Electrical Power for AV
A typical electrical branch circuit here in the USA is 120 volts (nominal) single phase terminated in a NEMA 5 or “Edison” outlet.
It consists of a hot and a neutral wire, and a ground. Old construction often used the conduit as the ground connection, but current code requires a wire be used for the ground. The neutral and ground are tied together at the service entrance where the power enters the building from the street and also connected to a ground rod into the dirt. Joining the neutral and ground at the service entrance is critical for safety, but the neutral and ground must never connect to each other elsewhere. The ground rod primarily helps prevent lightning from entering the building on the electrical service.
Most buildings have more than simple 120 volt single phase power entering the building. Residences and small commercial buildings will often have 240 volt single phase power entering the building. This has two hot wires (legs) and a common neutral wire. The voltage between either hot and the neutral is 120 volts, but between the two hots is 240 volts. The neutral entering the building is tied to a ground rod, and to the neutral and ground wires from the branch circuits in the building. Most 120/240 volt single phase breaker panels have every other breaker on the same leg of the incoming power.
Larger commercial power installations are usually 3 phase, and have 3 hot wires (phases) and a common neutral as they enter the building. There are different styles of 3 phase power, but the most common (Wye) has 120 volts from any of the 3 phase conductors to the neutral wire. Most 3 phase breaker panels have every third breaker on the same phase of the incoming power.
Very small and video sound systems can run on a single branch circuit breaker. Larger systems need more power than can be drawn from a single branch circuit (usually 15 or 20 amps per circuit max) and will need to be run from multiple branch circuits. It will help reduce the chances of certain types of hum and buzz issues if all the branch circuits used for the sound and video system are on the same leg or phase (for 3 phase) of the incoming power.
Other non-audio or video loads should not be run from the same branch circuits as your audio and video. The reason is that many types of electrical devices, and in particular lights on dimmers and motors, will put electrical noise back onto the branch circuit they get their power from. Ideally we would like such "noisy" loads kept off the same power leg or phase in the breaker panel as our audio and video. See if your electrician can move such "noisy" loads to other power legs or phases.
Very large A/V systems sometimes have their branch circuits spread across all three phases of the incoming power. The reason usually given is to spread the load evenly across all three phases, but at the expense of a serious compromise of AV system performance. Much better is to use an isolation transformer to derive single phase power from 2 of the phases of the incoming 3 phase service.
In the typical smaller church where only a few branch circuits are required to supply the power needed for audio and video, you can simplify your life and reduce your chances of hum and buzz by getting all your technical power branch circuits on the same leg or phase of the incoming power.
This information is provided for educational purposes only, and should not be considered a guide to installing power wiring. Please be safe and use a licensed electrician to perform electrical installations and modifications.
Ray A. Rayburn
Ray@K2Audio.com
http://www.K2Audio.com/
