PSU sizing and power-limiting for multi-GPU builds

A multi-GPU box asks three things of its power: enough watts from the PSU, the right number of cables to the cards, and a wall circuit that can carry the total for hours. The Catalog’s PSU table answers the first with one number, “GPUs supported”. This guide shows where that number comes from, where it stops being enough, and what the other two questions add.

How “GPUs supported” is counted

Every PSU in the Catalog shows GPUs supported = floor((rated W × 0.8 − 250) ÷ 300), and never less than 0. Three constants, each a choice this site made, not an industry standard:

• 0.8, the load factor. Only 80 % of the label rating is counted as usable. The other 20 % is headroom for the short spikes below, and keeps the unit away from running flat out for hours.
• 250 W, the system reserve. Watts set aside for the CPU, board, RAM, drives and fans before any GPU. AMD rates the Ryzen 9 9950X of the 48 GB build at a 170 W default TDP, which leaves 80 W for everything else. It is an estimate for a desktop or single-socket platform, not a measurement.
• 300 W per GPU. An RTX 3090 rated at 350 W, power-limited to 300 W with nvidia-smi -pl 300. That is the limit techpractice advises for four 3090s and the one every multi-GPU Tier on this site sets. The count assumes you apply it. Buying the cards used? The used RTX 3090 checklist includes a half-hour load test, which is also a good moment to read each card’s power limits.

The result is rounded down, so a part-GPU of headroom counts as none.

In the Catalog, 1200 W is the Corsair HX1200i, 1500 W the HX1500i, and 1600 W the Corsair AX1600i, the Seasonic PRIME TX-1600 and the be quiet! Dark Power Pro 13. The 2000 W Super Flower Leadex Platinum and the 2200 W Seasonic PRIME PX-2200 take 200 to 240 V input only (see the mains section below).

The 750 W row lines up with NVIDIA, which asks for a 750 W PSU for a single RTX 3090 system. A 1600 W unit buys no more GPUs than a 1500 W one under this count; it buys 80 W more headroom.

When to redo the arithmetic

The formula is built for power-limited 3090-class cards. With other cards, keep the 0.8 and the 250 W and put your card’s own figure where the 300 W goes:

• Unlimited 3090s. At NVIDIA’s 350 W, the HX1200i gives (960 − 250) ÷ 350 = 2.03, so still two, but with 10 W to spare: two cards plus the reserve come to 950 W against 960 W. That is why the 48 GB build sets the limit anyway.
• RTX 4090 at 450 W or RTX 5090 at 575 W. A 1600 W unit gives (1280 − 250) ÷ 450 = 2.29, so two 4090s, or 1030 ÷ 575 = 1.79, so one 5090, against three on the formula.
• Small cards. The 550 W row says 0, yet NVIDIA asks for a 550 W PSU for an RTX 3060 at 170 W. With 170 W in place of 300 W, 190 ÷ 170 = 1.12, so one card. The 0 is the formula being conservative for a card it wasn’t built for; the Starter build runs a 3060 on a stock 460 W unit, with about 280 W reported for the whole machine.

Transient spikes, and what a power limit doesn’t do

A GPU’s draw is not flat. Igor’s Lab warns that the RTX 3090’s short load peaks, at intervals of 1 to 10 ms, can trip a PSU’s protection and shut the system down even when the average is in spec. hav4ik reports his 3090s’ “power draw sometimes spikes up to 600W for brief periods”, and bought a 1600 W PSU for that reason. The 20 % the load factor holds back is for these, and it only stretches so far. Two cards at 300 W plus the reserve put 850 W on an HX1200i, leaving 350 W of its label rating: one card spiking to hav4ik’s 600 W uses 300 W of that, but two spiking at once would need 600 W, past the rating. What then decides is how the PSU handles excursions. Wikipedia’s ATX article says ATX 3.0 requires a unit to handle spikes of two times its nominal output for 100 microseconds; that window is shorter than the 1 to 10 ms Igor’s Lab describes, so it is no guarantee. The Catalog’s PSU table shows which units are ATX 3.x: the HX1200i, HX1500i, PRIME TX-1600, Dark Power Pro 13 and PRIME PX-2200 are, the AX1600i is not, and Super Flower states nothing for the Leadex Platinum 2000W.

nvidia-smi -pl 300 sets a card’s power limit in watts; it needs root, and nvidia-smi -q shows the card’s default and current limits. The limit does not survive a reboot or a driver reload. Puget Systems reapplies it at boot on a quad-3090 machine with a systemd service that turns on persistence mode (nvidia-smi -pm 1) and then sets the limit. Do the same, and check after every driver update that it still runs; the headless setup guide has a minimal service file. What a limit saves over a day is in the power and noise guide.

A power limit caps the average, not the millisecond peaks. jbarrow points out that nvidia-smi -pl 250 “does not set a hard cap at 250W” and recommends also limiting clock speeds, for example with nvidia-smi -lgc 210,1500, to avoid tripping the PSU’s overcurrent protection. That costs some speed; we have no measurement of how much.

Connectors: 8-pin and 12V-2x6

Count connectors before watts. Wikipedia’s PCI Express article gives 75 W from a x16 slot and 150 W from each 8-pin auxiliary connector, and 600 W for the 16-pin 12VHPWR connector, which 12V-2x6 revised.

