If you were not already familiar with the "West Virginia Law" the consequences of it came into play last night when the two projectors cut out. On further inspection, it turned out breaker 119 on the west wall is also connected to two other outlets nearby. It could be that someone plugged something into one of those outlets and that's why the capacity was exceeded, but at the end of the day, as demonstrated below, the rig should not have been constructed as it was.
The "West Virginia" formula is derived from Ohm's law and will come back to haunt us when we get to LED strings. Electricity is strange and you really shouldn't touch it (HA!), so water is often used to describe it's properties instead. Voltage is like pressure, amperage is rate of flow. Watts could be thought of as the capacity of a hose. Fire hoses can convey more fluid than a garden hose, a garden hose more than plastic tubing. Hooking plastic tubing up to a fire hydrant, may result in a leak...
On to our scenario from last night. House current is usually around 120V, and the breaker could be as much as 20A.
W = 120*20 = 2400
It's worth noting, that's the best case scenario! For one thing, household Edison outlets are often only 15 amps, and that juice is likely shared among a number of receptacles wired in parallel. Further, we used a voltage value of 120, though it is not uncommon to find the voltage running as low as 110. As the formula makes clear, a lower voltage or amps means less watts available for the equipment.
Most specs are easy enough to find online in user manuals. More importantly, any electrical device, by law, MUST provide its maximum power consumption on a label.
From the pics above we see that for the projector, W=120*7.2 or 864 watts.
For the tower, it's a little stranger since the power supply is an auto-sensing device which essentially means you can plug it into any common outlet and the power supply will configure itself, thereby not releasing any "magic blue smoke". In the case of 100 volts the machine can draw up to 12 amps, or about 1200W. At 240V, the consumption is 5A which is also 1200W. By design, it doesn't matter what the supply voltage is, the wattage remains the same.
There is the 80% rule of thumb to consider as well. In practice, Electricians advise not to go over 80% of the capacity of the circuit, which means that of the 2400W we started with, we would do well not to go over 1920W. The problem from last night, then, should be clear:
2 Projectors @ 860W/each, 2 Mac Towers @ 1200W/each and a camera for lets just say 100W...
equals 4220W and a tripped breaker. That, in a few words, is poorly thought out. It's nearly twice as much wattage as could reasonably be expected from a single outlet. This is exactly why breakers were invented in the first place. By interrupting the flow of current, permanent damage was avoided.
Based on the information provided on the tags: one projector, one computer and maybe one peripheral device per outlet is as much as the outlet can be responsible for. Another option might be two towers, or two projectors. Keep this in mind when you are designing exhibitions going forward. When you do a site survey, note the markings on the breakers, if they don't have any ask the building manager or an electrician. When you get onsite to install, include a power plan outlining what gets plugged in where.
When you spec projectors for a rig, keep those watt limits in mind. The projectors available in class are pretty pedestrian as far as power consumption goes. Take a peek at the ones in the closet next time you visit the shop. If that's not beastly enough for you, the ONE (1) projector in the lecture hall consumes about 8000W.
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