Last month, we talked about power supplies in the form of wall warts and line lumps. This month, we’ll examine how to determine generator power requirements for a P.A. system.
Don’t Blame the Groom
A friend of mine was telling me about a gig he did a few months ago: a wedding at an outdoor venue (pre-quarantine, of course) where electricity was supplied by a generator. He explained that several times during the night, power to the P.A. system was interrupted. As you can imagine, this made the bride and groom very unhappy campers. The culprit was an under-rated generator, which was not the fault of the band or sound crew but simply due to the fact that the event coordinator didn’t want to spend the money required to hire a generator with sufficient output. Needless to say, this was penny-wise and dollar-foolish.
Wattsup?
So just how much power do you need to run a P.A. system?
Power consumption of electrical appliances is usually rated in watts, and a trip to the rear panel of your favorite mixing console (or its power supply) usually reveals how many watts are required for use. For example, the Yamaha CL3 requires 170 watts of power for operation (see Fig. 1) — not much more than the amount of electricity used to run a few light bulbs.
Not all devices, however, have this information printed on the rear panel, so you may actually have to read the manual to get the information you need.
Some manufacturers take a different approach, rating the power consumption as the number of amperes (“amps”) required to power a device. An example would be the Crown XTi4002, the specs of which are shown in Fig. 2.
The XTi4002 requires 9.8 amps to run pink noise at 1/8 power into 4 ohms — which is typical of program material just below the clipping point. How many watts is that? It depends upon the local voltage supply.
Equation Alert!
If we use the formula:
P (power in watts) = V (voltage) x I (current)
and substitute the current required by the XTi4002, and the voltage (120 VAC in the U.S.), P = 120 x 9.8 = 1176 watts
Don’t Get Caught Short
Power consumption is all about how much work you need to do. In the event that you don’t push the XTi4002 anywhere near clipping, it will consume a lower number of watts. Of course, we all know that engineers never push a P.A. near the clipping point. And if you believe that, I have a bridge for you in Arizona…
One Yamaha CL3 and one XTi4002 will consume a total of 1,346 watts in the U.S., assuming 120 VAC. Suppose you’re building a small P.A. system that has three XTi4002s for stage monitors plus two more to run the mains, and you plan to run your speakers as 4 ohm loads. How much power do you need to run this P.A.?
Five XTi4002s at 1,176 watts each, plus another 170 watts for the CL3 = 6,050 watts. Don’t forget to add some extra wattage for the processing rack, the mini-fridge at monitor world, and the 27 phone chargers that will be plugged in at FOH.
In theory, you need a generator that can supply 6,050 watts. In practice, that ain’t gonna happen, for a few reasons. First of all, you won’t find a generator rated at that specific number, so look higher for a more typical rating like 10,000 watts (10 kW). Secondly — and this is really important — most generators can’t run safely at maximum output (a.k.a. “full load”) for long periods of time. Look at the “rated” power spec of a power generator, which is a more realistic indication of its output. Thirdly, audio amplifier power requirements vary depending upon demand.
Look again at the XTI4002 power consumption spec in Fig. 3, this time with the additional information on the bottom rows that I omitted the first time (yes, very sneaky).
When an XTi4002 is producing pink noise at 1/3 power (typical of an amp pushed well into clipping), the power consumption jumps to 18.9 amps — almost double the amount required at 1/8 power.
And how many watts is that?
Another Equation!
Use the formula P = V x I and substitute 18.9 amps for I:
P = 120 x 18.9 = 2,268 watts (per XTi4002)
If there are five XTi4002s in the P.A. system plus the Yamaha CL3, the total power consumption is 11,510 watts.
Again — Allow for “Headroom”
It’s easy to see that if you used a 10,000-watt generator to power this P.A. system, and the amplifiers were pushed well into the clipping point, the capabilities of the generator would be exceeded. Hopefully, the generator’s protection circuitry would shut the unit down to prevent damage and/or hazardous conditions.
Generators are usually rated in watts to provide an indication of how much work they can do, but you may find some rated in amps. If I were looking to rent a generator to power this system, I’d want a rated output of at least 20 kW, which would provide some “headroom.”
Perhaps more importantly, I’d want to make sure that this generator was dedicated to running only the P.A. system, and not the refrigeration system in the beer tent at that gig I did in Alaska many years ago. But that’s a story for another time.
A Final Word
In this day and age of digital consoles, it’d be a wise idea to use power conditioners and uninterruptable power supplies between the consoles and the AC service. This will give you time to save the files on your digital desks before you run for cover due to the fact that the P.A. has shut down.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
