[BBLISA] PDU Power Ratings?

[Bob Webber]

On Apr 2, 2014, at 12:30 PM, Scott Danek <scott_danek at terc.edu> wrote:

> Hi all,
> 
> We're looking into increasing our power in our colocation from 120V 20 amp to 120V 30 amp so that we can add more equipment. When looking for PDU's it seems that a PDU that will take a 120V input and output 30 amps is uncommon. Most 30 amp PDU's we've seen seem to take a 200V+ input. If we got a PDU that lists a 200V or 208V input and output, and plugged it into a 120V circuit, what would be the outcome? Would that be ill-advised? Thanks!

Yes, that would be ill-advised because it would be expensive and pointless.

In that case, the PDU would be unlikely to work, but would be unlikely to fail catastrophically. You probably will not be able to plug such a PDU into a 120V circuit without replacing the plug, which will be a strong clue that what you are doing is a very bad idea. You want a PDU that takes 208V or higher input and produces 110V output.

Another alternative would be to get the colo owner to provide a second 120V branch circuit for your use. Then you will need to manage your loads and distribute them across two 120V PDUs, each connected to its own branch circuit.

Many commercial buildings, and presumably your colo’s facility will be able to provide you with an appropriate 208V branch circuit and outlet. In household/domestic situations, 240V is more common. 

In specifying your power requirements to the colo you will want to know what kind of plug your PDU will have, some kind of “NEMA P-999” (where the “9”s are place holders) plug. There are a bunch of different types.

The reason it’s not a big deal to provide 208V or 240V is that buildings usually have more than one 120V circuit. 

Household 120V AC service connects the load to a wire carrying 120V at 60Hz to a neutral wire which is nominally at ground potential, electrically. A 240 V appliance load is connected between two “live” 120V AC 60Hz wires, where the sine wave of AC voltages of the two live wires are 180 degrees out of phase. So when live wire A is at +120V, live wire B is at -120V and the voltage across the load is 240V.

A typical 208V supply connects to wires carrying two of the three phases of a 110V 3-phase circuit. In this case the sine waves of the AC voltages on the two wires connected to the load are 120 degrees out of phase and the maximum voltage across the load is 208V.
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