Data Center Power (Part 2)
If you’ve ever looked at the circuit breaker panel at your home, you’ve noticed that each breaker has a designation printed on it’s face – 15A, 20A and 40A are most common. The designations are the maximum current ratings for the circuit that is connected to the breaker. That 20A breaker is connected to a circuit that is rated for 20 Amps, absolute maximum. If the current draw on the circuit exceeds 20A for any reason, even momentarily, the breaker will trip, shutting off power to the circuit to protect it from an overload. So the Rated value is simply the absolute max value (usually expressed in amps, but it can also be expressed in watts), above which overload and breaker trip will occur. Simple enough.
This is the safe maximum continuous power draw for a circuit. We’ve already established that the rated value is essentially the overload protection tripping threshold. It stands to reason that the De-rated value must be something below that, and in fact it is: as established by NEMA, the maximum safe sustained power draw is no more than 80% of the rated value. Therefor, on a 20A Rated circuit of any voltage, the De-rated value is 80% of that: 16A. On a 20A circuit, your continuous sustained draw may not be more than 16A. On a 30A circuit, the de-rated max is 24A. This can just as easily be expressed in watts: A 20A @ 208V circuit equals 4.16kW rated; 3.32kW de-rated. You can draw no more than 3.32kW sustained on that circuit. Why is this? Well, computer equipment rarely consumes power in a steady way. Drives spin into activity, fans kick on and off, and other functions creating changing demands in power. Any server will draw considerably more power during a boot up as compared to when it’s up and running. De-rating builds a safety margin into circuit usage to allow for these transient peaks while protecting the circuit.
When you purchase less than the full capacity of a power circuit, you are getting what is commonly referred to as metered power. Another way of saying this: you pay for what you use, rather than for what is available. Metered power can be an economical option when doing a smaller colocation that does not require a full circuit’s worth of power resources. Metered power is most commonly offered on shared power circuits that service two or more clients, though some providers will offer metered power on a dedicated circuit (but rest assured that you will pay for the benefit of a private circuit). Keep in mind that any provider offering metered power will generally want a reasonably accurate estimate of your power needs up front – this is necessary for them to manage power allocations and avoid potentially overloading circuits. Any metered power offer made to you by a provider should contain the amount of metered power included, with terms or rates for overages. Also, metered power is usually quoted at it’s de-rated value, but don’t assume that – be sure your provider specifies rated or de-rated in their quote.
When you purchase a full circuit, you are able to get it in an unmetered or “flat-rate” billing configuration from just about any provider. This simply means that you are allowed to use all of the available power supplied to the circuit without any additional charge. In most cases, flat-rate circuits are the best deal money-wise, so if your expected sustained loading is over 50% of the circuit rating, you’ll most likely want to go flat-rate. Flat-rate circuits should always be dedicated to just one user. Keep in mind that flat-rate circuits are almost always quoted at their rated value, so take 80% of that to find out what your maximum sustained loading will be (be sure to have the provider specify rated or de-rated in any quote).
Putting It All Together
Now that you understand amps, volts, watts and circuit ratings, you should be able to compare power offerings from providers to make sure you are getting adequate power in the configuration that best serves you. But how do you determine the power needs of your stack? As is often the case, there’s more than one way. We’ve put together a guide to address that topic as well – give it a read.
One final note: if you are shopping a smaller colocation project that requires less than a full circuit of power, you will typically have limited options with regards to power. Providers who rent shared circuits (typically offered in shared cabinet space situations) usually settle on a standardized power config for that offering – things like alternative voltages, A/B power and the like just may not be available to you.