Electrical codes and plugs/sockets was Re: [Beowulf] Opteron cooling specifications?

Bruce Allen ballen at gravity.phys.uwm.edu
Mon May 1 12:00:41 PDT 2006

In our experience it's a very good idea to observe the "don't operate a 
circuit at more than 80% of rated capacity".  We just had to replace a 
150A 3-phase breaker which died after being operated a bit too close 
(within 10A) of its rated capacity for a number of months.


On Mon, 1 May 2006, Jim Lux wrote:

> At 08:41 AM 5/1/2006, Mark Hahn wrote:
>> >
>> > We've being switching cabinets in one of our datacenters to 220volt
>> > service to support the sort of density we're seeing without running new
>> > conductors.
>> sure, there's no downside to 220 afaikt.  one thing I don't fully
>> understand is where the 80% figure comes from.  just a fudge-factor,
>> and what if you wind up always using that extra capacity?
>> presumably an L6-30 circuit is actually safe to run at 30...
> The electrical code usually requires that a circuit be wired (and overcurrent 
> protected) so that the expected load is no more than 80% of the circuit 
> ampacity.  If you don't have a specific design, then the expected load is 
> calculated by using a set of rules that turn square feet of floor space and 
> what that space is used for (offices are different than residential are 
> different than a parking lot).  There's also rules for certain specific kinds 
> of loads and locations (i.e. you have to have 20 Amp circuits for a 
> refrigerator in a kitchen) and rules about what kinds of loads can be shared 
> (no combining the lights and the receptacles in a kitchen).
> "L6-30"  is a NEMA designation for a particular form of pins in the 
> plug/receptacle. "L" for locking. 6 for the particular configuration of pins, 
> and it's nominal usage: The 30 refers to convention for what that 
> plug/receptacle configuration is used for (i.e. a 30 Amp circuit, intended 
> for use at 24 Amps)
> Bear in mind, also, that the various configurations will have designs 
> compatible with the nominal use. For instance, a configuration designed for 
> 30A service will accomodate AWG10 or AWG8 wires, typically larger on the 
> receptacle (female) than on the plug (male).  There's also voltage ratings 
> that apply to each configuration. That is, a connector designed for 480V 
> service will be rated for 600V, and have different clearances internally than 
> a connector designed for 120V service, rated for 300V.
> It is not unheard of to have NEMA plugs and receptacles used on circuits with 
> ratings different than the nominal. For example, some of the other 4 wire 
> connectors for 277 or 480V are used on 230V circuits, just to provide 
> different, non-inadvertently-interchangeable pin configurations.  For 
> instance, you might have a motor controller that is supplied with a L15-30 
> (30A, 3ph, 250V rating) from the wall, and the motor is connected using a 
> L17-30 (which is normally on a 480V 3ph line).  This is why you should always 
> check before blindly plugging something in.
> James Lux, P.E.
> Spacecraft Radio Frequency Subsystems Group
> Flight Communications Systems Section
> Jet Propulsion Laboratory, Mail Stop 161-213
> 4800 Oak Grove Drive
> Pasadena CA 91109
> tel: (818)354-2075
> fax: (818)393-6875
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