As of 2014, there are 8 circuits from the main electric panel, two 20A circuits going to the plugs in the wall, and six 30A circuits going under the floor. There are labels on the racks as to which circuit breaker the rack is on.
Circuit Breakers
Most of the power distribution units (PDU) in the racks have digital ammeters which show the current. These should also be labelled as to the maximum current.
To allow for startup current, the circuit breakers have a tolerance of about 20%. That is, a 15A breaker typically will not cut off until the current exceeds 18 amps. Do not exceed the rated capacity, even if the circuit breaker does not immediately shut off.
Reactive Power
Most modern computer power supplies are power factor corrected (PFC) meaning that the total power = volts x amps. However, with some equipment, the magnetics will push back current out of phase with the AC input. In this case, the current will be higher than would be expected for the wattage. This excess current will register on the ammeters, causes resistive heating in the power cables, and can trip the circuit breakers.
In theory, reactive power can be balanced with capacitors. We have tried this and it often just makes things worse, because it is difficult to balance properly. So we must reduce the load to keep the current within safe limits.
Voltage
Voltage should normally be between 115 and 125 volts RMS (root mean square). The Furman PDUs have a green "voltage range" light, which will turn orange if the voltage is below 115. To switch between voltage and amperage display, press the button next to the LED display. Normally we want to display amps.
The uninterruptable power supplies should switch to battery if the input voltage is below 106V RMS. In practice, they tend to switch to battery intermittently below about 112 volts. This wears out the batteries, and shoud be avoided if possible.
Low voltage can be caused by too much current through inadequately sized wiring. For example, 14AWG copper wiring has a resistance of appoximately .00515 ohms per foot (for two conductors). So a 10ft power cord would have a resistance of .0515 ohm. With a 15A load, the voltage drop would be 0.0515*15 = 0.77 volts. (also I²R = 11.6 watts of heat) With 10AWG wire, the resistance would be .02 ohm, and the voltage drop only 0.3 volts (and 4.5W heat). Most of the wiring under the floor is 10 or 12 gauge.
One of the uninterruptable power supplies was being particularly fussy about input voltage, and is now plugged into a voltage-regulating autotransformer. This is a single-winding transformer with multiple taps, in 5 volt increments. If the output voltage is below 117 or above 123, it automatically switches to the next tap.
3-phase power
The power coming into the room is 3-phase. The potential difference between the phases is 208V (although none of our equipment is connected this way). The three phases are split into single-phase (120V) circuits with a common neutral.
Most of our machines have redundant power supplies. Connect all power supply units to the same phase. Although in theory the power supplies should be isolated, in practice we have seen some odd failures when PSUs were connected to different phases.
UPS batteries
The uninterruptable power supplies use sealed lead acid batteries. These need to be replaced after 3-5 years. Do not leave batteries installed for more than 5 years. They will leak acid and cause damage. When replacing batteries, label them with the date.
Plugs
Most of our equipment uses NEMA 5-15 (Edison) plugs. Some have a 5-20 (20 amp) plug. This looks like the normal Edison plug except the neutral prong is rotated 90 degrees.
The plugs under the floor are L5-30 (30 amp). This is a round 3-prong plug. A few of the power cords have L5-20 (20 amp) plugs. These look similar to the 30 amp plugs, but have a smaller diameter. A L5-20 plug will not fit into a L5-30 receptacle.