Having designed and run colo data centers for many years my rule of thumb for calculating this for customers was to use the vendors tools if available or take 80% of power supply rating. Keep in mind that most servers will have a peak load during initial power on as the fans and components come online and run through testing. If a rack ever comes up on a single channel and the circuits are not rated right that breaker will just pop right back offline and you'll have to bring it up by unplugging servers and bringing them up in sets. Also most breakers are only rated at 80% load so you have to de-rate them for your load. So, e.g., a 20amp x 208 3 phase circuit really only has 8KW of constant power draw available.
+1. As tempting as it is to say "oh this server only needs full power when it boots" this may come back to bite you. As you grow usage in CPU and Disk, the power used by the server will increase substantially.
Yes. That's why they hardwire and use a breaker rated at 100%. In most jurisdiction, code requires breaker at 80% if a plug/receptacle is used. If you are hardwired you can use 100% of your capacity before tripping the breaker. You have incorrectly assumed that I suggested that you ignore breakers.
When you hardwire the circuit the electrical code allows you to use a 100% breaker.
This really is only a concern when you are paying a monthly recurring charge (MRC) by the breaker amp with many power drops.
For a deployment of this scale it should be metered power (For example 1 (or more) 3phase a+b drops to each cabinet) where you only pay a Non-Recurring setup Charge (NRC) and then the MRC is based on actual power draw.
3phase also means fewer physical PDU's (uses less space), but more physical breakers. Over-building delivery capability will eliminate any over-draw concerns for startup cycles.
Not really, although I agree with your reasoning. The other is issue is capex. When I deploy 240kW pods, if I use 80% breakers, I have to deploy 25% more PDUs than if I have 100% breakers.
Since my cabinet number is usually evenly divisible by N*PDUs, this impacts overall capital.
We are talking about 1 to 2 cab density here so capex doesn't carry that much weight.
Having a little headroom on your power circuits is also incredibility important, and not every facility will sell 100% rated breakers. It may make more sense to be in a facility with 80% rated breakers than 100%, even with the added capex of an extra PDU or two.
Goes back to my previous comment. What is important to you, at the pod / multiple pod level, isn't as important to the 1-2 cab deployment.
Similarly, ensure spare room in the cabinet for adjustments, that thing you forgot, and small growth. Much better to have 70% full and not need the space then to have no free RU and need the space.
