[Beowulf] Quick question...

Jim Lux James.P.Lux at jpl.nasa.gov
Sat Mar 17 18:41:06 EDT 2007


At 08:52 AM 3/17/2007, Robert G. Brown wrote:
>As I'm working up a server room and some UPS's, a question arises.  The
>APC units we have have a different plug and current rating from the
>circuits in the wall.  In particular, we have L6-30R receptacles and the
>APC requires only 20A.  I had thought that it would be perfectly safe to
>replace an L5-20P with an L6-30P and plug a 2200 VA UPS into a 30 amp
>circuit, but the electrician who was doing some of the rewiring said "we
>can't do that" because "the 30A circuit breaker might not blow in time
>to protect" the 2200 VA device from damage in the event of an internal
>short.

First off, the NEC really only deals with the permanent side of 
things.. (the utilization equipment side is dealt with by someone 
like UL or some other NRTL, and they would impose requirements for 
size of wire, etc.)

here's the deal on "receptacle ratings" from the NEC, bearing in mind 
that your local AHJ (Authority having Jurisdiction) may be different, 
and ultimately it's up to the inspector:

210.21 (B)(1) Single receptacle on an Individual branch Circuit
A single receptacle installed on an individual branch circuit shall 
have an ampere rating not less than that of the branch circuit.
Exception 1: In accordance with 430.81(B)
Exception 2: A receptacle installed exclusively for the use of a 
cord-and-plug connected arc welder shall....

(2) Total Cord and Plug connected load
<snipped>
(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


(430.81 has to do with motor controllers less than 3/4 hp)


>This argument seems quite senseless to me.  If this argument were
>correct we could never plug a 15 A device into a 20 A circuit, etc.  It
>would really simplify my life if I can just rehead the cable.  Do any of
>the electricity-savvy people here have any comments regarding the safety
>or legality of doing so?  I have always assumed that going up a size in
>circuit capacity is generally safe and legal...

Did you increase the size of the conductors in the cord to match the 
new plug? Is the fault interrupting capability inside the equipment 
rated to handle the higher peak fault current available from a higher 
rated branch circuit?

This is known as "coordination" of overcurrent protection (i.e. you 
can't put a big 100 A plug on 18 gauge lamp cord)


The NEC does have a bit to say about flexible cords and 
cables.  400.5 talks about minimum conductor sizes.  If you have only 
2 current carrying conductors in your cable, you can get away with 
AWG10 wiring in the cable. If you have 3 current carrying conductors, 
you'd need AWG8.

Now, you might be subject to the rules in Art.645 Information 
technology Equipment.

For instance, 645.5 requires that the ampacity of the branch circuit 
conductors be rated at not less than 125 percent of the total 
connected load (i.e. 2200 VA * 1.25, i.e. 25 A or so)

Also, you need to be aware of the requirement for a single switch 
that can disconnect power to all electronic equipment in the room 
(645.10) This also applies to the loads from the UPSes (actually, I 
think that you also need to have a "disconnecting means" to 
disconnect the battery from the load)  (i.e. the "big red switch" at 
the door).  UPSes rated at less than 750 VA (I think) are exempt from 
this requirement, but your 2.2kVA units will probably be subject to 
it. You might be able to have multiple controls, but they need to be 
grouped, and by the door.

{this is so that the fire fighters can hit a button and KNOW that 
everything in the room isn't still live}


>I'm planning to do the rewiring of cables today soon, so if you know
>something about his please respond "soon".
>
>    rgb
>
>--
>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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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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