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Power Utilization within a Colocation Environment

The most common rack's circuit arrangement is known as A/B redundant (aka: Primary + redundant circuit). It enables a total permissible continuous power consumption of 4,992watts in a single cabinet; at a combined 23A across both circuits (11.7A on one 208V/30A circuit & 11.8A on the other 208V/30A circuit).

Let's talk about a few more of the details:


In A/B redundant mode, either circuit must have sufficient headroom to support a full electrical load in the event one circuit fails...hence why neither 30A circuit should ever exceed 12A of continuous consumption.

Bursting & Continuous Load

There is a built-in buffer between bursty loads and maximum continuous rated loads: per the National Electrical Code (NEC), the maximum permitted continuous rated circuit capacity on a 30 amp nominal rated circuit is 24 amps (80%). That 80% requirement inherently buys you a 20% buffer between typically higher burst loads, and often lower continuous loads. This helps lower the risk of a circuit popping during bursty load times, but circuit breakers can and have been known to pop inside of the 24A-30A buffer. Such pops are exceedingly rare, but do occur.

The Dangers

The NEC's mandatory 20% buffer cannot be encroached upon for continuous/ongoing loads: the good colocation facilities frequently measures/audits usage; if you exceed more than 40% consumption of either circuit in an A/B redundant configuration, you would lose all reasonably expected benefits of electrical circuit fault-tolerance. As a failure of either circuit would inherently shift 100% of electrical loads to a single circuit...which too would pop; powering off the entire cabinet (if this occurs after-hours, colo personel would need to be onsite and remedy). Exceeding more than 40% per circuit in A/B redundant mode also means you'd can often be billed at the colos primary+primary circuit rate. There also could be some heating concerns if in a cabinet as greater than 5KW/cabinet of consumption can exceed the rated per cabinet cooling capacity.

Date posted: January 25, 2012

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