±400 VDC vs 800 VDC
At the upper end of the rack-power roadmap, the AC chain hits a current wall — I²R losses, breaker frames, and copper mass stop scaling. The ±400 / 800 VDC fork is no longer whether to go DC but which DC, and the two camps are betting on different things.
| Axis | ±400 VDC | 800 VDC |
|---|---|---|
| The bet | ride the lower-voltage EV supply chain at automotive volume | collapse the chain: one step from MV to the rail |
| Reference design | Meta / OCP Mt. Diablo bipolar sidecar | NVIDIA Kyber-class (~600 kW racks) + solid-state transformer path |
| Copper / current | halves current versus conventional rack AC for the same power | +157% power through the same copper versus conventional rack AC |
| Conversion chain | more stages than the higher-voltage path, fewer than legacy AC | SST prototype: medium-voltage AC → the rack DC rail at ~98%; ~87% utility-to-VRM end-to-end |
| Parts maturity | breakers, connectors, insulation class all shipping today | protection and arc-fault engineering on the higher-voltage path is genuinely new |
| Where AC stays | — | both camps agree: where conventional AC distribution closes for the named rack and for non-AI loads, it remains the default |
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
The lower-voltage path buys a supply chain; the higher-voltage path buys the shortest chain. If your roadmap stays in the range conventional parts can close, the EV parts bin is the pragmatic play; if you are underwriting ~600 kW Kyber-class racks, the single-step architecture is what the density demands.
What would flip it: If the higher-voltage component ecosystem (breakers, connectors, protection) ramps slower than Kyber-class racks need it, the lower-voltage path holds the volume longer than the roadmaps say.
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)