Perfecting the Power Link

Author: Wayne Harris Click HERE to view more articles by Wayne Harris. Originally appeared in the November/December 1989 issue of Car Stereo Review magazine. © All rights reserved.

A friend of mine from Dallas called me the other day, and in the course of our conversation, we began to trade war-stories about some of the crazier things we had seen over the past couple of years. One particular instance came to mind almost immediately.

It was the summer of '85 and I was on my way home from work. I had just crossed the rail-road tracks when I came across this new BMW pulled off to one side of the road. It wasn't really the two people jumping around wildly that caught my attention. Rather, it was the black smoke that could be seen billowing from the open doors that did the trick.

Fortunately, for the owner of the vehicle, the fire never took hold. Nevertheless, I'm sure that those first few seconds were some rather shaky moments indeed for the poor fellow. While he was recovering his wits, I decided I would hunt for the cause of the fire.

Looking inside the vehicle, I could see a line of charred carpeting leading from the floorboard on the passenger side, along the right side of the vehicle, and up and under the rear seat. Upon closer inspection, the remains of a stereo amplifier's power wire could be discerned within this charred area.

Apparently, the installer of this particular auto sound system had decided to run the amplifier's power wire underneath a metal brace in the rear seat. When the car had gone over the rail-road tracks, the jolt caused the brace to pinch the wire and cause a short-circuit. Everything from that point forward was burned to a crisp.

Curious as to why the power wire fuse hadn't blown, I proceeded to look under the hood. After several minutes of searching, it hit me; THERE WAS NO FUSE! Unbelievable, but true. Obviously, the installer of this system wasn't a professional; or was he? Don't let the above situation happen to you. The proper design of power distribution networks is critical to performance and safety in any auto sound installation.

STRATEGY

I believe that every task is easier if you have a strategy or game-plan to go by. Listed below is my ten point plan that I use whenever I design a power distribution network.

1. ALWAYS review safety considerations.
2. Calculate the wire gauge required for the system.
3. Select the appropriate insulation for the application.
4. Select the most suitable connectors.
5. Select the proper size and type of protection devices.
6. Examine special installation aids.
7. Map out the system on paper.
8. Review safety considerations again.
9. Perform the actual installation.
10. Miller Time

Safety

Always remember to think safety first. Always wear safety goggles and heavy long-sleeved shirts whenever working around batteries. No Smoking either. Batteries can produce Hydrogen gas, and unless you want to re-enact the final journey of the Hindenburg, sparks and flame should be avoided at all cost. Your workshop should also be well ventilated to disperse any Hydrogen gas that might accumulate.

Electrical shock is not a major concern when working with automotive electrical systems since the voltage is relatively low. However, most car batteries are capable of delivering in excess of 500 Amps. That's enough juice to turn a wrench into a white-hot stick of metal should it be shorted across the battery terminals. With this in mind, never wear ANY type of jewelry, rings, necklaces, etc. when working on your electrical system.

As a final note, always check with the dealership before attempting any modifications or alterations to your electrical system. Some vehicle manufacturer's warranties specifically state that the electrical system cannot be modified. Also, some vehicles have sensitive computer systems that could be damaged when connecting or disconnecting the battery. Still with me?

Let's do it.

WIRE GAUGE

Wire is sized by the American Wire Gauge (AWG) method. In this system, the smaller the AWG number, the larger the wire. To determine the proper wire gauge for a particular installation, it is first necessary to determine the maximum amount of current that the wire will be required to carry.

First, find the total RMS output power for each amplifier. This is accomplished by adding the power output of all of the amplifier channels together.

Equation 1
Power Out = Ch 1 + Ch 2 + ... + Ch N

Next, calculate the input power required by each amplifier. To do this, divide the power output of each amplifier by it's conversion efficiency.

[HINT: Most car amplifiers have an efficiency of about .5]

Equation 2
Power In = Power Out / Efficiency

Now, find the total input power to the system by summing the input power of all of the amplifiers.

Equation 3
Total Power In = AMP 1 + AMP 2 + ... + AMP N

Once the total input power is known, the total input current to the system can be found by dividing the total input power by the system voltage.

[HINT: In auto sound systems, the system voltage is 13.8]

Equation 4
Current In = Power In / System Voltage

To determine the proper wire gauge, we also need to specify the maximum voltage drop (Vdrop) acceptable across the power wire. This selection will always be a tradeoff between performance and wire size. Selecting a small Vdrop will result in less power loss but will require larger gauge wire. Conversely, a large Vdrop will waste more power but use smaller and thus less expensive wire.

