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QUESTION: I work with an electrical contractor in a data center in Minnesota. I am a Communications Tech, and am generally responsible for data center operations. We recently worked on a UPS upgrade, where we added a second UPS for redundancy.

My question is this...I had the electrician go through each of the six breaker panels which lead to receptacles under the floor in order to verify that none of the breakers is approaching the 40% maximum draw limit. These receptacles are pretty evenly split between L5-20 and L6-30 with a smattering of L6-20's thrown in for variety, and these are installed 1:1 so no breakers has more than one receptacle.

After doing his amp draw measurements, I put them into a spreadsheet to calculate total draw, and look at phase balance between the three phases.

Here's where I got confused. I can add up all the total loads going into a given UPS and came up with 60-70 amps per phase on either UPS, which is about what I expected. when I pull up the UPS management interface, I see total output current of about 30amps on each phase.

Obviously, I can't just add the total amps being drawn from each breaker together to calculate the total load on the UPS. Is there a formula I need to use?

Thanks much!

ANSWER: Justin,

I would think the UPS management system is taking its readings right after the inverters which would give you the total outgoing load per phase to the 6 panels. In other words with out the power that is being used before that to convert the AC to DC then convert it back to AC. So with out being there I can't say why its apparently reading half the load you think you have after the inverters.

Yes taking the readings on a breaker by breaker basis would be incredibly slow. I would think your electrician would take his readings off the "feeder wires" from the inverters that go to each panel to determine the load on each phase which is the best and fastest way to do it. In other words your getting all the amperage thats going through a panel from 3 wires, then do the same thing to the other 5 panels and you have an accurate picture of load and balance among the phases. Then compare that again to the UPS management.

Bob

---------- FOLLOW-UP ----------

QUESTION: Bob,

The reason we did a breaker by breaker inventory is to ensure that no breaker has more than a 40% load on it. If one UPS fails, or one rack PDU, then all the load that is on that UPS or PDU will suddenly shift to the other side, so any breaker running at 51% load (ie 11amps on a 20amp breaker) will suddenly exceed it's rated capacity (102% or 22 amps) when the full load is shifted.

Yes, the fastest way to check the load on the UPS would be to check the current at the panels, or even at the UPS output bus itself, but that wasn't the point of this particular exercise.

My original question was why does the total of the individual loads not match up to the load that the UPS thinks it sees? Can I measure the load on a 208V two-pole breaker by simply measuring the current draw of the individual legs and combine them, or is there a formula I need to apply (such as multipying this total by x) to get the "real" load? An example is this...on panel Y breakers 6&8 are a 30A 208V circuit. Can I measure the total of the amps drawn on the two legs and use this number, or do I need to somehow reduce it before I can combine it with other 208 loads?

Thanks again for your time.

Justin,

As I said earlier I don't know why the UPS system isn't measuring the loads closely to what your getting measuring manually but maybe its how your adding them up thats the issue.

Using your example of a 2 pole breaker lets take it a step further and say breaker 6&8 is on phase A&B. Lets say the current flowing is 10 amps on phase A&B. You don't combine the readings on A&B to get 20 amps. You simply have 10 amps on A phase and 10 amps on B phase.

Now lets say we have another 2 pole breaker in the 10&12 position under 6&8. This breaker also has 10 amps on it. So now we have two 2 pole 208 volt circits across A B C phase. You will have 10 amps on A phase, 20 amps on B phase and 10 amps on C phase. If we add one more 2 pole breaker in the 14&16 position under 10&12 and that circuit also has 10 amps, you will now have balance between phase A B C 20 amps on each. This is how you can approach adding up your loads per phase on a circuit by circuit basis.

Bob

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