My experience is that the 1-amp-per-rack-unit rule of thumb applies only if every server in the cabinet is under full load (CPU and all spinning disks). This is almost never the case, however: our average utilization is around 8 amps per phase on a 3-phase circuit. (Load limits are per phase, not total.)
You can also get 30-amp circuits in most data centers if you're concerned about it.
Keep in mind, too, that your load is usually balanced across 2 PDUs (assuming you're buying systems with redundant PSUs, which you should) and on 208-230V power, which is more efficient than 120V due to lower resistance. If you configure your systems correctly, the load will be shared across both PDUs under normal conditions. That said, you'll still need to ensure you don't overload the remaining circuit if redundancy is lost.
In summary, don't worry too much about overloading a 30A circuit; there are plenty of full cabinets in a DC for a reason.
My experience is that the 1-amp-per-rack-unit rule of thumb applies only if every server in the cabinet is under full load (CPU and all spinning disks). This is almost never the case, however: our average utilization is around 8 amps per phase on a 3-phase circuit. (Load limits are per phase, not total.)
You can also get 30-amp circuits in most data centers if you're concerned about it.
Keep in mind, too, that your load is usually balanced across 2 PDUs (assuming you're buying systems with redundant PSUs, which you should) and on 208-230V power, which is more efficient than 120V due to lower resistance. If you configure your systems correctly, the load will be shared across both PDUs under normal conditions. That said, you'll still need to ensure you don't overload the remaining circuit if redundancy is lost.
In summary, don't worry too much about overloading a 30A circuit; there are plenty of full cabinets in a DC for a reason.