For example, if we select a Vdrop of .5 volts, then there will be a .5 volt drop across the power wire when the system is producing full power. The voltage at the amplifiers (Vamp) will consequently be reduced by the amount of Vdrop.

Vamp = System Voltage - Vdrop
= 13.8 - .5
= 13.3 Volts

If the supply voltage at the amplifiers gets too low, the amps could run HOT or even self-destruct. Dynamic headroom will also be reduced resulting in distortion and lower Sound Pressure Levels.

Once the value of Vdrop has been chosen, the total resistance of the wire can then be calculated.

[HINT: The maximum Vdrop I recommend is .5 volts]

Equation 5
Total Resistance = Vdrop / Current In

To find the resistance of the wire in Ohms per foot, divide the total resistance by the wire length in feet.

Equation 6
Ohms Per Foot = Total Resistance / Wire Length

Finally, examine TABLE 1 to find an AWG wire size that has an Ohms per foot rating that is less than or equal to the one calculated in equation <6>.

COPPER WIRE SIZES

* DC current that produces a Vdrop of .5 volts

TABLE 1

Example 1

We are installing an auto sound system with one 75 watt per channel stereo amplifier and one 100 watt per channel stereo amplifier. In this particular installation, we only want to run a single power wire to feed both amplifiers. To find the proper wire gauge required for this installation, we need to find the total current required by the system.

Using equation <1> we determine the total power output for each amplifier.

AMP 1 Power Out = CH 1 + CH 2
= 75 + 75
= 150 Watts

AMP 2 Power Out = CH 1 + CH 2
= 100 + 100
= 200 Watts

By using equation <2> we can calculate the total power input for each amplifier. Since most auto sound amps are about 50% efficient, we will use this figure in our equation.

[HINT: 50% is represented as .5]

AMP 1 Power In = Power Out / Efficiency
=150 / .5
= 300 Watts

AMP 2 Power In = Power Out / Efficiency
= 200 / .5
= 400 Watts

Equation <3> can then be used to find the total input power requirements for the system.

Total Power In = AMP 1 + AMP 2
= 300 + 400
= 700 Watts

To find the total input current, use equation <4>. Let the system voltage be the standard 13.8 Volts.

Current In = Power In / System Voltage
= 700 / 13.8
= 50.7 Amps

Next, we calculate the maximum acceptable resistance for the wire by using equation <5>. For a good compromise between performance and wire size, let Vdrop = .5 volts.

Total Resistance = Vdrop / Current In
= .5 / 50.7
= .00986 Ohms

To find the resistance of the wire in Ohms per foot, use equation <6> to divide the total resistance by the length of the wire in feet.

[HINT: In a typical install, wire length = 20 feet]

Ohms Per Foot = Total Resistance / Wire Length
= .00986 / 20
= .00049 Ohms per foot

Finally, look at TABLE 1 to locate a wire size that has an Ohms per foot rating that is less than or equal to .00049. For this example, an AWG #6 would work quite nicely.

WIRE TYPE

After selecting the proper size wire, the next step is to decide on the type of wire you want to use.

RULE #1 - NEVER use solid wire.

It's hard to work with and it tends to break frequently. Instead, choose a wire with a stranded conductor.

One common misconception regarding stranded wire is conductivity. Some people believe that the more strands a wire has, the better it carries current. This isn't the case. An AWG #6 wire with 8 strands is the same as an AWG #6 wire with 8000 strands. Both conduct DC current equally.

While the number of strands may not affect the electrical property of the wire, the same can not be said for it's affect on the wire's physical attributes. Specifically, the number of strands in a wire greatly affects the wire's flexibility. For the most part, a wire with a great number of small strands is more flexible than one with a small number of large strands.

In an auto sound installation, an extremely flexible power wire is a very desirable feature. Since the wire must be routed through a never-never land of humps and bumps and twists and turns, a good flexible wire can really make a difference in installation time and effort.

The wire's jacket or insulation should also be taken into consideration. A suitable jacket will provide durability, flexibility, and resistance to the elements encountered in an automotive environment, as well as provide electrical insulation. Table 2 lists the resistive properties of several common plastic insulations.

