, and charge flows from the positive battery terminal to the negative terminal. (Yes, I know; the actual electrons flow the other way, but cut me a little slack.)

What Are Voltage, Current, and Resistance? These are the three major electrical parameters. A water analogy is often used to describe them. In this analogy:

  • Voltage, measured in volts, is the height of a waterfall of charge.
  • Current, measured in amps, is the amount of charge flowing over the waterfall.
  • Resistance, measured in ohms, is the diameter of the pipe the charge is flowing through.

So, you can have high voltage with low current (like a high waterfall with only a trickle of water), or low voltage with high current (like a low waterfall with a lot of water flowing over it).

There is another commonly-used analogy—that voltage is like electrical pressure. In this analogy, people say that volts push amps, squeezed by ohms. There’s a neat graphic representing it that has been kicking around the internet for years.

volts push amps
volts push amps

What is a Circuit? We said that electricity is the flow of charge. When charge flows, you have a circuit, starting at the positive battery terminal and ending at the negative terminal. A circuit needs three things:

  1. A voltage source. This is the battery (well, the battery plus the alternator, but let’s keep it simple).
  2. A path for the current to flow. This is the wires.
  3. A “load device” to offer resistance to the flow of current and to perform useful work. This is whatever the thing is you’re trying to power—the light bulb, the electric motor, etc.

A simple circuit, showing the voltage source, the load device, and the path for current to flow.
A simple circuit, showing the voltage source, the load device, and the path for current to flow.

What is a Short Circuit?
The question immediately comes up: If you have only wires but no load device, is that a circuit?

A degenerate circuit, with no load device
A degenerate circuit, with no load device.

The answer is: It is a circuit, but it is a degenerate circuit. It is a short circuit. If there is not a load device to offer resistance to the flow of current, the wires themselves act as the load device, but because their resistance is low, they melt.

To explain this, we’re going to have to use… math. Well, algebra.

Using Ohm’s Law to Explain Melting Wires
People hate Ohm’s Law because algebra makes their eyes cross. We’re going to talk about it very briefly because it lets you appreciate why a short circuit can cause your vintage car to burn up.

There is a relationship between voltage (V), current (I), and resistance (R) that is expressed in Ohm’s Law. The relationship is:

V = I*R (voltage equals current multiplied by resistance)

Now, because the voltage in a car is almost always 12 volts, it’s constant, and what you’re really interested in is how current varies with resistance. So, expressing Ohm’s Law a different way:

I = V/R (current equals voltage divided by resistance)

There is another equation, Watt’s power law, which states:

P = I*V (power in watts equals current multiplied by voltage)

Let’s try a simple example with a light bulb. This is useful, because the power of bulbs is expressed in watts, and people have a feeling for what the wattage of a light bulb means. So if you have a small light bulb with a resistance of 12 ohms, and put it in a 12V circuit, the current would be:

I = V/R = (12 volts) / (12 ohms) = 1 amp.

And the power would be:

P = I*V = (1 amp) * (12 volts) = 12 watts, about what you’d expect for a small bulb.

Current and power of a small light bulb
Current and power of a small light bulb.

So now we can explain why, in a short circuit, wires melt. You can actually look up the resistance of one foot of standard 10 gauge wire in a table. It’s about 0.01 ohms. Going through the same Ohm’s Law calculation as we did with the light bulb, for 12 volts flowing through one foot of wire with a resistance of 0.01 ohms, the current is:

I = V/R = (12 volts) / (0.01 ohms) = 1200 amps.

That is a ton of current, more than your starter motor probably draws. Now, your car battery probably isn’t quite capable of producing 1200 amps. The Cold Cranking Amp (CCA) rating of most batteries is about 850. But let’s just use the 1200 amp number. The power would be:

P = I*V = (1200 amps) * (12 volts) = 14,400 watts.

Current and power calculation for a short circuit
Current and power calculation for a short circuit.

Fourteen thousand watts. Think about that. That’s the power produced by one of these.

gator-14000 generator
Gator-14000 generator

And, since a piece of wire is not actually a load device—it’s not a motor spinning around, or a radio, or a light bulb—that power goes the only place it can: heat. And it’s not designed to be a heater. So it burns its insulation off, then melts.

Actually, you hope it melts. If it melts, then the path for current to flow is broken, and current stops flowing. Ironically, the thicker the wire, the longer it’ll carry current before it melts, and the more damage it causes.

Now, in practice, no one would ever intentionally connect a piece of wire across the battery terminals. However, in a car, the negative battery terminal is connected to the body of the car, and every electrical circuit in the car uses the body as part of the ground path.

Every circuit in your car includes the car’s body in the ground pathEvery circuit in your car includes the car’s body in the ground path.

So if any wire in front of the load device carrying current chafes off its insulation and touches the body of the car, it will cut the circuit short, and current will flow through it to ground. It is for this reason that what we routinely call a “short circuit” has a more technical name—“short circuit to ground.” If there’s a fuse in the short circuit, it’ll blow. But if there’s not, wires will melt.

In the figure below, we show a simple circuit that, in addition to having a load device, also has a fuse and a switch. The switch is in the open position, so no current should be flowing. We show two possible paths for a short circuit

  • If a short to ground occurs along the green path (short #1), the current will first flow through the fuse, causing the fuse to blow, opening the circuit, and stopping the flow of current before any damage is caused.
  • But if the short to ground occurs along the red path (short #2)—the one before the fuse—instead, the fuse isn’t part of the circuit, and won’t protect the wiring. This is why frayed power wires touching ground cause so much damage, and can burn your ride to the ground in a minute.

Two different shorts to ground, the first protected by the fuse, the second unprotected, resulting in melted wires
Two different shorts to ground, the first protected by the fuse, the second unprotected, resulting in melted wires.

In a vintage car, it’s actually pretty frightening how much of the wiring isn’t protected by fuses. Any wiring running directly from the battery to either the alternator or the fuse box is typically unfused. This is why it’s so important to inspect these wires for chafing against the body.

(Next week, you’ll see why a “short to ground” is one of five types of circuit failures.)

Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. His new book, Ran When Parked: How I Road-Tripped a Decade-Dead BMW 2002tii a Thousand Miles Back Home, and How You Can, Too, is available here on Amazon. In addition, he is the author of Memoirs of a Hack Mechanic and The Hack MechanicGuide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.