Re: Circuit maximum load
continous load: a load where the maximum current is expected to continue for 3 hours or more.
Many receptacle locations do not meet this definition. The computer may be running for more than 3 hours, but it is not always at max current. A lighting fixture for example will take about the same amount of energy to keep the lamps illuminated for the entire time. But a computer's power usage continues to rise and fall over time, depending on the demands put on it. So you will have to determine if your computers run near max current for more than 3 hours at a time, most likely they do not, although some servers may.
So the 80% rating of a circuit breaker is for continous loads, which in all likely hood you may not have. Then you could use the full 20 amps of the circuit. It sounds to me like you are doubling up the load by taking the power supplies twice tho. If you have two 350w power supplies for one server, the max load for that server should still be 350w, because the power supplies are redundant. On the servers i have worked on, you can hot swap the power supplies, so each power supply needs to be sized for the full load of the server. But you do not add the (2) 350w to get a 700w load. It sounds like you plug one power supply into one 20 amp circuit and the other power supply into another 20 amp circuit. IF this is the case for all the devices, then you could load each circuit up to 16amps continous or 20amps non-continous and not have a problem if either circuit dies.
A computer with a 350w power supply does not necessarily draw 350w of power. It simply can handle upto 350w. So a lot depends on how you are coming up with your loads. From experience we have found that computer rooms such as you are talking about consume somewhere between 25% to 40% of nameplate ratings. If you add up all the nameplate ratings from your devices, then look at your UPS for actual load being drawn, you will be able to determine what your Percentage of Nameplate actually is. Once you determine this, you will be able to go to a rack and get a better idea of what the actual draw per rack would be.
hope this helps, it would be much easier if i could actually see the installation. I am assuming that you have UPS powering the PDU's
[ April 24, 2003, 11:40 AM: Message edited by: jschultz ]