480V Farm Questions

Status
Not open for further replies.
N

NotFaraday

Member
Location
USA
Occupation
Engineer
Hey everyone!

The city is now providing my property with 200A 480v 3phase service from what I’m pretty sure is 4 wire closed delta. I hired an electrician to come out and go over the topology with me prior to any work being completed, by a professional of course.

I think most of the plan he walked through with me makes sense, but I had a couple of theoretical questions I’d love answered by someone with expertise in this field (I’m an engineer myself and I can’t help but be curious).

A crude overview for reference:

kkHrRAPbTdebzZDa0Bcogg.png


At each location in the pic below he intends to have an appropriately sized pull box with polaris connectors to splice each segment to one another and/or branch off to an appropriately sized fused 3phase disconnect to connect a transformer.



My questions are:
  • Why can’t I reduce the wire gauge on a branch circuit? He was adamant about it being the same gauge throughout the entirety of the run , which leads me to believe I’d need to have quite a few home runs to avoid needing 1000s of ft of say 2/0.
    • i.e. why can’t I run 2/0 to the barn, connect either to 2 fused disconnects or to a small breaker panel, and have one appropriately sized 3ph breaker with say 2 gauge to the 3 phase transformer and another 3ph breaker with just 2 gauge continuing to downstream loads like a subpanel in a house? (note: made up the gauges)

  • Do the disconnect fuses prior to the primary of the transformer have any sizing requirement, I think I’ve seen OCPD requirement of 125% max amperage if <9 A online?

  • If I choose to do a closed delta 3ph transformer for my barn/shop, in order to have full 240v for my welder, is it true I can pull say ~70% of the transformers rated power through just 2 of the hots in the secondary? He reassured me it’s obviously not like it’s just ⅓ of the power per “phase” but I’m curious what that limit is.

  • Why do I need to run the neutral to the sites where there’s just a smaller transformer, I understand I don’t need the 3rd hot if it’s just a single phase transformer but what’s the neutral for? Is it just bonded to earth/the secondary neutral tap to establish a consistent ground reference?


I do trust him, may still get a 2nd opinion, but he has good endorsements and has extensive experience with industrial in the past. I really look forward to learning more about 3 phase from you guys and working with him.
 
Is your service 480 3 phase delta corner grounded?
Or 277/480 3 phase 4 wire?
It would be easier to answer your questions if they were numbered, not bulleted
Your drawing is not what we typically see, it’s confusing and not clear. Have your electrician do a riser drawing and attach that
 
As Tom said, we need to know exactly what this service configuration is. In this day and age I can't imagine (even in rural areas) a POCO supplying 480 3 phase as anything other than 480/277 Y.
 
texie said:
As Tom said, we need to know exactly what this service configuration is. In this day and age I can't imagine (even in rural areas) a POCO supplying 480 3 phase as anything other than 480/277 Y.
Click to expand...
Whoops, looks like it is 480/277Y. Had to double check the panel.
 
tom baker said:
Is your service 480 3 phase delta corner grounded?
Or 277/480 3 phase 4 wire?
It would be easier to answer your questions if they were numbered, not bulleted
Your drawing is not what we typically see, it’s confusing and not clear. Have your electrician do a riser drawing and attach that
Click to expand...
It is 277y/480 3 phase 4 wire, I was initially incorrect.

I'll see if I can get him to do a proper drawing. In the meantime allow me to repost my questions in an enumerated fashion.

1. Why can’t I reduce the wire gauge on a branch circuit? He was adamant about it being the same gauge throughout the entirety of the run , which leads me to believe I’d need to have quite a few home runs to avoid needing 1000s of ft of say 2/0.
  • i.e. why can’t I run 2/0 to the barn, connect either to 2 fused disconnects or to a small breaker panel, and have one appropriately sized 3ph breaker with say 2 gauge to the 3 phase transformer and another 3ph breaker with just 2 gauge continuing to downstream loads like a subpanel in a house? (note: made up the gauges)


2. Do the disconnect fuses prior to the primary of the transformer have any sizing requirement, I think I’ve seen OCPD requirement of 125% max amperage if <9 A online?


3. If I choose to do a closed delta 3ph transformer for my barn/shop, in order to have full 240v for my welder, is it true I can pull say ~70% of the transformers rated power through just 2 of the hots in the secondary? He reassured me it’s obviously not like it’s just ⅓ of the power per “phase” but I’m curious what that limit is.


4. Why do I need to run the neutral to the sites where there’s just a smaller transformer, I understand I don’t need the 3rd hot if it’s just a single phase transformer but what’s the neutral for? Is it just bonded to earth/the secondary neutral tap to establish a consistent ground reference?
 
NotFaraday said:
  • Why do I need to run the neutral to the sites where there’s just a smaller transformer, I understand I don’t need the 3rd hot if it’s just a single phase transformer but what’s the neutral for? Is it just bonded to earth/the secondary neutral tap to establish a consistent ground reference?
Click to expand...
I'll take the easy question; You don't need the noodle on the primary side. You do need to run a EGC.
 
tortuga said:
I'll take the easy question; You don't need the noodle on the primary side. You do need to run a EGC.
Click to expand...
Yes but the EGC can be smaller than the 2 hots correct? Isn't there some 250.66 doc that specs grounding conductor size per primary conductor size?

When you guys are talking about 4 wire wye, that's 3 hots and a neutral. So if there's no noodle the electrician is going to bond what to what?
the transformer case to the the case of the main panel?
 
The Neutral must be taken to the Service Equipment. 3 hots and the grounded conductor from the POCO, or utility supply. ie. The neutral. A GES would be created and bonded at the SE. After that point, the neutral is no longer required if it is not needed. You will need an equipment ground from that point on. It may be reduced in size depending on the SCBC protection ahead of the feeders.
 
Guess I found some critics of my drawing/somewhat limited knowledge in this space. :rolleyes:

I can and ultimately will have to defer most of the decisions to my electrician but generally I'm a curious person who likes to have some knowledge in what to expect out of the system I'm paying for. Sorry for throwing a code number out, I haven't claimed to be well versed in all of them.
 
Yes with the lack of understandable drawings and questions I don’t see any way that we can assist. To the OP, you will need to work with your electrician(s). Permits and inspections will ensure compliance with the NEC
 
Status
Not open for further replies.
Top