Measuring voltages by the hundreds
800 VDC power is the clear next step, but there are two different paths towards that goal. The first is the approach proposed by NVIDIA, with power transmission cables consisting of three wires: +800 V, neutral, and ground. The second is the Open Compute Project’s Mount Diablo specification, with power transmission cables that add an extra wire: +400 V, -400V, neutral, and ground. Both of these approaches achieve the same 800 V range, but Mount Diablo effectively operates at half the absolute system voltage.
However, the differences between the two approaches are nowhere near as interesting as what both of them allow when compared to today’s 48 V power architecture. For the same load and conductor sizes, the higher system voltage means current reduces by a factor of around 17 and ohmic losses reduce by a factor of 172 (289) compared with 48 V systems. That allows for what was previously unthinkable: the AC/DC stage, PDU, and batteries can be moved out to a dedicated ‘sidecar’ rack.
El power sidecar racks can now be optimized in their mechanical arrangement for lowest conversion and internal distribution loss, and effective cooling. At the same time, the IT compute rack now has space for additional processor shelves, larger power connections, and advanced liquid cooling arrangements. Improving both power and compute density addresses a long‑standing tradeoff between power delivery and compute density, but 800 VDC architectures deliver exactly that — just make sure that your infrastructure can handle it.
For the Mount Diablo bipolar ±400 V distribution, things are a bit more complicated. You can use a high-resistance, mid-point-earthed (IT) scheme so each pole sits at about half the pole-to-pole voltage relative to earth. ETSI notes this reduces the voltage-to-earth to 50%, but it also means both conductors are live to earth and must be insulated. The same symmetry can improve EMC and make common-mode EMI filtering (common-mode chokes) easier. Because the bonding/return topology sets the common impedance in the ground network, ETSI’s guidance highlights that this is the path where DC power supply currents can circulate when systems are interconnected, so the earthing/bonding scheme strongly affects common-mode behavior. In ETSI’s preferred arrangement, the mid-point terminal is connected to the Main Earthing Terminal (MET), which ETSI describes as the common earthing point for the mid-point and protective earthing conductor (a defined reference point rather than ad-hoc local chassis earthing).
Considerations: inside the rack
800 VDC is routed to each shelf inside the server rack, and insulation is a major consideration as the requirements are far more onerous than with 48 V. This affects connector/cable ratings and creepage and clearance distances on PCBs and around busbars, requiring changes throughout the rack. One effect is that down-conversion DC/DC stages must be isolated to appropriate safety agency standards, such as IEC/UL 62368-1 system evaluation, depending on architecture. This goes against recent trends to eliminate isolation at this point and means that the entire server rack would need to be evaluated by a safety agency now.
Another effect is that the down-conversion DC/DCs typically have generated an output intermediate bus of 12 V from a 48 V input, regulated or unregulated. However, they must now operate from the increased DC voltage input or utilize series-stacked input stages so that the highest-possible efficiency switching devices can be used. Converters with these specifications are not common for data centers yet, but it may be possible to leverage similar products from electric vehicles in the short-term.
Considerations: everywhere in between
Another key consideration within the rack is that elimination of the 48 V bus removes the natural point to connect traditional 48 V backup batteries, necessitating new approaches to power storage. For lithium-ion systems, moving to a higher-voltage bus is mainly a matter of updating pack architecture, using more modules in series to reach the required DC link voltage. If a system requires short-duration ride-through or very frequent cycling, capacitor-based energy storage such as supercapacitor banks can be a good fit. They are typically low maintenance, tolerant of wide temperatures, and capable of very high cycle life — although they offer shorter hold-up time because they store less energy.
No matter how you choose to store power, 800 VDC also makes it simpler to incorporate onsite power generation. The 800 V bus allows more direct alignment with various microgrid or renewable DC bus architectures, which typically operate in a similar voltage range, making the interface simpler.
