ICEinLAVA's Domain

I want to give a big “thanks” to all those who sent me e-mails on power distribution basics (May 2003) and enjoyed reading the rarely mentioned subject material. Upon follow-up with many sound system operators, I have found a common problem scenario. They are happily mixing away in mid-show when a circuit breaker overloads, causing an unscheduled set-break and the quandary of how it happened in the first place. This article shall try to lead you through appropriately loading AC circuits, and help you to understand the causes of overloads. The best way is to show by example, and this example is vivid in my mind after a summer of civic outdoor street dances.

In my example, consider a real life small town street dance where I had one 50A, 120/240VAC receptacle (NEMA 14-50R) to do the whole bandstand–meaning FOH power, instrument backline, main and monitor power amps, and a modest amount of stage lighting. Now the standard contract clearly reads, “100A, 120/240VAC service within 100 feet of the stage,” but the local officials are handing me the well-worn line, “It always works for the other bands.” Right then and there, my mind trips into those two accordions and a tuba, or at least a garage band and a box mixer. Being resourceful, I look up the power pole where the receptacle medium voltage (120/240VAC) comes from, and from there I say “Drats,” as I spot no beefier than 6-gauge feeder wire heading down to the breaker box feeding my receptacle. So my hope of 2-gauge 100A service to tap directly onto withers away.

Load Sharing—By the Numbers

Pulling out my now wimpy 50A club distribution sub-panel (distro pack) and a 25-foot NEMA 14-50 feeder extension (6 gauge, 4 conductor); I begin to ponder the thought, “50 Amperes per hot-wire and three 20-Amp breakers per hot to pull it through.” With six 20A breakers each feeding duplex Edison receptacles (NEMA 5-20R), the task is also bit more complex, as I want to balance feeder currents to minimize neutral feeder hum, buzz, and just plain not have to worry about distro noises the whole gig. On the bright side, I have a separate ground wire (from the bonded neutral bus in the main panel) and every branch circuit in my subpanel uses it. Figure 1 depicts some of the thinking.

The first items to connect up are my main mid/high power amps and four mixes of monitor amps (driving four 12”+1” JBL passive wedges). Since my BSS minidrive, QSC PLX-3402 (bi-amp mids), and QSC PL218 (bi-amp highs) for the main speakers all run through a Furman PM-PRO for protection, I have a LED bargraph visual for current monitoring from the FOH position. All this goes on one 20-Amp breaker, with the monitor mix amps (two QSC PLX-3402s) on the opposite hot feeder. My rationale here is that both main mid-highs and monitors get very similar signal inputs, so placing them on opposite feeder hots balances out the neutral current fairly well. Another fact is that each PLX-3402 is spec’d to draw typically 7.6 Amps with 8-ohm loading per channel, and 11.6 Amps with 4-ohm loading per channel. The PL218 was driving eight ohms per channel, and typically drew six Amps. So both branch circuits were under the load limits of 20 Amperes. And because the monitor amp circuit was well under the limit, I ran my FOH power from it since it added only a couple more amperes.

The next two balanced circuits were the subwoofer amp circuit and the backline circuit. This is somewhat trial by fire in that you may or may not know what the act’s total backline power consumption may be. Yes, you can check instrument amplifier fuse or breaker ratings, but they are likely to 30 to 100% higher than the nominal power consumption. Fortunately, I knew the whole backline was mostly combo amps and did not challenge a 20 Amp breaker, but occasionally proved to trip a 15 Amp residential breaker. The subwoofer amp I had for this gig (a Lab Gruppen fp6400) prefers the solitary use of a 30 Amp, 120VAC circuit to give the subwoofers everything it has. But fortunately, with a little backing off the kick and bass guitar low end, it has proven to run reliably on a 20 Amp breaker. These are the kind of calculating things you do to ensure a smooth show without outward display of stress to the performers and patrons.

The last two circuits I reserved for stage lighting. Now remember, I have 50 Ampere main breakers at the power pole panel and my distro pack subpanel, and have allocated the better part of 40 amperes in the previous four breakers. In my rig, I have 16 fixtures (four light trees with four cans per tree) with ratings between 250 and 300 watts per fixture. Now I beg the forbearance of the editors and readers to briefly dive into stage light karma, even though this magazine is dedicated to live sound. Unfortunately, smaller productions do press the sound crew into running the lights when budgets are tight.

My stage light philosophy bends towards setting scenes, as opposed to the more traditional “flash ‘em all” shows I see today. Fixture gel selection is composed of red, blue, and amber hues with me using single colors or color pairs for variety and mood setting. I also make sure that Rosco 02 gel (bastard amber) gets liberally applied since it is a flesh tone that allows the audience to view the expressiveness of the performers. With that said, it means that no more than two or three fixtures of any four-can light tree are on at any one time. Also, by chaining each side of the stage’s front and back tree dimmer packs together on one circuit, each light circuit tends to have the same color washes on at the same time, thus further reducing any dimmer buzz and imbalanced neutral currents.

Going for a Trip

When loading up circuits, it helps to understand how a circuit breaker protects the circuit elements (wiring, receptacles, plugs, and cables). Most common breakers use a thermal method of determining when to trip. Thus a small known amount of resistance in the breaker creates a modest power loss that is converted into heat. The contact element that opens the circuit does so based on a buildup of temperature upon itself. Under normal circuit loading, surrounding elements wick away heat (heatsinking) to prevent too high a temperature on the contact element. What results is a curve of circuit current versus time in seconds before the breaker trips. On a typical 20-Amp Square D Q0 breaker, 25 Amperes of continuous loading would take about two minutes to trip, at 40 amperes about 20 seconds, and at 100 amperes about two seconds.

Given that you can cheat a bit for a short period of time, and that modern music reproduction loading tends to average 50 to 80% of its peak current draw (we are talking about the AC side of power amplifiers here), most overloads tend to be gross infractions of load current estimation. Stage lights are the classic exception in that “on” is full current draw, and they tend to have nasty turn-on current spikes when “bumped.” These flashes or bumps tend to have about double to triple the spike current draw of their constant on current. Thus the worst-case situation would be all fixtures pulsing on and off simultaneously, and doing this for many seconds could cause a breaker trip if the on current is close to the breaker rating.

Cords at Your Service

One of my pet peeves is the neglect for choosing proper AC service cords (extension cords) for use in performances. The first faux pas I see is the substitution of orange or blue plastic extension cords for the desired black rubberized plastic service cords. The advantage of black is not only the better quality of material, but also the psychological fact that black cabling hides well on stages and bandstands, so the audience is focused on the more colorfully illuminated performers and their instruments. Friends don’t let friends use orange cords at shows.

Another blunder I see with service cords is the wire size or gauge used. Because gig to gig they may be used to limits of breaker capacity, no smaller than 12-gauge cabling should be used, and 10-gauge is recommended if you need long runs at 20 Amperes. I buy my AC cable by 250-foot spools and attach premium quality nylon plugs and receptacles to make up bunches of 100-foot, 50-foot, 25-foot, and 12.5-foot length service cords. Another requirement that applies to theatrical stages, and should be applied to music performances, is the use of “extra hard service” (Type S) jacketing instead of “junior hard service” (Type SJ). The National Electric Code (NEC) requires all theatrical stage service cords of 20 feet or longer to be of the extra hard service variety. Look for designations like 12-3 SOW instead of 12-3 SJOW when cable shopping. The OW suffix stands for oil-proof and weatherproof. The rationale for extra hard service is because we know those street dance gigs will have vehicles, road cases, and people trampling on them until they are packed away for the next gig.