Data Center Part 2: Power, A-side and B-side

You are sold a 30 amp feed. You can use 24 of them. If you also want redundancy, you can use 12.

Power is the part of the datacenter that punishes vagueness hardest, because the failure mode is not a warning in a dashboard — it is a breaker opening and half a cabinet going dark instantly. This part is the arithmetic, the connectors, and the two mistakes that account for most cabinet-level outages.

A-side and B-side, properly

Your cabinet receives two feeds. They are called A and B, and the entire point is that they share nothing upstream: different breakers, different distribution panels, ideally different UPS systems and different generator paths.

   utility ──┬── UPS-A ── panel A ── breaker A ── whip A ──┐
             │                                              │
             │                                       ┌──────v──────┐
   generator ┤                                       │   PDU-A     │
             │                                       │  (0U, left) │
             │                                       └──────┬──────┘
             │                                              │
             │                                     ┌────────v────────┐
             │                                     │  server         │
             │                                     │  PSU1 ─── A     │
             │                                     │  PSU2 ─── B     │
             │                                     └────────^────────┘
             │                                              │
             │                                       ┌──────┴──────┐
             │                                       │   PDU-B     │
             │                                       │ (0U, right) │
             │                                       └──────^──────┘
             │                                              │
             └── UPS-B ── panel B ── breaker B ── whip B ───┘

The whip is the flexible armoured cable that drops from the overhead busway or comes up through the floor and terminates in a socket at your cabinet. The PDU plugs into it.

Colour convention is near-universal and worth trusting: A is usually blue, B is usually red, on the PDU bodies, the whips and often the power cords themselves. It is not a standard, but every facility I have seen documentation for does something like it, and it makes "both cords are in the same side" visible from three metres away instead of invisible forever.

The 80 % rule, which is where the capacity goes

In the US, the National Electrical Code treats anything drawing current for three hours or more as a continuous load. Every server is a continuous load. A continuous load may not exceed 80 % of the breaker's rating.

So a 30 A circuit gives you 24 A of usable continuous draw. Not 30. The last 6 A exists so the breaker does not sit permanently on the edge of tripping from thermal drift.

This is a US code framing — elsewhere the equivalent constraint comes through IEC practice, the PDU's own rating and the facility's contract rather than the NEC — but the practical outcome is the same everywhere: you do not get to use the number on the label. Ask your facility what they will actually let you draw, and get it in writing. Many PDUs will alarm, and some facilities will bill or shut you off, well before the breaker itself opens.

In kilowatts, for the feeds you are most likely to meet:

Three-phase looks like magic until you see the formula: kW = 1.732 × volts × amps. The 1.732 is the square root of three. It is the reason a three-phase 30 A feed carries roughly 73 % more than a single-phase 30 A feed on the same amperage number, and it is why dense cabinets are three-phase.

208 V rather than 120 V is the norm in datacenters and is slightly better than 120 V for the same equipment, because server PSUs run marginally more efficiently at the higher voltage and you get more watts per amp of breaker.

Redundancy costs you half

Here is the part that surprised me most, and it is the single most useful thing in this post.

If the point of A and B is that either one can carry the cabinet alone, then the total cabinet load must fit inside one feed's 80 % limit — not inside the sum of both.

Two 30 A feeds do not give you 48 A of usable capacity. They give you 24 A of usable capacity, delivered over two paths.

In normal operation the load splits roughly evenly, so each feed carries about 12 A — 40 % of its rating. That looks like waste right up until the moment one side drops.

The thing that goes wrong when people ignore this has a name: swing load. Both feeds are running at, say, 70 %. The A-side breaker opens. Every dual-corded server instantly moves its entire draw to the B side, which is now asked for 140 % of its rating. The B breaker opens too. You have just converted a single-feed failure into a total cabinet outage, and you did it by using capacity you were sold but could not actually have.

   Designed correctly (2N):          Designed wrong:
   A: 40%  ████                      A: 70%  ███████
   B: 40%  ████                      B: 70%  ███████

   A fails ↓                          A fails ↓

   A:  0%                             A:  0%
   B: 80%  ████████  holds            B: 140% ██████████████  TRIPS
                                          → whole cabinet down

If you take one number from this series, take this one: in a 2N cabinet, target 40 % per feed and treat 40 % as full.

