Background and Challenge

Customer is looking to deploy high density racks, 20 servers (possibly more for future growth) per rack. Each server is estimated to consume 1200W per server, split between 2 power supplies. The estimated load per rack is 24kW. Planning to install 2 power strips each rack for A and B feed.

Problem

Their colocation provider could only supply 2 x 3 phase 50A (MAX) feeds per rack.

Solution

Our experience in data center power allowed us to solve this tricky situation.

Based on our server deployment chart (Fig 1), with 2 power strips per rack, the customer will need at least 2 x 3-phase 50A circuits in order to fully support the 12kW per feed.

Rack power strip manufacturers provide cabinet power strips with 3 phase 50A plugs with two different rated currents: either 40A or 35A.

35A units only require 3 circuit breakers, which would limit the total load per branch. 40A units employ 6 breakers per strip which would allow the customer higher loads per branch.

Mirapath evaluated a number of cabinet power strips solution in the market and provided a detailed comparison chart (Fig 2). Using the units in the comparison chart and their respective number of breakers, Mirapath walked through a 3-phase load calculator (Fig 3 and Fig 4) to simulate the load on the strip and the impact on load balancing and breaker overload.

case3Figure 3
Figure 4

On Fig 3, a 3 phase 50A power strip with 3 x 20A circuit breakers. The customer can only connect up to 18 servers since one of the 20A circuit breaker will trip after server #19 connects with the PDU.

On Fig 4, a 3 phase 50A power strip with 6 x 20A circuit breakers. The customer can power up to 22 servers which is 2 extra servers for future growth due to no limitation at the branch level.

In the end, we have recommended a 3-phase Delta 50A PDU with 6 circuit breakers, which offered 14.4kW per strip instead of 12.6kW, a 14.3% increase in capacity.

Takeaways:

No two cabinet power strips are the same. In addition to evaluating cabinet strips based on their features, don’t forget to look at their internal construction such as how many fuses/breakers it employs. It will have a significant impact on how much you load you can sustain in your rack.