Understanding Power for POWER
As servers have become more powerful, the requirements for power connections and the rules about power distribution units (PDUs) have become more constrictive. IBM publishes maximum power usage for each of its servers in the technical overview manuals, but the rules confuse many people. These details are typically provided in the operating environment section. I’m asked about electrical power all the time and much of the confusion is centered on planning for power and what’s actually being measured, as well as why IBM requires so many PDUs when the measurements show less power is actually being used. Hopefully, I can clear this up and I’ll start by covering some terminology.
The details provided typically consist of watts, kVA (kilovolt-amps) and voltage and are either per server (scale out) or per node (scale up). Numbers provided in the Redbooks publications are the maximum power consumption but you should still plan for that in case you end up using all the resources at some point.
Volts is a measure of the strength of an electrical source at a given amperage (similar to water pressure in a pipe). Amps (amperes) is a measure of the total electrical current that can flow (similar to water current). kVA (kilovolt-amps) is the key number typically provided by vendors for power planning and is 1,000 volt-amps. A volt-amp is the unit used for apparent power in an electrical circuit. Volt-amps are used with AC (alternating current) power. Other calculations are available for DC (direct current) power.
I normally plan using kVA but if you want to plan using Amps then the following calculations can be used:
Single phase Amps = ((kVA x 1000)/voltage) or (Watts / (voltage x power factor))
Typically we divide by either 200 or 208 for voltage. If using three phases then calculations can be found at the useful power formulas link provided.
PDUs and Line Cords
The first key thing to understand is that power planning is required to adhere to the National Electrical Code. Because of past issues with overloading and fires, etc., PDUs have been derated. This means that a 60 amp PDU is only allowed to have up to 48 amps (9.6 kVA) of equipment connected to it and a 30 amp PDU is derated to 24 amps (4.8 kVA). This is a critical consideration in all of your power planning.
As a starting point, let’s look at IBM PDUs and their associated line cords. IBM currently provides two PDUs—the 9188/7188 and the 5889/7109. All four PDUs have six pairs of IEC320-C13 outlets rated at 200-240 volts. Each outlet is rated at 10 amps and each pair of outlets is connected to a 20-amp circuit breaker in the PDU and each circuit breaker is derated to 16 amps. The full amperage rating for the PDU (derated) is 16 amps, 24 amps or 48 amps, depending on the line cord chosen. The difference between the PDUs is that the 5889/7109 combination provides power-monitoring capabilities whereas the 9188/7188 doesn’t.
The two most common line cords to the PDUs that we see in the United States are:
6492 14' 200/240v/48A UTG0247 IEC309 60A
6654 14' 200/240v/24A UTG0247 L6-30P 30A
The 6492 uses a plug type 363P6W, which means the electrician needs to provide a 60-amp circuit with a 363P6W receptacle. The 6654 uses a NEMA L6-30P plug, which requires a 30-amp circuit and a NEMA L6-30R receptacle. There are multiple other line cord options so it’s important to discuss these with your electrician and data center personnel to ensure the correct ones are ordered. If they provide the PDUs, then you’ll need to know the voltage and derated amperage of the PDUs to plan for your power needs. Details and pictures can be found at the line cord reference.
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