[Beowulf] newbie's dilemma

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Mar 2 06:33:21 PST 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.