[Beowulf] Quick question...

Robert G. Brown rgb at phy.duke.edu
Sun Mar 18 06:31:35 PDT 2007

On Sat, 17 Mar 2007, Jim Lux wrote:

> (3) Receptacle Ratings. Where connected to a branch circuit supplying two or 
> more receptacles or outlets, receptacle ratings shall conform to the values 
> listed in Table 210.21(B)(3), or where larger than 50 amperes, the receptacle 
> shall not be less than the branch circuit rating.
> Table 210.21(B)(3)
> Circuit Rating  Receptacle Rating
> 15              not over 15
> 20              15 or 20
> 30              30
> 40              40 or 50
> 50              50

A big round of thanks to all who replied.  A last second google before
working turned up this code table, so I wired it correctly with an L5
20P matching an L5 20R matching a 20A breaker.

Of course now it all makes sense thanks to the "weedy cord" replies.
I tend to think of faults in the device, which are almost certainly not
going to depend on the load capacity of the breaker, but the power cable
having a window where it can burn but the breaker not break hadn't
occurred to me.  In fact, thinking more carefully about it, there is
little risk in plugging in a 50A welder into a 15A circuit -- it will
just blow the fuse or breaker and "should" be quite safe as it doesn't
work.  There is a lot of risk in plugging in a 1A lamp with a 15A cord
into a 50A welder circuit because you can easily heat the whole thing up
to welding temperatures without blowing the breaker.  So my whole
picture of "risk" was inverted.

So the table above makes sense.  It isn't about "protecting the device"
from its own internal current overload as if it will break (especially
since it is probably aggressively internally fused), it is all about
fire protection on the cable, which CANNOT be fused unless it were in
the plug itself.  So an L5-20R to L6-30P converter would be safe only if
IT were fused or had an internal breaker at 20A.

So now all I have to worry about is a UPS kill switch (which for the
benefit of future archive searches, isn't called a "kill switch" -- it
is called an "Emergency Power Off" switch -- EPO).  I'm having a very
hard time finding anything like a clean description of just how to wire
all this up on the APC site.  I can find the master EPO switch, but the
manual for the latter is just about how to hang it on the wall and what
size wire to use to do this or that.  APC's tech forum is no help.
Anybody know of any good HOWTO or overview documentation resources for
this?  Again, (local) electricians seem to be clueless about this, and
APC doesn't seem to be very helpful either, given that this shouldn't
really be an "option" on any of their rackmount products -- they are ALL
over 750 VA.

Do people just ignore this (out of ignorance or otherwise)?  I'd think
APC would have a very clear link on every one of their server room UPS
products to the REQUIRED EPO controllers, along with a clear and cogent
description of why you must have one and how it must be wired.


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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