Electrical Power Planning for Server Rooms: Best Practices

Last updated  November 11th, 2013 by Steven Jordan.

Server room power requirements:  Strategy for electrical circuits and outlets.

     What's so complicated about power?  Plug in the machine and watch the LED lights come to life.  Unfortunately power supply planning is a bit more involved.  Larger server rooms typically have 110V and 220V outlets.  To simplify the process we'll examine typical 110V outlets and how they supply appropriate power to the sever room.

     Assessment Planning:  Layer 1 may not be glamorous but it is the essential foundation for all server room dependencies.  Don't assume that server room outlets can support every device you can throw at it.  Likewise, it's poor practice to substitute power strips when there appears to be a shortage of electrical outlets.  An assessment of available electric resources will avoid these pitfalls.

    Visio.  Begin the assessment by conceptualizing the server room into a Microsoft Visio diagram.  The diagram does not have to be to scale.  We're simply mapping electrical outlets and basic server locations.  This is an example of a server room circuit map I had previously created:

N.B., 110 outlets are represented with a singular power symbol.  220 outlets are represented with the power symbol and with "240V".  The 240V stencil is nominal for either 208 or 240 Volts.  APC UPS units, however, can be configured with inputs and outputs of nominal 220, 230, and 240 voltages.  Like wise, 110V usually refers to nominal 120V. 
      Power Formula.  Visio mapped the 110V and 220V outlets in the server room.  Each outlet should then be assessed to ensure a sufficient supply of electricity.  On average, 110V outlets can support approximately 1440 Watts, or 1.4kW.  The standard power formula is used to estimate an appropriate devices-to-power ratio:

     Electrical power is the measurement of output.  Server consumption of electricity is ratted in Watts (W) (CDX, 2013).  The power formula measures consumption:  

P = I x E 
P = Power, I = Current, E = Volts  

     Use the power formula for server room electricity consumption:

W=A x V

W = Watts, A = Amps, V = Volts     

     Why is this formula important?  Consider a 110V run provides a nominal 120V to each outlet.  The maximum theoretical output is 1800W:

1800W = 15A  x 120V

     Do not cross the 1800W threshold!  Design load should be approximately 80% for the maximum output to prevent overloading (power outages):

1440W = (15A x 120V) .08

     The design load is only an estimation and does not account for every situation.  Circuit breakers do not limit Watts; rather Amps.  Consider a single device can draw more than 15A (i.e., don't use toasters or hair dryers in the server room).  It is however, important to determine reasonable consumption for all devices in the server room.

    Excel Spreadsheet.  Use an Excel worksheet to document each server and its average power consumption.  Columns represent individual circuits.  Rows represent miscellaneous servers and devices.  Average power consumption is then recorded to corresponding cells as Watts.  Precise measurements are preferable but not always be available. Use rough estimates in lieu of recorded statistics.  Remember, estimates can be updated to reflect closer approximation at a later date.  Armed with this information, Excel formulas can determine total output for each circuit in the server room.  

    There are a number of methods to estimate power consumption.  Amazon sells a variety of tools to determine electrical load.  A popular (and low-tech) device is called the kill-a-watt and sells for $30:

     Server tools may provide additional power consumption data.  Dell's IDRAC management card provides accurate PSU data.  For example, the Dell Open Management Server Administrator (OMSA) determined our Power Edge R420 used on average, 66 Watts. 

Dell also provides a power assessment tool at their website:  http://www.dell.com/html/us/products/rack_advisor_new/

     I found the online estimates useful, albeit overly conservative.  According to Dell, the same R420 server was expected to consume an average of 224.2 Watts.  That's a 158W difference of capacity planning!  There is an important distinction between maximum rating and actual consumption.  In reality, most servers never experience more than 300W at any given time.  It should also be noted that data for Amps, as well as Watts are provided.  

     APC UPS management cards may also provide power consumption data.  The following UPS log shows the output for Voltages and Watts (as a percentage):
     How can this data be used to estimate appropriate circuit load?  The maximum rating for the Smart-UPS 1500 is 1050W (available on APC's website).  Quick math determines actual consumption:

1050W x .37 = 388.5W

    Total consumption from this UPS averages 388.5W.  Our maximum target per circuit is 1440W.  Data indicates we can tipple the current load without crossing over the 1440W threshold.

    The logs provide additional information to help our assessment.  Recall the power formula?

W = A x V       
     We can determine the average Amps:

388.5 = A x 118
388.5 / 118 = 3.29A

     What is the significance of 3.29A?  We've already determined we can add additional devices to the circuit.   Additional devices will increase current which can risk overloading the 20A breaker.  Exercise caution and incorporate a cushion for growth.  Server upgrades, including memory, CPUs, additional virtual machines, etc.. will increase the overall electrical load.    

     Electrician's Help.  An electrician can be of substantial help through the assessment process.  I recommend an electrician's help to ensure the server room has a 1:1 ratio of circuits-to-outlets.  An unexpected power outage is a particularly harsh way to discover your design only allows half of its intended power.  Comparatively, the 220V outlet with a 40A breaker is robust and forgiving.

     A qualified electrician can install 220V outlets.  The disadvantage to 220V outlets is that existing UPS units and servers most likely use NEMA 5-15 and C13 power cables.  Change of outlets requires a change of UPS units.  Consider this is a one time expense.  The 220V UPS can be paired with a step-down transformer to allow a 110V PSUs (server) to connect.

     An electrician should also review of the breaker box to determine the main amperage rating.  The main amperage rating is different from the ratings of individual circuits.  Several circuits that run near capacity can trip the the main (DIY, 2013).   The electrician can also ensure 20A breakers are used for the 110 runs.  Most importantly, if the circuit breakers are not clearly identified, have the electrician label all breakers and outlets.  Electricians can be expensive but power mitigation in priceless.

    Planning for appropriate server room power is the precursor to assessing uninterrupted backup power supplies...   




  1. Hey Steven,
    For any server configuration of your choice, how do you calculate the amount of heat dissipated from it in terms of BTU or Watts? For example, if a server specifies 200 Watts as it's operating power, then how much of power is actually drawn and how much of power dissipates as heat? Any help in this would be appreciated

    1. That's a good question. From what I know, heat dissipation varies across vendors and models. This is mostly because each generation of technology becomes more energy efficient that the previous (i.e., Koomey's Law).

      Most vendors publish heat dissipation rates in device manuals. For example, Dell R730's BTU rates are found here: http://dell.to/2pqtZmY

      Explanation of Koomey's Law: http://www.stevenjordan.net/2013/12/assessment-model-of-byod-adoption-of.html

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