- This conversation is for power typically used in the US.
- 120V power is one phase wire to neutral wire.
- 208V power, across 2 phase wires, is still considered single-phase.
- There is no "two phase" power in common use these days. Trust me.
- It is really important to balance use of the phases of 3-phase power.
Most data centers use large UPS systems for the room (before distribution of power to power panels and racks). Smaller server rooms sometimes use rack-based UPS systems (after distribution to racks).
The step-down transformer sometimes used taking 208V down to 110V wastes energy. I ballpark it at 10%. In most server rooms 208V power is used for equipment like servers and storage. Here are a few examples of how that power is often distributed and how to balance power across the phases in each case:
- A 3-phase power panel with 208V circuit-breakers wired to outlets near each rack. This method is the usual method in older data centers, and requires an electrician for every change, which can be expensive over time.
- Balance power by placing gear in racks using under-utilized phases. For example - Phase A is under-utilized by B and C phases are over-used. Use racks with AB and/or CA to boost use of A more than B and C.
- 3-Phase from Starline Busway or equivalent to 208V plugin modules which provide outlets. This type of distribution is quite flexible and a bit more expensive up front. No electrician is needed for minor power distribution changes over time.
- Balance power with a short outage and switch modules to change phases in use for the outlet.
- 3-Phase to the PDU/power strip in the rack. The equipment to do this is typically more expensive than other power distribution equipment.
- Balance power by changing which outlets you use in the rack. Power must be balanced in each rack.
The real question in balancing 3-phase power is this: how much out-of-balance is too much? Our standard is that power must not be more than 10% out of balance. It is "ok" if a very temporary migration takes power use 15% out-of-balance for less than a few hours.
I have seen power panels 25% or more out of balance. This typically causes some problems:
- Heat - this condition is detectable as heat
- Circuit Breaker outages - circuit breakers can flip
- Waste of power - Power will be induced in the neutral wire and be wasted
- Loss of capacity - this wasted power is lost and cannot be used, lowering the maximum capacity available.