Here's another informative column by Mark Amundson, reprinted from the Aug. 2009 issue of FOH. -ed.
Being in the live sound business, and especially towards the ankle-biting side, I get a lot questions of how much audio power or amperes of loading a typical circuit can handle before tripping the circuit breaker.
The true amount can be affected depending on the kind of music being performed and how much gear is connected onto any one branch circuit, or circuits. This column is to help understand the situation from the breaker point of view, as to how much power we can get away with before the breaker starts tripping out.
Breaker Types
There are two main types of circuit breakers commonly used in AC power distribution: magnetic breakers and thermal breakers. Magnetic breakers are popular in industrial control situations where large turn-on current surges are typical, and thermal breakers may not hold during the short-term heavy current surge. Magnetic breakers are also found in better quality power conditioning units for audio/video racks for surge purposes and can be fairly compact in mounting onto rack chassis assemblies.
Thermal breakers are the usual suspects found in residential and commercial breaker panels for common power distribution. They are the black plastic devices with a beefy reset switch and sometimes a red paint trip indication, depending on if they encountered a short circuit or an overload condition. One could define a short circuit as a sudden load of more than 10 times the rated current. Most common brands of thermal breakers are Square D, ITT, General Electric, and Cutler-Hammer.
Overloading
When a circuit breaker trips in most live sound applications, it tends to be from a slightly overloaded circuit. I would define "slightly overloaded" in terms of a 20 ampere-rated circuit loaded to 25 to 20 amperes continuously. Easily identified with constant stage lighting loads, this kind of slightly overloaded abuse would normally result in a circuit breaker trip after a half-minute to several minutes in duration. For the tolerance band for tripping a Square D 20 Ampere Q0 circuit, see Fig. 1.
These kinds of slow trips are fair warning that the operator should shed about half the load or back off severely on the intensity of the musical performance volume. What I find fascinating is the transition between a slight overload and a moderate overload. As the graphic in Figure 1 illustrates, you could load up a 20-ampere breaker with 60-ampere spikes, and as long as those spikes are less than a second in length, you can get away with them without a breaker trip.
Audio Loading
What these moderate overload conditions are telling the average live sound operator is that a lot of branch circuit abuse can be tolerated by thermal breakers way beyond the trip ratings, as long as the average currents are below the rating. This works well to a small extent with audio power amplifiers, as the audio program power delivery gets smoothed out by the amplifier's power supply circuits. Thus sudden crescendos of music performance are covered by both the breakers and the supply capacitors within the audio power amplifier.
There are also prudent things you can do if you are working 20-ampere breakers and associated wiring with spikey audio loading. If possible, use a well-maintained Portable Power Distribution Unit (PPDU) so that those spikes are melded into 6 gauge or heavier conductors designed to handle those higher currents continuously. Another thing to try is to use short length 10- or 12-gauge service cords between power amplifiers and the receptacles with freshly cleaned interconnects that make very low ohmic contacts to avoid arcing and carbonizing problems.
One other trick that sharp-eyed viewers of my column may have noted on my "volts-wagon" PPDU is that I took advantage of my breaker panel size and spaced out my branch circuit breakers so there are some air gaps between breakers. Since thermal breakers heat up before tripping, having air gaps between breakers helps raise the trip threshold a couple of amperes. That can mean a big difference in outcomes if the circuits are loaded just at, or slightly above, rated trip thresholds. A lot of wise sound persons already know that a warm breaker will trip quicker than a cold breaker, especially if you cannot readily shed some of the load on the branch circuit.
I cannot give you more (branch) circuits to work with, but I can at least make you understand how the breaker looks at surviving your loading situation.
