help with transformer

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wireperson

Senior Member
Location
Florida
I have a condensing unit , rla = 23 , Volts=460 phase = 3 there is a LRA = (I don’t remember right now)

Panel to feed this unit : 200a 3 phase 120/240open delta.

Question 1 : how do I calculate the transformer size to feed this CU ,is this correct? 23A (condensing unit RLA) x 480 x 1.732 = 18.32 kva == 25 kva , 120/240 delta primary , 480 Y secondary transformer . should I go for a 30 KVA?.

Question 2 : OCP , I read 450.3(b) , how do I do the transformer OCP and CU OCP ? in terms of installation , do I need a sub panel on the secondary side then feed CU and disco at the CU? , or I can go straight from transformer to CU disco , this is a dedicated transformer , just to feed this CU.

Question 3 . do I use 310.15 for conductor sizing? It’s 100ft from transformer to CU.
 
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wireperson

Senior Member
Location
Florida
can I use a 120/ 208 primary (even if feeder panel is open delta high leg ) - 680 secondary? I doub it but , that's what suppliers have available in stock.
 
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Cold Fusion

Senior Member
Location
way north
I wasn't ignoring you, I was mostly hoping some one else would pitch in cause:

I'm really not a good person to ask. I haven't ever sized a transformer to run a single motor. I have never sat down and figured out the voltage drop on motor starting for a dedicated transformer. I don't know how far one must oversize the transformer to get the Vd down to a reasonable level.

I can tell you this, most brands of three phase transformers I'v seen come in a 30kva size, not 25kva.

As for the xfm primary OCPD, I have rarely had trouble sizing a thermal-magnetic at 125% fla.

location of the secondary disconnect: That's a 240.21.C question - lots of variables.

Secondary conductor sizing: Use 310.15? Sure, why wouldn't one?

There have to be lots on here that have done this before and can advise you better than I.

cf
 
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wireperson

Senior Member
Location
Florida
thans cold , anyone else with an opinion? I talked to an electrical engineer friend of mine , he said 25 kva is ok ,he agreeed with my calculation , but ,I'm open to other opinions.
 
R

RonPecinaJr

Senior Member
Location
Rahway, NJ
wireperson said:
I have a condensing unit , rla = 23 , Volts=460 phase = 3 there is a LRA = (I don’t remember right now)

Panel to feed this unit : 200a 3 phase 120/240open delta.

Question 1 : how do I calculate the transformer size to feed this CU ,is this correct? 23A (condensing unit RLA) x 480 x 1.732 = 18.32 kva == 25 kva , 120/240 delta primary , 480 Y secondary transformer . should I go for a 30 KVA?.

Question 2 : OCP , I read 450.3(b) , how do I do the transformer OCP and CU OCP ? in terms of installation , do I need a sub panel on the secondary side then feed CU and disco at the CU? , or I can go straight from transformer to CU disco , this is a dedicated transformer , just to feed this CU.

Question 3 . do I use 310.15 for conductor sizing? It’s 100ft from transformer to CU.
Click to expand...


Question 1 : how do I calculate the transformer size to feed this CU ,is this correct?

The size of the transformer depends on the connected load. Check the condensing units nameplate data for minimum circuit ampacity and for maximum fuse or circuit breaker protection. This is how you determine branch circuit size and OCPD. Also, the find out the connected voltage rating. You need this information before you can do anything else, especially sizing a transformer.

Question 2 : OCP , I read 450.3(b) , how do I do the transformer OCP and CU OCP ? in terms of installation , do I need a sub panel on the secondary side then feed CU and disco at the CU? , or I can go straight from transformer to CU disco , this is a dedicated transformer , just to feed this CU.

You need to know full load amps and multiply that number by 125% to calculate for primary OCPD. It'll depend on how far away the CU is from the transformer to know if you'll need the sub panel or not.

Question 3 . do I use 310.15 for conductor sizing? It’s 100ft from transformer to CU.

Nope, you need to use table 310.16 and it depends on the insulation on the wire. Never use the 90? column for anything other than derating conductors.
 
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wireperson

Senior Member
Location
Florida
does anyone have experience reversing a transformer?
I was shopping for a step up 240 to 480
and the supplier showed me a transformer catalog from acme electric corporation where there is a 480 primary 240 secondary,delta delta and the catalog say they are reversible.No neutral needed for this aplication
 
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C

Cold Fusion

Senior Member
Location
way north
wireperson said:
does anyone have experience reversing a transformer?
I was shopping for a step up 240 to 480
and the supplier showed me a transformer catalog from acme electric corporation where there is a 480 primary 240 secondary,delta delta and the catalog say they are reversible.No neutral needed for this aplication
Click to expand...
The 240D primary is fine. The 480D secondary will need to be corner grounded or have ground detectors. Check the requirements of 250.20, 250.21. I'm partial to ground detectors - but I only do industrial.

RonPecinaJr said:
---Question 3 . do I use 310.15 for conductor sizing? It’s 100ft from transformer to CU.

Nope, you need to use table 310.16 ---
Click to expand...
Ron -
As I recall, 310.15 is the section that tells you to use T310.16.

cf
 
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