How to determine the best size wire for your amp's power and ground
by Crutchfield's Buck Pomerantz
In order to deliver the best sound, an amplifier needs its power and ground wiring to be large enough to accommodate its demand for electrical current. In this article, we'll tell you how to figure out the best wire size for your amplifier.
U
nderstanding the needs of your car audio system can help you know when to choose 4-gauge wire instead of 8-gauge wire. We've done all the math and specification-matching, so all you need to do is consult our wire size chart below. Of course, if you're looking for a new car amplifier, we list the recommended amp wiring kit with each amp.
What size power and ground wire do I need?
Determing the proper wire gauge (thickness) you should use for your car amplifier's power and ground wiring depends on three factors:
- how much power you want your system to put out
- how much current it will try to consume
- how long the wiring run will be
T-Spec 8-gauge power/ground wire
Add up the total amplifier wattage
First, calculate the total watts RMS output power of your system. Multiply the number of channels by the number of RMS watts per channel. A 4-channel amplifier with 50 watts RMS per channel has a total output of 200 watts.
If you have multiple amps, add up the total RMS wattage figures to arrive at a grand total. Use RMS power ratings only, never peak or max numbers.
We factor in efficiency, voltage, and resistance
Generally speaking, there are two kinds of amplifiers — Class AB and Class D — so there are two formulas for calculating the output power that results from a given current draw due to the different efficiencies of the two technologies. (You can read the detailed explanation below the chart.)
Use the numbers in the column that apply to your amplifier. If you don't know what Class your amplifier is, or have a mix of different amplifiers in a large system, use the Class AB calculations for the safest result.
Wire size calculator
Note: This chart is for stranded copper wire only. Copper-clad aluminum (CCA) wire cannot handle the amount of current that a copper wire of the same size can. Using CCA wire usually means you'll need a larger size to carry the same amount of current, especially in some of the longer wire runs.
AWG: Please note that our sizes are AWG (American Wire Gauge) sizes. The smaller the gauge number, the larger the wire. For example, 4-gauge wire is larger than 8-gauge wire. 1/0 ("one-aught") is the common name for a 0-gauge wire; 2/0 ("two-aught") for a 00-gauge wire.
Load Capacity: The larger the wire, the more current it can carry, so the amp can put out more power. This is sometimes referred to as the wire's "ampacity." That's the basic premise at play here. The more power in your system and the greater the distance you need it to travel, the larger the wire you need to get it there.
Tinned copper wire with some tin scraped off.
Fusing:For safety, power wires need to be fused as close to the battery as possible. For amplifier wiring kits, always use the fuse the wire manufacturer includes in the kit. For more information, check out our article about Car Amp Fuses.
The formulas we used
Class AB amplifiers: Power output (watts RMS) equals current draw (amperes) times voltage (13.8V) times amplifier efficiency (50%).
Class D amplifiers: Power output (watts RMS) equals current draw (amperes) times voltage (13.8V) times amplifier efficiency (75%).
A more detailed explanation
Here is an explanation about the formulas we used, in case you want more details. Calculating the amplifier's total power is straightforward, but the other parts can be confusing.
Wire is rated for how much current it can carry.
Amplifiers, although rated in watts output, depend on a particular amount of input current to produce a particular amount of output wattage. Different amplifier technologies operate at different efficiencies. There're also resistance factors to consider, due to wire size and length, that affect power delivery.
For the wire gauge chart, we've done all the math for you.
Crutchfield's 4-gauge power wiring kit works for amplifiers with outputs up to 1100 watts RMS.
Calculating wattage using Joule's Law
Power (watts) equals current (amperes) times voltage (volts).
But no amplifier is 100% efficient
The above formula, by itself, doesn't take into account the inefficiency inherent to power production. That needs to get factored in. An amplifier's efficiency is the ratio of what it puts out divided by what it draws from the electrical system. No amp is 100% efficient, putting out exactly what it draws, nor can an amp put out more power than it draws. The power that doesn't make it to the output terminals is wasted energy that turns into heat. The different amplifier classes produce their outputs with different efficiencies.
Power (watts) equals current (amperes) times voltage (volts) times efficiency (X%).
By factoring in this inefficiency for each class of amplifier, we arrive at the two formulas listed above:
The formula for Class AB amps
For optimum safety, we'll assume a typical Class AB amplifier is 50% efficient, which means about half of the power it generates is turned into audio output while the other half of the power is lost as heat. So if the amplifier is putting out 400 watts, it's actually drawing about 800 watts of power from its source, and the amp's wiring needs to be big enough to handle that draw.
- A Class AB amplifier's output power (watts RMS) equals its current draw (amperes) times the supply voltage (13.8V) times 50% efficiency.
The formula for Class D amps
A conservatively rated Class D amplifier is about 75% efficient, which means about three quarters of the power it generates is turned into audio output while one quarter of the power is lost as heat. So if the amplifier is putting out 400 watts, it's actually drawing about 533 watts of power from its source, and the amp's wiring needs to be big enough to handle that draw.
- A Class D amplifier's output power (watts RMS) equals its current draw (amperes) times the supply voltage (13.8V) times 75% efficiency.
Rockford Fosgate's 1/0-gauge dual amp wiring kit comes with a 300-amp fuse and can handle a total output up to 2700 watts RMS.
Automotive voltage is neither 12 volts nor 14.4 volts
And the 13.8? Yes, vehicles have a 12-volt electrical system, but we're assuming that the vehicle is running — which means its alternator will bump up the system voltage to about 13.8 volts. This is a better real-world representation of the vehicle's electrical supply. Dividing by 12 results in a larger number, which could point to a larger wire gauge, but it's often in the same color range in the chart. Manufacturers use 14.4 volts, when they spec their gear, to exaggerate their power ratings.
Resistance increases with wire length
The reason different cable lengths bear different ratings is because the electrical resistance, inherent in all wire, builds up as the cable gets longer, until it forces the voltage to drop below a useable level. At that point, up-sizing the power cable will restore the voltage to its intended level.
Wire size matters for current flow
Finally, according to our tech support guys, the primary performance limitation in most amplifier installations is in the current delivery — either a weak ground or insufficient wire gauge.
Installing too small of a wire gauge results in poor performance, potentially shorter service life of connected components (your amplifier and speakers), and a potential safety hazard.
On the other hand, installing too large a wire gauge doesn't really have any downside, and there is the potential for better performance. Obviously, there's no need to buy 2-gauge wiring when 10-gauge will do. That kind of overkill would be a waste of money. But if the chart could lean either way between two sizes, going with the larger wire size would be the smart choice.
Crutchfield's 12-gauge wiring kit works for small compact amps and high-power car stereos.
What size speaker wire do I need?
Speaker wiring matters too. The signal and power coming out of your amplifier must not be impeded on their way to your speakers and subs. When you replace or run new speaker wiring, we recommend using:
- 18-, 16-, or 14-gauge wires for speakers
- 16-, 14-, or 12-gauge wires for subwoofers
As with the power wire, the longer the run and the more current you're pushing through it, the larger size you should use. For example, if your amp is in the trunk and you're sending 100 watts to your front speakers, 14-gauge speaker wire is a good call. But if the amp is only 50 watts, 16-gauge would be fine.
Stinger PRO 16-gauge speaker wire
Let us help you get what you need
Now that you have some idea of how much amp wiring you need, shop our selection of amplifier wiring and accessories. We have amp wiring kits, distribution blocks, and everything else you need. And if you have any questions about putting together a shopping list, contact our advisors via phone or chat. If you want to learn more about amplifier installation, read our Amplifier Installation Guide.
