Transformer job

[electron_ryguy]

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?

 

[electrofelon]

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.

 

[electrofelon]

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.

 

[augie47]

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)

 

[infinity]

Like EF stated your secondary needs to land on an OCPD.

 

[electron_ryguy]

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.

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.

 

[electron_ryguy]

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)

thanks! its not a maximum of 25'?

 

[electrofelon]

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

 

[infinity]

thanks! its not a maximum of 25'?

The 25' rule is typcially applied to 480-208Y/120 volt and transformers.

 

[augie47]

You can review the various lengths & requirenents in 240.21C