possible issue with APC 7968 Switch PDU

[Anonymous user]

We have set up an APC PDU Model # 7968
we ordered a 3phase 208V circuit 40amps each phase

we did this because our unit has a 35amps per phase, and we figured we can drive it up to 30 - 33 amps and we would block the breaker on the PDU before we tripped the breaker in the data center.

here is the issue - now we have it all configured, we have 17amps per Bank and the total on the PDU shows 20amps

did we miss calculate here on something?

any help please

[BillP · Accepted solution]

hmm, i am not an expert on three phase power but the electrical specifications sheet for this model (http://www.apcmedia.com/salestools/ASTE-6Z6K7J_R1_EN.pdf) indicates that it has an output current rating of - 15A/C13, 16A/C19; 16A/ bank. Thus, that's why the banks have 20A breakers on them.

i found some info that I have on a different PDU but I think it is the same explanation here:

In regards to the AP7998, each bank references two phases:

Bank 1 - Phase 1 and 3
Bank 2 - Phase 2 and 1
Bank 3 - Phase 2 and 3

The breaker on each bank is rated for 20 amps. Here is an explanation how the phases are distributed:

"Bank 1 draws current from phases 1 and 2 simultaneously. Bank 2 draws current from phases 2 and 3. This means that phase 2 is supplying current to both banks 1 and 2. However, like the phases, the banks are 120° out of phase. This means that part of the time that current is flowing one direction on bank 1, it is flowing the same direction on bank 2 and the two currents increase the draw from phase 2 to over 20 A. However, at other times, when current is flowing one direction on bank 1, it is being returned on bank 2. This means that you can't just add the loads on the two banks to get the load on phase 2.

Here is another way to look at it. We are in essence supplying three 120 V phases, but we are powering three 208 V loads. If the loads are 20 A, the total power consumed is ( 3 banks) x (208 Volts) x (20 Amperes) = 12480 Watts. The current on the input must then be (12480 Watts)/(three phases x 120 Volts) = 34.6 Amperes."

hope that helps!