stacking pdu

Is there a way to stack PDUs? like, with 30A 220, we need more plugs
than power but I'd like them to communicate to make sure we don't over
power the circuit. Do any APC or Triplite systems support this?

How many plugs and what style do you need? What is the facility
providing receptacle wise? L15-30A three-phase ?

Isn't it against the NEC and the fire code to stack power strips? We
all do it, but isn't it against code?

Bill Herrin

It sounds like he's asking for a PDU specifically intended to be used with managed expansion/stacking type modules. That would be permissible per NEC and most local fire codes as the device would be specifically listed for such usage. I've seen something like this inside blade/modular racks like IBM Flex systems, but unfortunately I don't know of any such off-the-shelf parts.

It would seem entirely reasonable to take a 30A/240V feed on a 6-30 or 14-30 type connection and break it out to numerous either C13 or 6-15/5-15 style sockets, possibly on a modular basis using some sort of proprietary power+data connections for the breakout. E.g. if you needed to mix 120V and 240V devices in a rack, you could get 5-15 and 6-15 breakouts with each individual outlet switched and maintain whole-rack metering.

Sounds like a really handy device, if one exists COTS.

You could run a PDU in paralallel so that you don't use more current than
the wires are rated for (although the PDU should trip the circuti anyways
in case you overload it). Only problem is matching the receptacles. You
probably don't want to half-ass it, so I'd just add an extra PDU and run an
extra ethernet cable so you can monitor it.

If I read it correctly, stacking PDUs is perfectly acceptable so long as the breaker feeding each subordinate PDU is sized no larger than the maximum current that can be safely delivered to that PDU.

However, adding extension cords or other “temporary power extenders” on a permanent basis is also unacceptable and most PDU stacking of the type being contemplated here involves deploying the subordinate PDU in a way that ends up being essentially an “extension cord” for this purpose.


William Herrin <> writes:

Isn't it against the NEC and the fire code to stack power strips? We
all do it, but isn't it against code?

Sorry to be late to the party (I plead vacation), but no, afaik it is
not. About as close as the NEC comes art 400.8 - you can't use
flexible cord as a substitute for permanent wiring (think of some of
the shenanigans you've seen with extension cords standing in for NM or
MC on or similar sites).

Rack wiring is not "permanent", but I would not go so far as to claim
it is subject to the "qualified personnel" rules (OSHA subpart S and
NFPA 70E). Datacenter workers who could pass a test on LOTO
procedures and routinely utilize proper PPE (even gloves, safety
glasses, and steel toe shoes) are the exception rather than the rule.

As always, when someone asserts that "X is against code" whether in
the form of a statement or a question, the proper response is
"Citation, please!"


This takes me back to the days of old with bread racks full of modems and
the mess of wall-warts and power-strips.


MC on or similar sites).

iftfy. :stuck_out_tongue_winking_eye:


Well, I was kinda thinking this would turn out to be a dumb question / have
an obvious answer. Apparently not. But it seems I can't go buy a solution
either. I guess there isn't much of a market (though I am just talking
software - maybe someone could make an update :slight_smile: ).

The fire marshal that regularly inspects our building will cite us if he sees an extension cord in use - even temporarily - or sees a temporary power tap/surge suppressor connected to another. Meanwhile, in another city, I see government and commercial buildings violating these rules for years. Perhaps there's some amount of interpretation allowed or some inspectors are more aggressive than others.


I was dinged for power strips connected to a cube tap. I don't have
the citation handy, but I looked it up at the time and it was
definitely against code.

Bill Herrin

Or the local ordinances block daisy-chaining. I've run into this in several parts of the country, while other parts don't have local regulations -- particularly in "commercial" spaces, which include offices.

Every Authority Having Jurisdiction (AHJ) is their own fiefdom. Although
there are a few model national codes, its the locally enacted law and AHJ
interpretation that rules.

And, yes, the effectiveness and knowledge of AHJs varies greatly. It wouldn't surprise me if there were some places with no building codes
or inspectors.

APC does make some 'half rack' PDU's that take a C20 inlet so they could
hang off a C19 outlet on another PDU:

On the software side, just use a "master" PDU with metering. These "sub"
ones are also metered but you would want to look at the total utilization
on the "master".

No comment if its to code...

I was pretty much thinking the same, get a switched/metered outlet PDU. APC, ServerTech, et al have them, then daisy chain something like a Dell AP6015 off the outlet. No clue about NEC/local laws, but the Dells are pretty much setup for that type of setup.

Server tech makes some devices that are powered independently but can be managed as a single unit. We have 2pdus per cabinet on separate circuits, which share a management controller and can provide stats for both circuits.