• RTX 3090 Founders Edition: two 8-pin cables through the 12-pin adapter in the box, 2 × 150 W plus 75 W from the slot against its 350 W.
• Partner RTX 3090s: many take three 8-pin cables. Two of them on the 96 GB build’s HX1200i need six cables against its five, and four of them need twelve. Rebalance which cards hang off which PSU, or pick the PSU by its connector count, not only its wattage.
• RTX 4090: NVIDIA calls for three 8-pin cables to its adapter (it supports three or four) or one 450 W or greater PCIe Gen 5 cable.
• One cable per input. Run a dedicated cable from the PSU to each 8-pin input; don’t feed two inputs from one cable’s daisy-chained pigtail.
• What the PSUs have. The HX1200i and HX1500i list five 8-pin PCIe connectors and two 12V-2x6 cables, but the 12V-2x6 cables plug into the same PSU sockets, so you can’t use both counts in full. The AX1600i lists ten 8-pin connectors and no 12V-2x6, so 12V-2x6 cards need adapters. Four Founders Edition 3090s need eight 8-pin cables, more than any single HX unit has.

The Catalog’s PSU table shows both counts for every unit.

Two PSUs: sync and the back-feed rule

When one unit can’t cover the cards, or can’t run on your mains (below), a second PSU feeds some of the cards. The 96 GB build does this: an AX1600i powers the board, CPU, drives and two cards; an HX1200i feeds only the 8-pin inputs of the other two. The two must switch on and off together. The Thermaltake Dual PSU 24Pin Adapter does that from the board’s 24-pin connector; the level1techs build uses an Add2PSU relay instead.

Split the load by what each card draws from where. A card’s slot power (the PCIe and risers guide covers how it reaches a card on a riser), up to 75 W, comes through the board, so from the first PSU, even when its 8-pin cables hang off the second. In the 96 GB build that puts about 1000 W on the AX1600i (250 W system, 2 × 300 W, 2 × 75 W of slot power) and about 450 W on the HX1200i, each inside its own count.

The level1techs thread is about the risk of current flowing the wrong way between two PSUs. One reply reports: “I have actually cooked a pair of 3090s with power flowing the wrong way through them when the system was “shut off””. The thread doesn’t settle how big the risk is, so treat it as real:

• Keep all of a card’s 8-pin cables on one PSU, and feed a powered riser or slot adapter from the same PSU as its card.
• Never power one PSU off or unplug it while the other is on. PSUs keep standby power live while plugged in, so when the machine is off, switch off or unplug both together.
• Keep both on the same circuit, so one breaker can’t trip and leave the other unit feeding half the machine.

The wall circuit

The PSU delivers DC; the wall supplies more than that. Divide by the PSU’s efficiency: 80 Plus Platinum requires 89 to 92 % between 20 % and 100 % load on 115 V, so at an assumed 90 % the 48 GB build’s 850 W at load (2 × 300 W + 250 W) is about 944 W at the wall from its HX1200i.

Then compare with the circuit. For the US, Michael Bluejay gives the rule that a continuous load (one running more than three hours) stays under 80 % of the circuit’s rating:

• US 120 V, 15 A: 1800 W, so 1440 W continuous. 20 A: 2400 W, so 1920 W.
• UK 230 V: a plug takes at most a 13 A fuse, about 3.0 kW per plug; check the fuse actually fitted. German Schuko: a socket is rated 16 A, about 3.7 kW. We know of no matching 80 % rule for those; this site keeps the same 80 % margin there anyway, which still leaves room, but the circuit behind a socket is shared with others.

Two GPUs fit a US 15 A circuit: jonready measured his two-3090 machine peaking at 1.04 kW at the wall. Four do not fit comfortably: the 96 GB build’s estimate is up to about 1.57 kW at the wall with all four cards busy at 300 W, over the 1440 W line; at 250 W per card it is about 1.35 kW. Those figures assume 93 % efficiency for the AX1600i and 91 % for the HX1200i at their loads (the 96 GB build’s Power & noise section shows the working). jbarrow ran four power-limited 3090 Tis from one 1650 W PSU and still tripped the breaker: “It only tripped when I was running an OCR job while making tea.” By this site’s count a 1650 W unit supports three GPUs ((1320 − 250) ÷ 300 = 3.57), so his build ran past it; his page doesn’t give his per-card limit, and what tripped was the circuit, shared with a kettle, not the PSU. On 120 V, a four-card box wants a lower limit or a 20 A circuit. Don’t run two PSUs from one power strip either; they then share the strip’s rating, which is often lower than the circuit’s.

Mains voltage changes the rating

A PSU’s label rating can depend on input voltage. Corsair’s power table for the HX1500i reads “1200W (100V - 115V)” and “1500W (115V - 240V)”; the two ranges meet at 115 V, so on 100 to 115 V mains plan on 1200 W, and it counts as an HX1200i. For the AX1600i Corsair says it “delivers 1600 watts of continuous power on 15A circuits of 115V and higher” and states no rating below 115 V. The 2000 W Super Flower and 2200 W Seasonic take 200 to 240 V only and won’t run from a standard US outlet. By wattage, on 230 V mains, either can carry four power-limited cards alone (four and five GPUs supported), which removes the second PSU and its back-feed risk. By connectors, only one can: the Super Flower lists fourteen 8-pin connectors, enough for four 3090s with two 8-pin inputs each, but the PX-2200 lists four 8-pin connectors plus four 12V-2x6 cables, two of which share the 8-pin sockets, so it cannot feed four such 3090s natively. On 120 V, two synced units are the way to four cards.

Parts in this guide

Store links may earn us a commission at no cost to you. As an Amazon Associate we earn from qualifying purchases.

Related

• PSU
• 48 GB build
• 96 GB build
• Corsair HX1200i (2025, ATX 3.1)
• Corsair HX1500i (2025, ATX 3.1)
• Corsair AX1600i
• Seasonic PRIME TX-1600 (ATX 3)
• be quiet! Dark Power Pro 13 1600W
• Super Flower Leadex Platinum 2000W
• Seasonic PRIME PX-2200 (ATX 3.1)
• Thermaltake Dual PSU 24Pin Adapter Cable (AC-005-CNONAN-P1)

Last reviewed 2026-11-12