±400 VDC vs 800 VDC
At high rack power, current drives I²R losses, breaker frames and copper mass. Compare bipolar ±400 VDC and two-wire 800 VDC offers against the named AC route before committing to DC. Bipolar ±400 VDC puts the full 800 VDC span across a load, so it carries the same ideal current as a two-wire 800 VDC output at equal delivered DC power. An AC comparison also needs phase and conductor counts, power factor, route resistance, temperature and conversion losses; there is no universal halving of AC feed current. Grounding, fault clearing, service isolation and the complete conversion chain decide which offer works.
| Axis | ±400 VDC | 800 VDC |
|---|---|---|
| The bet | use a midpoint-referenced output when its normal and fault rail-to-earth duties, detection and isolation meet the rack's requirements; select solid or high-resistance grounding explicitly | use a two-wire output when its specified end bond, high-resistance ground or isolation scheme meets the same requirements; the voltage label does not select an SST |
| Reference design | Meta / OCP Mt. Diablo v0.7.0: 800 VDC two-wire (optional; isolated from PE). Read the offered midpoint bond or isolated two-wire option; the project name alone does not select grounding | NVIDIA Kyber-class roadmap: ~600 kW on its dated roadmap, not an installed rating; qualify the offered grounding, conversion and rack interface |
| Copper / current | 800 VDC across the full-span load gives the same ideal current as the two-wire option at equal DC power; separately account for any midpoint load and return conductor | NVIDIA's +157% power-per-wire-gauge comparison uses three-wire DC (POS/RTN/PE) versus its named four-conductor AC reference; it does not establish an equal-conductor-count advantage for both arrangements |
| Pole-to-ground stress | an ideal solid midpoint sets ±400 VDC to earth normally; qualify fault clearing and insulation duty separately, and study the different first-fault behavior of high-resistance grounding | bonding one end places the opposite pole at the full bus voltage to earth; isolated and high-resistance schemes need their own first-fault current, detection and insulation study before crediting continued operation |
| Conversion chain | an AC-fed sidecar retains the actual upstream transformer and UPS where fitted, then AC/DC and downstream DC/DC conversion; compare every included package at the same output boundary | direct MV/DC can package transformer and rectification functions, but rack DC/DC and final conversion remain. SST prototype evidence: ~98%. SemiAnalysis's ~87% utility-to-VRM model is separate from the Chapter 4.1 guide fixture |
| Parts maturity | EV/industrial breakers, connectors and insulation systems are candidate supply; qualify installed voltage, DC interruption, isolation, thermal duty and service procedure before counting them as available | require the same offered-part evidence for full pole-to-earth duty and clearing behavior; qualification and delivery dates decide whether the component pool meets the project schedule |
| Where AC stays | retain AC when its named rack interface, current, thermal, fault and service envelope closes at the required usable capacity, cost and delivery date | apply the same test to the competing DC offer; retain AC for other loads whose qualified interface and lifecycle cost favor it |
Quantitative cells are the guide's canonical figures — each is date-stamped and sourced in the numbers register and derived in the chapters below.
How the decision falls
Choose either DC offer only after its normal and first-fault behavior, clearing, service isolation and usable capacity meet the design basis. Compare route and conversion loss at the same output boundary, installed and lifetime cost, and delivery against each other and the qualified AC route. A midpoint bus wins when those conditions close with the better cost and schedule; a two-wire offer wins when its qualified interface and complete conversion route do better. Neither a brand nor an EV parts catalogue proves that outcome.
What would flip it: Reverse the selection if the chosen offer misses its required fault/service qualification or delivery date, or if the alternative meets the same usable-capacity boundary at lower lifetime cost. Credit a sidecar or SST loss/footprint advantage only from the offered complete route; an SST package alone does not eliminate downstream conversion.
Model this fork with your own numbers: Build-cost calculator →
Full derivations, worked examples, and the numbers behind this matrix: The DC power revolution (Ch 4.7) · The I²R wall: busway to rack (Ch 4.6) · Transformers and the SST (Ch 4.4)