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Ask the Engineer - How much power should I provision for my cabinets?

Posted by Robert Faulkner on Dec 30, 2009 8:57:00 AM

This is a common question I receive: How much power should I provision for my cabinets?

This can be a very complex question depending upon your goals for today and plans for the future. At the top end, you might provision enough power per cabinet to fully utilize your cooling capacity. For instance, if you can cool 500kW and have 100 cabinets, then you can provide 5kW to each cabinet. This is usually too simplistic because you may have certain high-density cabinets. But, the intent of this post isn’t to cover all of the infrastructure concerns. It is simply to focus at the cabinet level.

You need to start with the question of how much do you need today? You must, of course, identify all of the equipment you intend to put into each cabinet. The trick is then to determine how much power each piece of equipment will draw, that is, what amperage and at what voltage input. Also, if you can get the value of the power factor for the equipment supplies, you will get a better answer. There are various methods to figure the equipment draw.

Method 1: The simplest way is to assume a worst case in which the equipment will draw the maximum that the power supply can handle. This is the “name-plate rating”. For instance, the Dell PowerEdge R900 is supplied with hot-swappable 1570W (180V-240V) 90% efficient power supplies. One thing to note here is that the 1570W rating is the maximum output of the supply. You must divide by 0.9 (the efficiency) to find the maximum draw. So the R900 could draw 1745W using this method.

Method 2: Alternatively, use of the Dell Power Calculator will come up with a much more accurate value if you know the exact configuration. For example, the default configuration for an R900 HE w/ 2.5” drives draws only 709.4W (or 3.41A at 208V).

A third method to determine power draw is simply to measure it while in service. This often requires the use of outlet level monitoring CDU’s such as Server Technology’s POPS units which provide high accuracy and resolution measurement of amperage, voltage, and wattage. POPS CDU’s can be combined with the Sentry Power Manager to provide trending information at the outlet or device level thus allowing a user to find the maximum power draw from actual use. Note that it is important to use the maximum power draw not the average power draw to prevent overloads from occurring on the circuit.

Now that you have determined the power draw per device, it is time to look at the total per cabinet. Remember that the calculation for available power per circuit is found by W = V x A x pf where W is wattage, V is voltage, A is amperage (for 208V 3-phase Delta, multiply the single line rating by 1.732 for total), and pf is the power factor. Per NEC in North America, we then de-rate by a factor of 0.8. In most cases, we don’t know the power factor of the attached equipment. It is common in the industry (including device spec sheets) to ignore the effect of power factor thus treating it as value 1.0; however, real equipment is typically in the 0.75 to 0.85 range. The reason this is normally acceptable is that the equipment in a cabinet is rarely at maximum draw all at the same time and the safety de-rating will generally cover the temporary surges in current draw. Once the summation of the per cabinet device draws are complete, a simple determination of the circuit that will meet the needs can be done – 5.0kW for a 1-phase 30A circuit, 8.6kW for a 3-phase 30A circuit, 14.4kW for a 3-phase 50A circuit, or 17.3kW for a 3-phase 60A circuit.

Using method 1 above, a 30A 1-phase 208V circuit provides 5.0kW safety rated allowing for 2 servers, a 30A 3-phase 208V circuit provides 8.6kW safety rated allowing for 4 servers, a 50A 3-phase 208V circuit provides 14.4kW safety rated allowing for 8 servers, and a 60A 3-phase 208V circuit provides 17.3kW safety rated allowing for 9 servers. Using method 2 above, this allows for 7 servers on a 1-phase 30A 208V circuit, and 12 servers on a 3-phase 30A 208V circuit. (At 4U per server, more than 12 servers in a cabinet is highly unlikely, so additional calculations will not be done here.) The result is that it is highly advantageous in terms of efficient utilization of space to get a best estimate of true power draw for any given device. In fact, method 2 results in three times the density (servers per cabinet) as method 1. Some of the major manufacturers including Dell, HP, IBM, and Sun have power calculators to help determine true power draw.

Double up the circuits for redundancy and you are ready to go…

…Unless you need to plan for growth. In many initial build-outs, cabinets are being provisioned and devices are being apportioned in a low-density configuration. Be sure to account for more and newer devices in terms of power usage and in terms of available CDU outlets. Consider as example the HP DL380 G5 came with 800W power supplies and the DL380 G6 can include 1200W supplies.  – Food for thought.

Topics: data center power

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