Liebert AC UNIT

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ERIC LEDBETTER

ERIC LEDBETTER

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I was contracted by a local mechanical contractor to bring power to a 125 amp libert ac unit at 480 volts . They made a mistake and ordered a 208/240 at 155.5 amps unit at a coast of 50,000 returning it is not an option . SO I set a 45k 208 volt transformer. on start up the ac is at 211 volts (nice) however the commissioning of the unit requires the ac and DE humidifier and heating element to run at the same time as the ac this is causing the voltage to drop to 198volts He still has more items that need powered at the same time and my fear is he will lose even more voltage..
Any suggestions before i get back to this guy Monday morning .
 
ERIC LEDBETTER said:
I was contracted by a local mechanical contractor to bring power to a 125 amp libert ac unit at 480 volts . They made a mistake and ordered a 208/240 at 155.5 amps unit at a coast of 50,000 returning it is not an option . SO I set a 45k 208 volt transformer. on start up the ac is at 211 volts (nice) however the commissioning of the unit requires the ac and DE humidifier and heating element to run at the same time as the ac this is causing the voltage to drop to 198volts He still has more items that need powered at the same time and my fear is he will lose even more voltage..
Any suggestions before i get back to this guy Monday morning .
Click to expand...
Ask why they need to load it that way, even if you had the 480 volt unit you may still have had similar problems by loading it to more than the system was designed to supply to it.
 
Ok

Ok

The technician said it is a 16 hour start up and he follows the guidelines in a book from the manufacture. At this point i want to cover my actions and make sure i get paid(i will ask for all the manuals in the am .
 
ERIC LEDBETTER said:
I was contracted by a local mechanical contractor to bring power to a 125 amp libert ac unit at 480 volts . They made a mistake and ordered a 208/240 at 155.5 amps unit at a coast of 50,000 returning it is not an option . SO I set a 45k 208 volt transformer. on start up the ac is at 211 volts (nice) however the commissioning of the unit requires the ac and DE humidifier and heating element to run at the same time as the ac this is causing the voltage to drop to 198volts He still has more items that need powered at the same time and my fear is he will lose even more voltage..
Any suggestions before i get back to this guy Monday morning .
Click to expand...

Does the AC have the option of being configure as 208 or 240v? Did you say that you are supplying 208 or 240v which I may have missed this your OP?
Have you asked the Liebert what the acceptable operating voltage range is? With NEMA frame motors the motors operate satifactory at +-10 of the NP voltage and am currrious is Liebert may have an acceptable operating rang also.
My point is first assure if you in fact have a problem.
 
ERIC LEDBETTER said:
I was contracted by a local mechanical contractor to bring power to a 125 amp libert ac unit at 480 volts . They made a mistake and ordered a 208/240 at 155.5 amps unit at a coast of 50,000 returning it is not an option . SO I set a 45k 208 volt transformer. on start up the ac is at 211 volts (nice) however the commissioning of the unit requires the ac and DE humidifier and heating element to run at the same time as the ac this is causing the voltage to drop to 198volts He still has more items that need powered at the same time and my fear is he will lose even more voltage..
Any suggestions before i get back to this guy Monday morning .
Click to expand...

Liebert field guys can be a lot sharper than normal and that's usually a good thing. Anything out of spec I would expect to get escalated to the office engineers directly.

Depending on the application it is possible for the AC to run in dehumidification mode and then reheat is necessary to balance the space temp. I've seen it, heat and AC on at the same time, especially with bad control system setup. It could also be a useable feature of the system or a design requirement for the application. Like driving your car in the rain, run the AC and heat at the same time for dry heat.

Needs a load calc with the heat and AC both on to see if the feeder and tranny are big enough for continuous loading. If the feeder or the tranny fail the load calc .. You would need the application requirements, does heat and AC have the option of running at the same time or are they mutually exclusive.

Second, the Liebert units dual voltage 208/240 rating, sometimes the only change to the unit is a tap at the control transformer for 208/230, nothing else. If that is the case you may be able to set the distribution transformer taps higher. Taps are usually every 2.5% so you may be able to get 5% or 10% higher system voltage going to the higher taps.
 
3 phase unit ?

155.5 amps at 208 3 phase is more like 56 kva. Why a 45 kva transformer ?
 
