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Power Delivery As The Next Physics Wall In AI Datacenters
Brief
Sekar lays out why power delivery is the next “physics wall” for AI datacenters and why the industry is moving toward 800V DC rack architectures. The core physics is simple: for fixed power, lower voltage means much higher currents, which amplify resistive losses (I2R), force massive copper busbars, and erode voltage-headroom. Using his worked example, a 600 kW rack at 48V needs ~12,500 A, a 1% voltage-drop budget implies ~38 µΩ total resistance, ~6 kW dissipated, and roughly 1,000 cu. in (~147 kg) of copper busbar per rack. Moving to 800V reduces current to ~750 A, lets busbars be ~1"×1" (~84 cu. in, ~12.3 kg), and cuts dissipated power to ~0.56 kW.
Those physics drive architectural choices: keep voltages high as long as possible and perform point-of-load (PoL) conversion—vertical power delivery—close to GPUs. Conversion chains run from high-voltage grid lines (100 kV) down to medium/low voltages (10–30 kV substations, 400–480V AC halls) then to rack DC (48V historically, now 800V in new builds) and finally to VRM voltages (~0.8–1V). Each conversion stage adds loss (e.g., 97% per stage → ~91% over three stages), and isolation rules (SELV 60 V DC) force transformer-based topologies above 60 V, shaping which semiconductor and converter designs compete. Early 800V rollouts require greenfield or DC-sidecar approaches (OCP Mt. Diablo, NVIDIA Kyber), and analysts and vendors (Wood Mackenzie, SemiAnalysis, Citrini, ON Semi) expect sharply higher power‑semiconductor content and supply-chain shifts as deployments scale.
Why it matters
Vikram Sekar (Vik's Newsletter, published 2026-05-06) argues datacenters are shifting to 800V DC because resistance and I2R losses make low-voltage (48V) rack power impractical at AI scale.
Key details
- Example math: a 600 kW rack at 48V requires ~12,500 A. With a 1% voltage budget (0.48 V) the allowable resistance is ~38 µΩ; I2R losses at that current and resistance are ~6 kW and busbar copper volume/weight becomes ~1,000 cu. in and ~147 kg per rack.
- At 800V the same 600 kW needs ~750 A. A 1% budget (8 V) yields ~11 mΩ headroom; practical busbars can be 1"×1" (~84 cu. in, ~12.3 kg) and I2R losses drop to ~0.56 kW (order-of-magnitude improvement over 48V).
- Architecture/standards implications: any DC stage above the 60 V SELV threshold requires galvanic isolation (transformer), so 48V sits just below the SELV ceiling allowing non-isolated downstream converters; this shapes conversion topologies and vendor sockets.
- Market and implementation notes: early 800V deployments (OCP Mt. Diablo, NVIDIA Kyber racks) use DC sidecar racks; analysts (Wood Mackenzie, SemiAnalysis, Citrini) and ON Semiconductor (CEO: ~10× power‑semiconductor content at 800V) flag a structural supply-chain inflection.