RESISTIVE PROPERTIES OF PLASTIC INSULATION

P = poor F = fair G = good E = excellent O = outstanding

TABLE 2

Probably the most common insulation for auto sound purposes is that of the PVC variety. This type of jacket features excellent durability at a moderate cost and is a good choice for most power distribution networks.

CONNECTORS

When installing a power distribution network for your auto sound system, try to keep the power wire in a single continuous piece. The goal here is to provide maximum power transfer with minimum power loss. To accomplish this, splices in the power wire must be kept to an absolute minimum.

RULE #2 - Keep power wires splice-free.

Spliced power wires are bad news. Unless they are VERY GOOD, they will eventually cause problems of one sort or another. Remember, your power wire will only be as good as it's weakest link.

In a properly designed power distribution network, the main power wire will only have one connector at each end. If the power wire feeds multiple amplifiers, use buss bars or a power distribution block (more on these later) to distribute power to each individual amplifier. Never use a rats-nest of spliced wires to provide power to the amplifiers. Not only is this un-sightly, but it poses a fire-hazard as well.

Currently, there are only two types of connectors that I truly feel give satisfactory results. These are the compression-type connector and the ratchet-type crimp.

Compression-type connectors are usually made of brass or copper and consist of two separate parts; the terminal portion and a threaded compression fitting. Installation of this type of connector is a relatively simple and straightforward process.

1. Strip the wire
2. Insert the bare wire into the compression fitting
3. Screw the wire/fitting into the terminal
4. Tighten with a wrench

The beauty of this connector, other than it's good electrical connectivity, is that it may be re-used if you make a mistake or ever decide to make a new power cable.

Ratchet-type connectors, on the other hand, make for a very permanent connection. The connector itself is usually made of a soft metal alloy similar to that of a battery post. Installation is a bit trickier and requires the use of a huge pair of costly ratchet-driven crimpers.

1. Strip the wire
2. Insert the wire into the connector
3. Crimp with MONDO sized ratchet crimpers

This type of connector makes by far the best connection I've seen. It not only boasts superior electrical and mechanical properties, but provides an air-tight seal in the crimp area as well.

POWER DISTRIBUTION

If the installation incorporates more than one amplifier, chances are some sort of power distribution setup will be necessary. Again, the goal here is to provide maximum power transfer with minimum power loss. That means that the distribution system needs to form an almost perfect bond between the main power wire and the power wires that feed each amplifier.

Best results for distributing power can be obtained with the use of a Power Distribution Block (PDB). This device is a rectangular shaped block of metal, usually made of brass or aluminum, that accepts a large power wire at one end and provides multiple smaller power output connections at the other.

Usually, the connectors incorporated in a PDB are of the compression variety. In the most common application, the larger input power wire from the battery is inserted into a compression fitting which is then threaded into the input end of the PDB. Similarly, the power wires from each amplifier is inserted into it's own smaller compression fitting which is then, in turn, threaded into one of the outputs at the other end of the PDB.

Buss bars provide an alternative method for distributing power. These devices are little more than metal bars made of aluminum or copper with threaded holes along their entire length. In this type of scenario, you would actually bolt the connector from the incoming power wire to the buss bar and then connect all subsequent power connections for the amplifiers to this bar as well.

PROTECTION

To protect your vehicle - your system - and the safety of you and your passengers, it is absolutely essential that fuses or circuit breakers be installed in each and every auto sound system.

RULE #3: Always install protection

There should always be some sort of protective device in-line with the power wire. Additionally, this device should be installed as close as practical to the battery itself. In this manner, your vehicle will be afforded protection against shorts should they occur anywhere along the length of the power wire.

Each amplifier should also have it's OWN separate fuse or circuit breaker. Since different amplifiers have different current requirements, adequate protection will be provided only if each and every amplifier possesses it's own individual protection device.

The most popular form of protection is the fuse. This little device, in simplest terms, is nothing more than an electrically conductive filament that melts when a specified current is passed through it for a given amount of time.

In order to standardize fuse ratings, Underwriters Laboratories has developed a set of specialized tests for evaluating fuses under differing load conditions. For automotive glass tube fuses, the test is very simple. Current at 110%, 135%, and 200% of the rated fuse value is passed through the fuse while a timer measures the amount of time it takes for the fuse to blow under each load condition. Table 3 lists some common fuse data as well as the maximum recommended current rating I recommend for each fuse type.