Will check control taps in the am that will be great if it is that simple . and on the sizing i came up with a 45 k out put 178 amp after the 1.25% for continuous duty/ as for wire size and lengths are sort runs all under 20 feet and wire sized and stepped up one just because it was short and i didn't want to risk any thing.
 
ERIC LEDBETTER said:
Will check control taps in the am that will be great if it is that simple . and on the sizing i came up with a 45 k out put 178 amp after the 1.25% for continuous duty/ as for wire size and lengths are sort runs all under 20 feet and wire sized and stepped up one just because it was short and i didn't want to risk any thing.
Click to expand...

You'd better check your math.
At a power factor of 1.00, 155 amps is 56 kva. At 178 amps that is 64 kva.

You transformer is undersized.
You should have used a 75 kva transformer.

Suck it up and fix your mistake before you have to replace the AC unit.
 
I agree, I show a 45kVA at 208V to be 124A full load capacity. 75kVA is 208A, that would have been the minimum size were it me, don't know where you got 178A from. Plus you also need enough capacity to avoid a severe VD when the motor starts, so I might have even gone to 112.5, but you don't say the size of the motor or the starting method, so that's still an unknown.

Transformer calculator
 
Changing taps may bring up voltage when loaded, it will also result in over voltage when not loaded.

I also agree transformer is probably too small. You possibly forgot to multiply VA by 1.73 for three phase when you selected the transformer?
 
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U R ALL RIGHT

U R ALL RIGHT

I TRUSTED THE C E S SAILS MAN TO GIVE ME A 200 AMP 208 XFORMER AND I BELIEVE HE SIZED IT FOR SINGLE PHASE THANKS WILL ORDER A 112 IT IS MY MISTAKE
 
Is the transformer going to be mounted in the 'conditioned' space?
Right now I do not see a good case for needing to go to a 112.5kVA transformer.
 
"Suck it up and fix your mistake."

I am tired of having to do this! That is, having to redesign the electrical, and add a transformer, because the HVAC guy "accidentally" ordered the wrong unit. Or, he shows up at the last minute and claims 'this is all they had.'

Indeed, I'm not able to recall ANY job where the A/C guy actually brought in the 480v unit specified. Does the HVAC trade simply refuse to order them?
 
renosteinke said:
"Suck it up and fix your mistake."

I am tired of having to do this! That is, having to redesign the electrical, and add a transformer, because the HVAC guy "accidentally" ordered the wrong unit. Or, he shows up at the last minute and claims 'this is all they had.'

Indeed, I'm not able to recall ANY job where the A/C guy actually brought in the 480v unit specified. Does the HVAC trade simply refuse to order them?
Click to expand...

I think they don't even think to look at power requirements, what is heating or cooling capacity is all they care about.
 
renosteinke said:
"Suck it up and fix your mistake."

I am tired of having to do this! That is, having to redesign the electrical, and add a transformer, because the HVAC guy "accidentally" ordered the wrong unit. Or, he shows up at the last minute and claims 'this is all they had.'

Indeed, I'm not able to recall ANY job where the A/C guy actually brought in the 480v unit specified. Does the HVAC trade simply refuse to order them?
Click to expand...

His mistake wasn't believing the HVAC guy, it was sizing the transformer.
 
tkb said:
His mistake wasn't believing the HVAC guy, it was sizing the transformer.
Click to expand...
And the mistake there was letting someone else select what he needed instead of calculating it himself. Transformers are sized to kVA not amps, he asked for a 200 amp transformer and it got miscalculated based on that. I guess I can't speak for other suppliers, but places I would buy this from, I would probably be giving them the catalog number of the unit I wanted, otherwise the risk of getting something else increases, and on occasion they will still get me the wrong item.

Now who eats the original transformer?
 
STILL A MESS

STILL A MESS

We installed a 75 k but phase a under load only puts out 198 volts not 208 . trace it out further the 277 primary from street only has 274 on phase a. is it worth up sizing this thing again at the expense of the H V A C guy. without a load we run only 201 one a 205 on b and c the primary run is less than 110 feet and the wire is over sized . Looking for help
 
ERIC LEDBETTER said:
We installed a 75 k but phase a under load only puts out 198 volts not 208 . trace it out further the 277 primary from street only has 274 on phase a. is it worth up sizing this thing again at the expense of the H V A C guy. without a load we run only 201 one a 205 on b and c the primary run is less than 110 feet and the wire is over sized . Looking for help
Click to expand...

Have you checked and adjusted the taps on the transformer ?
 
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