Rack PDUs: distributing power inside the cabinet

A rack PDU is a strip of outlets with a plug on one end, which is why it is often treated as an accessory. It is in fact the point where the facility’s power design meets the equipment that uses it, and the place where a design that looked fine on a single line diagram turns out to have stranded half its capacity.

The nameplate is not the usable capacity

A circuit’s rating is not the amount of load you can plan to put on it continuously. Electrical code treats a load that runs for extended periods differently from an intermittent one, and the practical effect is that the capacity you can design to is meaningfully less than the number on the breaker. The exact requirement is in the Canadian Electrical Code, Part I, currently CSA C22.1:24, and the authority having jurisdiction has the final word.

This matters in a cabinet because it is the difference between planning to a number that exists and planning to one that does not. A rack designed to the full circuit rating will either trip or will have to be de-loaded later, and de-loading later means moving equipment.

Know what happens on one feed when the other is gone

Most critical cabinets are fed twice, A and B, with dual-corded equipment drawing from both. The question that decides whether that redundancy is real is simple: if one feed disappears, can the remaining feed carry everything in the cabinet?

If each side is loaded past half its usable capacity, the answer is no, and the failover that was supposed to protect the cabinet is what takes it down. Single-corded equipment makes this worse, because it has to be fed from one side or put behind a transfer switch, and either choice needs recording rather than assuming.

The other half of this is knowing which outlet is on which feed. That information survives in documentation or it does not survive at all, and cabinet work is exactly when it gets lost.

Metering is how you find stranded capacity

Four levels of instrumentation are common, and they answer different questions.

A basic unit distributes power and tells you nothing. An input metered unit tells you what the whole strip is drawing, which is enough to see whether a cabinet is near its limit. Branch metering tells you the same per circuit, which is what you need when a strip carries more than one. Outlet metering tells you what an individual device draws, which is what you need to charge a tenant, to find a machine that is idle, or to prove which device caused a step change.

The reason to care is that rooms routinely run well below their design capacity while individual cabinets sit near their limit. Without per-cabinet numbers that imbalance is invisible, and the usual response is to buy capacity that already exists somewhere else in the room. If metering is going to be used for billing, the accuracy figure is part of the specification rather than a detail.

Switched outlets are an operations decision

A switched PDU can power an outlet on or off remotely, which is genuinely useful for a remote site where the alternative is a drive. It also introduces two questions worth answering before purchase.

What is the outlet state after the unit loses and regains power? A default that energises everything at once can present a large inrush, and a default that leaves everything off can leave a site dark after a utility event.

Who can operate it? A device that can cut power to production equipment over the network is a control system, and it deserves the same treatment as one: restricted access, supported firmware, and a change record. A PDU on a flat network with default credentials is a way to turn a minor intrusion into an outage.

Plugs, outlets and the mundane failures

Most of what goes wrong with a rack PDU is not electrical theory.

The input plug does not match the receptacle at the position, so the unit is on site and unusable. The outlet mix is wrong, so half the cabinet needs adapters, and adapters are the things that work loose and heat up. The vertical unit fouls the cabinet’s cable management or the rear door will not close. A three-phase unit is installed without anyone tracking which outlets sit on which phase, so the cabinet becomes unbalanced as it fills and one phase reaches its limit while the others idle.

None of these is difficult to avoid. All of them are expensive to fix after the equipment is racked, because fixing them means unracking it.

Certification and what to ask a vendor

A rack PDU installed in Canada falls under CSA C22.2 No. 62368-1, the harmonized standard with UL for information and communication technology equipment, which applies to equipment rated up to 600 V. The current edition is based on the fourth edition of IEC 62368-1. Certification to it is a reasonable thing to ask a vendor to evidence, and the answer should be a certificate rather than a statement on a datasheet.