[Beowulf] newbie's dilemma

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Mar 2 09:33:21 EST 2006


At 04:10 PM 3/1/2006, Josip Loncaric wrote:
>Robert G. Brown wrote:
>>On Tue, 28 Feb 2006, Don R. Baker wrote:
>>
>>>for 8 years, but consider myself to still be a beginner.  I have a room
>>>with 4, 15 amp circuits and a 20 000 btu air conditioning unit installed
>>>that I can use for the next 2 years, but after that I may need to find
>>>another home for the system.
>>Let's see.  20KBTU is a bit more than 1.5 tons of AC, call it the
>>ability to remove 5800 Watts total.  4 x 15 x 120 is is 7200 Watts peak,
>>or about 5000 Watts RMS.  In my opinion this is going to leave you a bit
>>light on AC if you run the circuits fully loaded, and don't forget warm
>>bodies (60 W) and built in light bulbs etc. on other circuits (maybe
>>several hundred W more).  You have to not only remove the heat as fast
>>as it comes in but get ahead some, correct for heat that infiltrates
>>through the walls, and get the room temperature down below 20C (68 F) if
>>at all possible.  15-16C is more like it -- cold enough to just be
>>uncomfortable.
>
>Sensible conclusion, but: A 15 amp circuit should deliver up to 15 amps 
>RMS (otherwise, a 15A heater would immediately trip a 15A breaker). Peak 
>currents during the cycle can be higher.  This fine point is academic, 
>though, since in this example the air conditioning capacity limits maximum 
>power dissipation.

And, as a practical matter, the voltage at the load end (receptacle) will 
be less than 120V (typically 110-115V), depending on how big and long the 
wires are from the distribution panel to your load.  Be aware that a 
switching power supply is a constant power device, as the line voltage 
drops, the line current increases.  However, if you use the "nameplate" 
line current from the power supply you'll be always safe, because that's 
the maximum current the beast should draw.

When calculating what sort of overcurrent protection you need, you're 
supposed to only load it to 80% of the rating.  That is, a 20A circuit 
should be loaded to no more than 16A, and a 15A circuit to no more than 
12A.  And, in theory, the wiring should be designed to 20% more than the 
overcurrent protection level.

Circuit breakers have a "time to trip" that varies inversely with the 
amount of overload.  The actual "must trip" level is somewhat above the 
rated current, and they're typically rated at 125%, 200% and sometimes even 
higher.  They'll trip a lot faster at 200% than at 125% overload.


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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