Rack PDUs for GPU Servers: Metered vs Switched, C19 and 3-Phase

The rack PDU is the last thing ordered for a GPU deployment and one of the first things to cause a delay. The servers arrive, the power cords are C19 to C20, and the PDUs in the rack have two C19 outlets each. Or the outlets are right but a single-phase feed cannot carry the load. Three decisions settle it: metered or switched, how many C13 and C19 outlets, and single-phase or 3-phase input.

Metered vs switched PDU: which should I buy?

A basic PDU is a power strip for a rack. A metered PDU adds current measurement, shown on a local display and, on networked models, reported remotely. A switched PDU adds remote on and off control of individual outlets, usually with metering included. Metered-by-outlet models sit between the two, reporting each outlet individually, which helps with per-server power accounting and spotting a supply that is not sharing load.

• Choose metered for most data center racks. The value is knowing how much of each feed you are using before a new server trips a breaker, and having real numbers for capacity planning. Examples: the Eaton ePDU G3 Metered 0U, the Vertiv Geist GU10012L and the APC AP7822B, all rated 7.4kW, and the 120V Tripp Lite PDUMH15 at 1.4kW.
• Choose switched when you need to reboot hung equipment remotely, stagger power-on to limit inrush current after an outage, or lock unused outlets so nobody plugs in unplanned load. That matters most at edge sites, remote colocation and labs. Examples: the APC AP7921B (8 x C13, 3.7kW), the APC AP8958, the APC Easy PDU EPDU2116S and the 3-phase APC AP8886.
• GPU servers have out-of-band management through their BMC, so a switched PDU is less often the only way to recover a node. It is still the last resort when the BMC itself stops responding.

Do GPU servers need C19 outlets?

Usually, yes. IEC C13 and C14 connectors are rated 10A (15A under North American UL listing), and C19 and C20 connectors 16A (20A under UL). As an illustration only: a 2,700W power supply at full output on a 208V feed draws more than 13A once efficiency losses are included, which is beyond a C14 inlet's rating. That is why high-wattage server supplies generally use C20 inlets, which must plug into C19 outlets. One-rack-unit servers, switches and storage generally use C13.

Now count. A GPU server in 4U or larger commonly has four or more power supplies, split across the A and B feeds. Two such servers can need eight C19 outlets across two PDUs. Look at what typical PDUs carry:

• Eaton ePDU G3 Metered 0U and Vertiv Geist GU10012L: 20 x C13 and 4 x C19 each.
• APC AP8886 switched 3-phase: 36 x C13 and 6 x C19.
• APC AP8659EU3: 21 x C13 and 3 x C19.
• APC AP8958: 7 x C13 and 1 x C19.

For a GPU rack, the C19 count is almost always the binding constraint, not the total outlet count. Adapting C20 cords into C13 outlets is not a fix: it overloads the outlet's rating. Plan PDUs from the C19 count up, and confirm the power cord type each server ships with.

When do I need a 3-phase PDU?

When the rack's load is more than one single-phase feed can deliver. PDU capacity follows from the input: voltage times current for single-phase, and voltage times current times 1.732 for 3-phase. North American practice also limits continuous load to 80% of the breaker rating. The PDUs in our range line up with those numbers:

• 120V 15A single-phase: the Tripp Lite PDUMH15 is rated 1.4kW. Suitable for network gear and a few small servers.
• 230V 16A single-phase: 3.7kW, as on the APC AP7921B with its IEC C20 inlet.
• 230V 32A single-phase: 7.4kW, as on the Eaton ePDU G3 and Vertiv Geist GU10012L (both on IEC 60309 plugs), the APC NetShelter APDU10150ME (32A) and the AP7822B.
• 208V 3-phase 30A on a NEMA L21-30P plug: the APC APDU9965 is rated 8,600W, the common US 3-phase rack feed.
• 230V 3-phase: the metered 0U APC AP8881 is rated 11kW.
• 400V 3-phase 32A: 22kW, as on the switched APC AP8886.

A single multi-GPU server can draw several kilowatts, so a rack of them quickly outgrows single-phase feeds. 3-phase delivers more power per circuit and per cable, which keeps the number of PDUs and whips manageable. The trade-off is balancing: the load must be spread evenly across the three phases, which means planning which server supplies plug into which outlet bank. 3-phase PDUs group outlets by phase and branch circuit breaker for this reason. For US 120/208V feeds, the APC APDU9970 is a switched 0U unit rated 30A at 120/208V. Browse inputs on our 400V 3-phase 32A and 230V 32A IEC 60309 pages.

For a GPU rack, plan PDUs from three numbers: C19 outlets needed, kilowatts per feed, and whether that fits one phase.

How do I size PDUs for redundant A and B feeds?

With dual power supplies split across two PDUs on independent feeds, each PDU must be able to carry the entire rack if the other feed fails. In normal operation that means each PDU should run at no more than about half its rated capacity. A rack with a 7.4kW PDU on each feed can therefore support about 7.4kW of total load, not 14.8kW. Check this against the server's power supply redundancy mode too: a server configured so that all supplies are needed at peak will not survive losing a feed, however large the PDUs are. Also watch the branch breakers inside the PDU: a 32A or 3-phase PDU splits its outlets into banks, each behind its own breaker, so one bank can trip while the PDU as a whole still has headroom. Spread GPU server supplies across banks as well as across feeds.

What else should I check before ordering?

• Inlet and plug: C20 inlets (see the 230V 16A C20 input page), IEC 60309, NEMA L21-30P or hardwired. The facility's receptacle has to match.
• Mounting: 0U vertical PDUs mount in the rack's side channel and carry more outlets; 1U and 2U horizontal units such as the APC AP7820B (8 x C13, 12A at 208V) use rack space.
• Rack depth and width: tall 0U PDUs need the channel space, and deep GPU servers can block access to outlets if the rack is too shallow.
• Networking: metered and switched models on a network port need an address, credentials and monitoring integration planned in advance.
• Cord lengths and retention: C19 cords are heavier; locking cords or retention clips help in high-vibration, high-density racks.

Our guides to cabling high-density GPU racks and H100 server power and cooling cover the rest of the rack plan. Nexus Compute supplies new metered, switched and 3-phase rack PDUs from APC, Eaton, Vertiv and Tripp Lite through authorized distribution, with the full manufacturer warranty. Send your server models, feed type and rack layout, and we will match PDUs to the outlets and load and quote within 48 business hours.