Transformer job

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electron_ryguy

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Hello all, new contractor here. I have a three phase 480v 3 wire panel. I'm doing a transformer job with a 45kva transformer which will step down to 120/208v. the distance from the main distribution panel is 250' away. I'm using a 70amp breaker with 6awg and a EGC #8awg in a 1" raceway for the primary side. The secondary calculations are as follows: transformer protection size is 150amps 1/0 awg with a 6awg SSBJ in a 2" conduit.

Questions that I have are:
1- NEC requires a disconnect and then into the transformer correct? and if so how would I size that? do I size to the primary or secondary voltage? also how many amps would I size it to?
2- coming out of the secondary to the new panel I understand I need to terminate to a over current protection device. will it be okay to come into a 125amp sub panel being that my conductor ampacity would be 150amps?
 
There are typically three things going on that you need to coordinate the OCPD 's to:
1. Transformer protection 450.3(b)
2. Transformer secondary conductors 240.21(c)
3. Panel board protection 408.36.

Device on the secondary may be doing all three of these things. I would start with which transformer protection scheme you are using.

You don't need a disconnect at the primary of the transformer, it can be remote as long as it is labeled and I think it has to be lockable.
 
What is the detail of the secondary ocpd? You can come right into a 150 amp main breaker panel, or you could hit 150 amp fused disconnect and then over to a main lug panel. Note with the latter you would need at least a 150A MLO panel to comply with 408.36.
 
Your primary protection meets the 125% rule and the 70 amp breaker can serve as the disconnect as long as it is in the same building and lockable as electrofelon indicates.
A disconnect or main breaker panel is needed on your secondary (see 408.36) and your secondary conductors need to meet the 240.21(C) rules (normally 10 ft)
 
electrofelon said:
What is the detail of the secondary ocpd? You can come right into a 150 amp main breaker panel, or you could hit 150 amp fused disconnect and then over to a main lug panel. Note with the latter you would need at least a 150A MLO panel to comply with 408.36.
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Honestly I am just planning the job the engineer over here is very green. so i am kinda picking up the slack and trying my best to figure it out.
 
augie47 said:
Your primary protection meets the 125% rule and the 70 amp breaker can serve as the disconnect as long as it is in the same building and lockable as electrofelon indicates.
A disconnect or main breaker panel is needed on your secondary (see 408.36) and your secondary conductors need to meet the 240.21(C) rules (normally 10 ft)
Click to expand...
thanks! its not a maximum of 25'?
 
I would probably come into a MB panel. Note that you do not need secondary transformer protection in this case, so you could upsize your secondary conductors to 200 amps and hit a 200 amp main breaker panel - just in case that's more readily available than a 150
 
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