There is a softer variant. If your fleet is genuinely tolerant of losing a fraction of nodes — stateless web tier, three-way replicated storage — some operators deliberately run primary/primary, loading both feeds to 80 % and accepting that a feed failure kills half the cabinet. That is a legitimate engineering choice and it doubles your usable power. It is only legitimate if it is a decision, written down, with the blast radius understood. Drifting into it by accident is the failure mode described above.

PDU types, and which one you want

PDUs come in four escalating tiers, and the difference is entirely about what you can see and do remotely.

Basic. A metal strip with outlets. No intelligence, no network port, no measurement. Cheap. You have no idea what you are drawing.

Metered. Reports total current draw for the PDU, either on a local display or over the network. This is the minimum I would accept, because without it the 40 % rule above is unenforceable — you cannot manage a number you cannot see.

Monitored. Per-outlet measurement. Now you can tell which server is drawing what, spot a PSU that has quietly failed over, and do capacity planning that is not guesswork.

Switched. Per-outlet relays, so you can power-cycle an individual outlet over the network. This is the one that matters operationally, for a reason covered properly in Part 4: a switched PDU is the only remote reset that works when the BMC itself is wedged. It is the difference between "I can fix this from my laptop at 3 a.m." and "I need to open a remote hands ticket and wait."

Modern units combine tiers — "switched and monitored" is common — and most speak SNMP, plus increasingly a REST API. Get them into Prometheus. Cabinet power draw is a genuinely useful metric and almost nobody graphs it.

The connectors

This is the vocabulary you asked about, and it is smaller than it looks. The IEC 60320 family covers the cords between the PDU and your equipment, and there are really only four pieces.

The pairing rule is trivial once you see it: odd numbers are sockets, the next even number up is the matching inlet. A C13-to-C14 cord runs from the PDU to an ordinary 1U server. A C19-to-C20 cord runs from the PDU to something hungry.

Two practical notes. First, locking variants exist — C13/C14 with a latch, or the C15/C16 heat-rated version — and they are worth specifying, because the most embarrassing outage in this entire domain is a power cord walking out of a socket a few millimetres while someone works in the cabinet next to it. Many datacenter PDUs have a built-in retention clip. Use it. Second, C15 is not C13 with a different name: it is rated for higher temperatures and has a notch, and a C15 plug will not seat in a C13 socket.

The other family is NEMA, which is the plug on the input side of the PDU, going into the facility's whip. You will see:

• L6-30P — the twist-lock plug for a 208 V, 30 A single-phase feed. Extremely common.
• L6-20P — same but 20 A.
• L21-30P — five-wire twist-lock for 208 V three-phase wye, 30 A.
• L15-30P — four-wire, three-phase delta.

The L means locking: you push and twist, and it cannot be pulled out. You will never handle one of these. You will absolutely have to quote one when ordering a PDU, because a PDU with the wrong input plug is a paperweight — and the whip is fixed infrastructure that the facility installed, so the PDU is what has to match.

What to actually do with all this

Four things, in order of how much trouble they save.

Know your budget in kilowatts, not amps. Vendors quote server power in watts. Facilities quote feeds in amps. Convert everything to kW once, at 80 %, and keep it in the same document as the rack elevation.

Add up nameplate, then measure. Nameplate PSU ratings are worst case and are usually far above real draw — a 750 W PSU in a 1U web server might pull 150 W idle and 300 W under load. Plan on nameplate so you never trip a breaker; measure real draw with a metered PDU so you know how much headroom you truly have. Those two numbers can differ by a factor of three, and the gap is where your remaining capacity lives.

Audit the cording. For every dual-PSU machine, confirm one cord is in A and one in B. This is a perfect remote hands job and it is worth doing once as an explicit ticket, with photos, rather than assuming. Ask for a photo of each PDU's outlet strip with the labels visible.

Know which machines are single-corded. Some things — cheap switches, older appliances, small NAS units — have one PSU. In a 2N cabinet these are your true single points of failure, and the mitigation is either a transfer switch (a small unit with two inputs and one output that switches automatically) or accepting the risk deliberately. Either is fine. Not knowing which machines they are is not.

The one-line summary

Two feeds, sized so either can carry the whole cabinet: 40 % each is full. C13/C14 for normal kit, C19/C20 for hungry kit, odd is socket and even is inlet. Get switched-and-monitored PDUs so you can see draw per outlet and power-cycle without a ticket.

Next up: Part 3 — cables, optics and transceivers, which is where DAC, AOC, SFP28 and QSFP28 stop being noise and start being a decision you can make.