On Assuring Adequate AC Power

How to assess your audio system’s full AC power line requirements, and how to determine whether your AC power line capacity is sufficient.

By Bryan Geyer

It’s important to confirm that you have an adequate AC current reserve to optimally power your audio system. This means more than just being confident that you can push the volume up without causing a circuit breaker to trip. The function of a circuit breaker relates to safety—not line quality or line stability. To assure that your AC supply is not plagued by load-induced voltage drops it’s desirable to verify that your audio system presents a drain that’s within the traditional guideline for power line current density. The conventional “good design” limits for this are as described here: http://www.powerstream.com/Wire_Size.htm.

You can generally determine your household’s wiring gauge by inspecting your circuit breaker. The current practice is to use AWG 14 Romex for a 15 ampere circuit, and AWG 12 Romex for a 20 ampere circuit. Because 20 ampere circuits are habitually routed only to kitchen, bath, and laundry areas, it’s likely that your listening room utilizes 15 amp circuits, hence AWG 14 wiring. By reference to the above, you can see that the classic (it’s ultra-conservative) design guide for AWG 14 wire is 5.9 amperes maximum. If it’s your intent to dedicate that specific circuit exclusively to audio system use, then 5.9 amps max. current drain should be your goal. If you exceed 5.9 amps you will potentially risk “modulating the line”, meaning you could induce brief AC line voltage fluctuations due to the inevitable load changes that your power amplifier imposes as its output shifts. Regardless, I feel that some 50% more current drain is both safe and acceptable if it truly reflects operation at full power output (with no other loads on same circuit). That makes 9 amperes my personal full power ceiling for AWG 14 in-wall wiring with 15 ampere AC circuit breakers.

To calculate your system’s maximum AC current drain, sum the net power (as stated in Watts*) consumed by your components, and divide the total by 120 Vac. That result will approximate the maximum AC operating current. Component power consumption is generally listed in the related performance specifications. Take care to assure that power drain listed for your main amplifier(s) refers to power consumed when operating at full power output into the applicable load impedance.

Better yet, directly measure the net current drain for yourself. Use a Kill-A-Watt P4400 meter; refer: https://shop.p3international.com/p/kill-a-watt.

A NOTE: You can buy top quality molded SJT-type power cords, AWG 14, 16, or 18, custom made to any length, at http://www.stayonline.com/molded-cord-configurator.aspx. The price for a custom SJT power cord is low when compared to the price for a high-end “audiophile-market” power cord, but it’s functionally identical, and it will be properly sized, without unwanted excess.

*Watts apply when considering most audio equipment, but not for AC motors, and not for Class D power amplifiers. (Most self-powered subwoofers use Class D power amps.) Use the Kill-A-Watt P4400 meter to measure the current drain or VA (Volt-Ampere) consumption of turntable/tape motors and Class D amps.

BG (January 2019)