- Power Density
- Delivery Method
In managing your power requirements, there are three points of consideration to review:
Density is improved by allowing more power to be delivered in a single plug. A 3 pole circuit provides 1.73 times more watts than a 2 pole circuit. Ex: 30A 2P (208V) gives you 6240 VA or 6.2 kVA. 30A 3P gives you 10795 VA or 10.8 kVA. You can see that one 3 pole power circuit delivers almost as much power as 2, 2 pole circuits. This allows greater density because you can ultimately fit more PDUs into less space.
We are seeing a trend in the very high density computing world to move toward 60A 3 pole circuits. One of those is capable of delivering 21.5 kVA to a rack. Two of those can deliver 21.5 kVA redundant or 43 kVA without redundancy. Four of those can deliver up to 86 kVA without redundancy. That is a lot of power!
The most dense rack we have in one of our data center facilities has 2 60A 3 pole circuits in it and they don’t use all of the power. But, in order to deliver equivalent power, we’d have to drop in the same rack 7 208v 30A or 21 110V 20A circuits (which at that point half of the rack is used just to rack PDU’s leaving little room for servers).
The way the PDU’s work that connect to these 3 pole circuits is that they take the 3 pole circuit and turn it into 3 smaller 2 pole circuits. Typically you see 3 circuit breakers on the PDU itself where it’s being broken down. There are several manufacturers that make good PDUs for 3 pole circuits. APC probably has the most options.
There are a variety of ways to deliver power to your equipment. For full redundancy, you will need dual feeds that are not loaded over 40% so that upon failover the maximum load on one side is 80% capacity. Anything over 80% capacity could trip the circuit breaker. A rack-mounted ATS (automatic transfer switch) can take care of single-power supplied devices.
Power strips deliver the power to the cabinet and come in vertical or horizontal models. The type of power strip chosen can affect the required front-to back cabinet length and the server racking kits that can be used. Horizontal power strips take up less space, but vertical power strips deliver power outlets to the entire height of the cabinet and are more convenient when plugging in equipment. More high-end models provide remote power cycling, monitoring and real-time amperage.
To understand why 3 phase power is more efficient than 1 phase power you have to understand the internals of how AC power works and Line to Line vs. Line to Neutral efficiency. In brief, when you go Line to Neutral (110V) you end up with more power lost to heat than if you go Line to Line (2 pole and 3 pole circuits). Line to Neutral is your 110V power and Line to Line is your 208V power. It should be noted that a 3 pole circuit is not more efficient than a 2 pole circuit. They both function in exactly the same way.
Some people will say that with 208V power your server uses fewer amps than on 110V power. This is true, but the amount of power that the server consumes is the same on either voltage. Power is a measure of VA (volts * amps) and so if you increase the volts you decrease the amps if the VA stays constant.
1A * 110V = 110VA
1A * 208V = 208VA
208VA / 110V = 1.89A
110VA / 208V = .53 A
Since we’re talking in VA here, we’re not taking into consideration power factor or phases, which we would also need to consider to convert into watts (units of work). But, it should give you a rough understanding and a basic attempt at a proof to demonstrate that power usage is the same regardless of delivery voltage.
Here is how to account for 3 pole circuits.
To determine VA for a 3 pole circuit, just multiply by 1.73. 3 Pole can only occur in 208V setting (not in 110) with US power delivery, so here’s how to determine the VA on a 208V 3 pole circuit with 1 amp of load:
1A * 208V * 1.73 = 359.84 VA
You can write the same conversions as before with this formula, you just need to adjust for the 1.73 multiplier used for 3 pole power.