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

Jim Lux James.P.Lux at jpl.nasa.gov
Mon May 1 09:50:33 PDT 2006

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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