Calculating Needs Based On Power
It is essential to estimate the power consumed by a car audio system to properly determine the size of the power cables, alternator, and auxiliary battery. For my system, I knew the current demand for the audio system would be high, so I chose the most powerful alternator available as a factory option, a Bosch with 200 amps of rated output. Because my system could draw as much as 336 amps of current, a larger alternator, if available, would've been preferable. Nonetheless, the 200-amp alternator has provided adequate service, since the system is never played at maximum output.
It's vitally important to use appropriately sized power cables. Not only is it a matter of achieving the desired sound quality, it's a matter of safety. IASCA's rules provide guidelines for selecting the proper power cable size, or gauge, based on current demand and length of power cable.7 In former rulebooks, IASCA specifically stated the maximum permissible voltage drop in any current-carrying conductor was 0.5 volt over its entire length.8 For systems operating nominally at 12 volts, this represents a voltage loss of 4.17 percent over the length of the conductor, a reasonably conservative specification. The current rulebook uses the same "Power Cable Calculator" chart as in previous rule books; therefore, the maximum voltage drop per conductor still applies.
For those who wish to calculate the maximum permissible length of their conductors, Lmax, according to IASCA's rules, the following equation may be used: where Vd is the maximum permissible voltage drop, (Greek symbol psi) is the resistance of the conductor in Ohms per foot, and I is the current flowing through the conductor in amperes.
Table 4 provides tabulated values for for conductors commonly used in mobile audio applications.9
For example, the current demand for the front soundstage in my system was estimated to be 226 amps for the amplifiers plus 7.5 amps each for Alpine's DVI-9990 DVD-Audio/Video tuner and PXI-H990 Multimedia Manager, bringing the total current consumption to 241 amps. From Table 4, it can be seen that a 0-gauge, or larger, cable could be used. The maximum permissible length was estimated to be:
The actual length of the conductor was about 19 feet (5.8 meters) and, therefore, complied with IASCA's rules. Similar calculations were performed with each of the principle power conductors in my system and tabulated in Table 5. Table 5 shows that each of my principle power conductors complied with IASCA's requirements. It should be noted that these calculations are rather simplistic and do not take into consideration applications at elevated temperatures, or when wires are bundled together. Consider your application carefully and increase the wire gauge, or use wire with high-temperature resistant insulation, or both, if necessary, to improve the safety of your wiring. Remember, IASCA awards 0 or 5 points for appropriately sized power wires. One miscalculation or oversight could result in the loss of 5 points.
Although a number of manufacturers produce high-quality power cables, I chose from Radix Wire's Sil-A-Blend 200 family of power cables for my large gauge needs.10 These extraordinary power cables use tin-coated copper conductors and silicone insulation reinforced with fiberglass braid. Unlike most car audio power cables, Sil-A-Blend 200 cables are rated for continuous usage at 200 C (392 F), and ideally suited for routing within engine compartments. For small gauge wires in the engine compartment and dashboard areas, I used GXL automotive wiring, which is insulated with cross-linked polyethylene and rated for continuous usage at 125 C (257 F). For other small gauge wires in the cockpit area, I used Ancor's marine-grade primary wire, which uses tin-coated copper conductors and a premium vinyl insulation rated for continuous usage at 105 C (221 F). In future articles, I'll discuss wire terminations, routing, and protection.