Cost for stepping up.

[PM cold-n-mn]

I have a customer who got a deal on storm damaged RTU's. He bought them from an insurance company assuming he could use them. The only problem is they are 480v and he has a 120/208v system in his building. I can not deceide the most cost effective way to go. Assuming the feeders for the old RTU's are large enough to feed step up transformers I have several options:

Option #1 I could set them on the roof next to the RTU. I am not sure if I need a disconnect on the primary side or if the breaker would due. If the breaker would due this seems to be the cheapest.

Option #2 I could set them inside the building next the panel. I could save money by reusing the conduit and wire to the roof but I believe I would have to add secondary overcurrent protection down by the transformer.

A seperate issue is that the units stamped rating is 16amps at 480v. This puts me on the line between a 15 & 30 kva transformer. Which way would you go.

Any opions would be appreciated..

 

[augie47]

The key may be in the plural unitS
You may want to look at the comparative cost of a separate transformer for each unit or a larger transformer to feed more than 1 unit.

 

[PM cold-n-mn]

There are three total but by the time you add a 480v panelboard on the secondary side of one large transformer it seems like it would almost be a wash.

 

[augie47]

I'm thinking "at the unit". Of course you would need to address environment (may need to add rain kit). If you use the existing branch circuit (including breaker) the OCP will probably exceed 125% of the transformer rating thus secondary protection will be required.
Since you need (or may have) secondary protection at the unit, you can use that.
You might check out the grounding situation ref 250.30 to see if that affects your location.

 

[PM cold-n-mn]

Thanks,
I thought that would be the most cost effective. I wasn't 100% sure that I did not need a primary disconnect with the transformers on the roof. Do you think a 15 KVA would work? The charts show them at 18 amps and the min.circuit ampacity on the unit is 16amps.

 

[augie47]

To make sure, I'd get your A/C FLA & LRA and check with the transformer manufacturer. The MCA has some oversize from the FLA, but the LRA on A/C units can be high.

 

[PM cold-n-mn]

Thanks
I will check it out..

 

[mikeames]

I dont knwo all the actual numbers so I may be too negative but it sounds like someone was trying to save money, bought used equipment and is now paying roughly the same as new to make the old work. :roll:

 

[PM cold-n-mn]

Thats pretty close but the units are only a couple years old and the sheetmetal had hail damage. Pretty hard to pass that up if he got them for a good price.

 

[charlietuna]

Sounds to me that your customer's "free equipment" will cost him more money to retrofit than their original "NEW" cost--and then be stuck with a "White Elephant" on this building. The power loss across the transformer alone seems to make it inefficient!

 

[weressl]

I have a customer who got a deal on storm damaged RTU's. He bought them from an insurance company assuming he could use them. The only problem is they are 480v and he has a 120/208v system in his building. I can not deceide the most cost effective way to go. Assuming the feeders for the old RTU's are large enough to feed step up transformers I have several options:

Option #1 I could set them on the roof next to the RTU. I am not sure if I need a disconnect on the primary side or if the breaker would due. If the breaker would due this seems to be the cheapest.

Option #2 I could set them inside the building next the panel. I could save money by reusing the conduit and wire to the roof but I believe I would have to add secondary overcurrent protection down by the transformer.

A seperate issue is that the units stamped rating is 16amps at 480v. This puts me on the line between a 15 & 30 kva transformer. Which way would you go.

Any opions would be appreciated..

RTU? It is commonly used as Remote Terminal Unit in Utility Power Distribution SCADA terminology.

http://www.acronymslist.com/search2.php

Reading further I discover you're talking about - possibly - a refrigeration/AC unit? Depends on the number of units, it seems to me that you need to investigate a single large transformer that will result in a smaller transformer than the sum of all individual transformers. The individual transformers would need to be oversized to allow the starting of the motors without the voltage drop high enough - as the result of the inrush - that it would stall the motor. The single, common transformer could be sized for the sum of the RLA-1 plus the RLC of a single unit. This is rather simplistic way of approaching the issue, but only in much larger units would require you to run a load flow study.

 

[iwire]

RTU is 'Roof top unit' to us knuckle draggers.:wink:

 

[LarryFine]

RTU is 'Roof top unit' to us knuckle draggers.:wink:

"Hey! I resemble that remark!"​