UPS feeder power

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JJME

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Hi all,

I had a discussion with the CTO from the company I work at the other day regarding the amount of power a UPS consumes. We have this scenario: the load is 364kVA so I naturally put in my design plans a 400kVA UPS to feed this load. The UPS itself is being fed by a switchboard, and my conclusion is that this switchboard sees a load of 400kVA (the UPS and its batteries) and we consider 10% for battery recharge, so the total load would 440kVA. The switchboard doesn't care or know how much the final load is, it just sees a UPS to be energized. The CTO states that the total load is 364kVA plus the 10% = 364*1.1 = 400.4kVA.

Which one of us is right?

Thanks.
 
Usually, it's best to ask the UPS manufacturer how to size the feeds.

Doing a "back of the envelope" calculation-
(actual load being fed) + (conversion loss in the UPS) + (battery recharge demand) = the total load. If we assume 364kva @ 95% efficiency = 384kva just to make things run and with a charge rate of 10% of 400kva, that comes to 414kva (the UPS charge rate will be determined by how the charger is configured, not by the demand of the load being powered). However unless you're quite sure that nothing would be added..... I'd build for the total allowed load of the UPS, not the immediate load (400 + 21 + 40 = 461kva).

Don't forget any necessary cooling/venting for the UPS itself.
 
Chances are the load will be somewhat (maybe substantially) less than the stated maximum loads.

I'd be inclined to put in a feeder that could support at least a 500 kVA load though.
 
After an outage it is likely that the battery charge load will be much higher until the batteries have been completely recharged. That can require a substantially larger feeder size.
 
The UPS will, in general, only draw what the load requires but you have to also include the LOSSES in the UPS. If you connect 100 kVA of load to the UPS, it will have to draw this 100 kVA plus the battery charging needs, plus the UPS losses. The losses are not trivial.

But, to size the incoming feeder, you have to consult with the UPS supplier or data sheet to get maximum INPUT power required, including maximum battery charging current and UPS losses. It will be MORE than 400 kVA. The load on the incoming feeder is the UPS, not the downstream loads fed by the UPS.
 
don_resqcapt19 said:
After an outage it is likely that the battery charge load will be much higher until the batteries have been completely recharged. That can require a substantially larger feeder size.
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How did you figure that?
Any appliance/device that is manufactured and sold to the public is mandated (as stated in the nameplate) to carry the necessary power demand, voltage rating etc .
It is mandated by ANSI. (American National Standards Institute.)
Your assumption simply VIOLATES the Law of Energy Conservation as stated by the law of PHYSICS - formulated by Isaac Newton.

If that faulty assumption (of yours) is unchecked at rampart--all energy- consuming product/gadget on the market will just rob all the power that is readily available.
 
Fishbrain said:
How did you figure that?
Any appliance/device that is manufactured and sold to the public is mandated (as stated in the nameplate) to carry the necessary power demand, voltage rating etc .
It is mandated by ANSI. (American National Standards Institute.)
Your assumption simply VIOLATES the Law of Energy Conservation as stated by the law of PHYSICS - formulated by Isaac Newton.

If that faulty assumption (of yours) is unchecked at rampart--all energy- consuming product/gadget on the market will just rob all the power that is readily available.
Click to expand...
It is not an assumption and does not violate any laws of physics. It appears to me that you do not understand how these systems work.

I suggest you download and read the specs. After the batteries have been discharged the input power supply must both carry the load and recharge the batteries. I have see the manufacturer's specified input current on larger units that approach twice the output current.
 
Refer to a site planning sheet for a 400kVA UPS, such as Vertiv/Liebert
They can cap the battery recharge lower than the maximum shown 548A, but just dealing with losses of 3.5%, you can try to interpolate the ratios to 364kVA if you wanted to try and nitpick something lower than the actual equipment rating.
The fact that the load is within 10% of the equipment rating is technically doable, but most would oversize the equipment a bit for potential future growth.
 
Hello all,

Thanks a lot for your answers.

Just to clarify, we take 10% as the battery recharge power for UPS as a rule of thumb. Our specs are with no brand in mind, apart that the product has to comply with UL, ANSI, NEMA, and other quality marks. Losses are accounted by 3%. (rule of thumb also).

My take after looking at the answers is, better to ask the equipment manufacturer and that there is no general rule of thumb (maybe for a quick calculation for a general idea, but not for specifying on the plans). Even if we don't specify to a specific brand, I still have to pick a brand to have a base for the design, so that's that.

