Power Capacity Planning for Data Center Colocation.

Power compromises a large percentage of the data center’s costs and understanding your power requirements will help determine what size circuits you will need to order.

Server manufactures may provide different power figures like max, startup, and running wattage per server configuration.  Max wattage, also known as Power Supply Wattage is the maximum wattage the power supply can provide to the server.  Many times the server may have two power supplies for redundancy, thus max load is usually calculated to run with one power supply.

Running loads are what is typically used to calculate power needs.  Startup load may run about 20% higher than the running load.  For power draw calculations best practices would be to add 20% to your calculated running load.  For example a server that averages a 200W running load should have a budget load of 240W to estimate max load during peak operations and planned server reboots.  See the chart below.

The following example uses a 20amp 120V circuit breaker.  Accepting that the 20amp breaker has a potential of tripping at 80% or 16amps, it gives a trip point that you should be below.  How far below is subjective; and due to budgetary factors, many try to push the circuits to maximum capacity without the breaker tripping.  So what is safe for your mission critical environment?  50%, 60% 70% 80% of a circuit?  Let’s assume you want to maximize the circuit and leave 20% power margin available for high server loads and planned server reboots. For calculation purposes we will convert amps to watts. (120V x Amps = Watts) or (120V x 16amps = 1,920Watts).  If you plan to leave 20% safe ramp-up margin and utilize the circuit to MAX 80% capacity your average running power capacity would be 1,600Watts allowing for a 20% spike in power. (1,600W x 1.20 = 1,920W or 16 amps.).  1,600watts = 13.3Amps.  In most cases a customers’ top average power usage is around 60% of circuit capacity, or for this example approximately 12Amps on a single 20Amp circuit.

Knowledge is key.  Knowing what size circuit you’ll use and giving yourself ample room to stay under the limits as set forth by the NEC, you’ll be able to find the one that best meets your needs.