FUSE DATA

TABLE 3

The most abundant fuse type for auto sound use is that of the 3AG or AGC variety. These fuses are about 1 1/4 inches long and are usually contained within a tubular glass cylinder. The rated values for fuses of this type range from 1/8 amp to 35 amps, but in reality, aren't good for more than about 15 amps.

RULE #4: Don't use AGC or 3AG fuses for more than 15 Amps.

The problem with these fuses is not so much the fuse itself, but rather the fuse holder. Most (All) fuse-holders for this type of fuse rely upon springs or clips to make the critical electrical connections. Under high current conditions, excessive heat build-up occurs at these connections and the fuse holder will literally begin to melt or burn up. For high current applications, you should always use a 5AG fuse, an ANL fuse, or a circuit breaker.

Circuit breakers are electro-mechanical devices that "open up" like a switch when a specified current is reached. Automatic reset breakers provide protection in applications where it is not possible or undesirable to manually reset a breaker. Upon an overload, the breaker will begin to cycle, alternately opening and closing the circuit until the fault is removed.

Manual reset breakers are similar to the auto reset type, except that upon an overload condition, the breaker will open and remain open until the reset button is depressed.

To calculate the proper current rating for any particular application, use the following equation <7> or Table 4.

Fuse Value = Current required x 1.25

[HINT: Fuse voltage rating is un-important in auto sound.]

GENERIC PROTECTION REQUIREMENTS

* Current required at 13.8 volts

TABLE 4

Installation Aids

At this point, you should know:

• The wire(s) size required for the installation.
• The most suitable type of insulation for the job.
• The type of connector(s) you will be using.
• The type of protection device(s) required.

Before you actually jump right in and get your feet wet, let me first introduce you to grommets, wire-loom, and heat-shrink tubing.

The purpose of a grommet is to prevent nicks and scrapes from occurring whenever a wire passes through a hole in a piece of sheet metal. Grommets come in various sizes and slightly resemble a small rubber "doughnut". To install a grommet, simply drill the appropriate size hole and then snap the grommet into the hole. A wire can then be threaded through the center of the grommet.

RULE #5: Always use grommets when passing through metal.

Wire-loom is used to provide additional protection to the wire. The most common form of wire loom resembles a black piece of ridged plastic hose that has been split lengthwise. This slit allows you to snap wires in and out of the loom without having to disconnect them. Wire-loom should always be used if there is a possibility of abrasion.

Heat shrink tubing is generally used to provide insulation. Normally, after splicing a wire, you would slide a piece of heat shrink tubing over the splice and then heat it with a heat gun or cigarette lighter. As the tubing gets hot, it shrinks down onto the splice. Most heat shrinkable tubing is capable of shrinking to 1/2 of it's original diameter.

INSTALLATION

The first step in the installation process is to map out the system on paper. This will help you to actually visualize the way in which the power system will appear when the installation is complete. Here are some tips to use when drafting your system.

• Draw a rough outline of the vehicle.
• Sketch in the battery.
• Draw the path the wire will be routed by.
• Include the size and location of all protective devices.
• Indicate the location of the PDB or buss bars.
• Show the size and locations of all the amplifiers.

Once this is done, you can now begin the actual installation process.

1. Disconnect the battery.

Remember to:

• wear safety goggles
• consult vehicle's owner's manual for procedure
• clean skin of spilled electrolyte (acid)

2. Route the main power wire.

Remember to:

• not let the power cable touch anything hot
• always use grommets when passing through metal
• use wire-loom where the wire touches metal

3. Install the PDB or buss bars.

Remember to:

• secure these devices properly
• provide adequate insulation

4. Install connectors.

Remember to:

• cut each wire to length
• slip heat shrink onto the wire FIRST

5. Install protection devices.

Remember to:

• locate the main fuse close to the battery
• provide every amp with it's own fuse
• secure devices properly
• make them readily accessible

6. Complete the installation.

7. Double check everything.

Remember to:

• make sure power wires are not shorted to Gnd

8. Re-connect the battery.

Remember to:

• wear safety goggles
• consult vehicle's owner's manual for procedure
• clean skin of spilled electrolyte

This concludes the installation process. Hopefully, you were patient and stuck with the ten point plan I outlined at the beginning of this article. If you did, you can relax in the knowledge that the power distribution network you have just installed is adequate for your needs, reliable, and safe. If you didn't, well, hopefully your car won't be the next one I see roasting on the side of the road.