Thanks again and have a good one.
 
Fishbrain said:
How did you figure that?
Any appliance/device that is manufactured and sold to the public is mandated (as stated in the nameplate) to carry the necessary power demand, voltage rating etc .
It is mandated by ANSI. (American National Standards Institute.)
Your assumption simply VIOLATES the Law of Energy Conservation as stated by the law of PHYSICS - formulated by Isaac Newton.

If that faulty assumption (of yours) is unchecked at rampart--all energy- consuming product/gadget on the market will just rob all the power that is readily available.
Click to expand...
IMO, what the law says it is, will never be the actual situations on the ground! Common sense dictates that after a long outage, the batteries will be discharged deeply. That DoD level will not be limited up to the 10% assumption. UPSs DoD (depth of discharge) depends on the type of batteries being used. If your UPS uses LiFePO4, then the depth of discharge can go up to 100%-->meaning, you have to double your power source capacity as you are delivering charging and load power at the same time when utility power comes back.
 
topgone said:
topgone said:
IMO, what the law says it is, will never be the actual situations on the ground! Common sense dictates that after a long outage, the batteries will be discharged deeply. That DoD level will not be limited up to the 10% assumption. UPSs DoD (depth of discharge) depends on the type of batteries being used. If your UPS uses LiFePO4, then the depth of discharge can go up to 100%-->meaning, you have to double your power source capacity as you are delivering charging and load power at the same time when utility power comes back.
Click to expand...
Click to expand...
Opinion s are exactly what they are. . . in essence they are plausible when they are based on science.
And then you alluded to the misunderstood notion that batteries will continue to supply power up to a point where battery power is reduced to zero.

And after a long outage (your word) and logic dictates that the battery would be DEEPLY DISCHARGED to zero.

This is where the LOGIC failed miserably. Lead-Acid Battery power will never be reduced to ZERO

Electrical Engineering principles state-- that current will only flow when voltage is present.
So, when the battery is DEEPLY DISCHARGED no remaining energy is left and the consequence is:

No remaining energy available. . . .therefore no current will flow.

Lead-Acid Batteries don’t supply power to a point that no remaining power is left. . . . it has a cutoff limit to prevent reaching the threshold of zero power.
 
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Fishbrain said:
Opinion s are exactly what they are. . . in essence they are plausible when they are based on science.
And then you alluded to the misunderstood notion that batteries will continue to supply power up to a point where battery power is reduced to zero.

And after a long outage (your word) and logic dictates that the battery would be DEEPLY DISCHARGED to zero.

This is where the LOGIC failed miserably. Lead-Acid Battery power will never be reduced to ZERO

Electrical Engineering principles state-- that current will only flow when voltage is present.
So, when the battery is DEEPLY DISCHARGED no remaining energy is left and the consequence is:

No remaining energy available. . . .therefore no current will flow.

Lead-Acid Batteries don’t supply power to a point that no remaining power is left. . . . it has a cutoff limit to prevent reaching the threshold of zero power.
Click to expand...
Hmmm. Putting words into my mouth is not the proper way to strengthen your case. Where in my post did I say "zero"? Please read back, again!
 
topgone said:
you have to double your power source capacity as you are delivering charging and load power at the same time when utility power comes back.
Click to expand...
I don't see where "have to" comes in. The charge rate is whatever the manufacturer has configured with regard to battery health, and IME it's seldom going to be as fast as the discharge rate (IIRC often more like 1/4 the discharge rate).

All this depends on the UPS and the batteries used.
 
zbang said:
I don't see where "have to" comes in. The charge rate is whatever the manufacturer has configured with regard to battery health, and IME it's seldom going to be as fast as the discharge rate (IIRC often more like 1/4 the discharge rate).

All this depends on the UPS and the batteries used.
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That is the maximum rule of thumb I can think of on sizing of power supplies to UPSs.
One school of thought recommends (depending upon the input rectifier configuration of the UPS, 6 or 12 pulse), especially if the establishment uses a generator is to size between 1.5 and 1.8 times the size of the UPS. The perceived setup is that the UPS continuously supplies the loads during a power interruption and then the backup emergency generator is ran to take the loads while there is a power interruption from the utility.
The other scheme is to size the power supply at 1.25 to 2 times the size of the UPS system if: a) the generating set has a modern electronic governor and, 2) The UPS is a high efficiency system with a high input power factor greater than 0.9pF.
 
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