6.9 KV?

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Desert Sparky

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Desert
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Electrical Engineer
I'm working on the design for a new building on a large industrial site. According to the owner, there is a 6.9 KV line that runs on site that we'll need to extend in order to serve the new building (this line is not owned by the utility company). In the projects I typically work on, the utility company extends power to our service entrance equipment, so this will be my first foray into medium voltage design. The only thing I know about this existing overhead line is that its 6.9KV. Here are my questions:

- Can 6.9 KV be stepped down to 208V, 3-phase?
- If so, can this only be done via pole mounted transformers, or can we provide a pad mounted transformer?
- Any rule of thumb for sizing this medium voltage transformer(s)? I know that if it were a utility transformer, the utility company would size based on "real world" loading, not code load.

Any insight on medium voltage would be much appreciated. Thanks.
 
There are more than one way to design something like this. Some of which plays in to the site's layout and their preferences. Some sites/managements want lines UG or padmounted transformers because it can be cleaner and hidden by small trees and brush. Making things look more aesthetic. Others could care less and are more concerned about not reducing usable SQ-ft so they want everything to stay overhead.

1) What is the service entrance design for the 208V service/feeder? UG feed OH Feed? Over 1000A? Do you need busduct or is it weatherhead?
2) What size transformer would you need to feed the service? Is it too large for a single pole? Typically (3) 100kVA transformers so 300kva total is nearing the size where you might need a two pole structure (platform). The max is (3) 500kva but you are going to need a couple H5s or similar for the vertical loading.
3) Existing conditions of the 6.9kV line and how your addition impacts it. Are the OH wires large enough? What is the existing load? etc. All the normal stuff.

You could also create something known as a customer substation or a small gated off area that resembles a substation and uses 6.9kv to 208V 3ph substation type transformer. It might take more leg work on the civil side to make and I would only suggest it if a padmount doesn't work for some reason. Like you are not allowed to dig for some reason..
 
Desert Sparky said:
I'm working on the design for a new building on a large industrial site. According to the owner, there is a 6.9 KV line that runs on site that we'll need to extend in order to serve the new building (this line is not owned by the utility company). In the projects I typically work on, the utility company extends power to our service entrance equipment, so this will be my first foray into medium voltage design. The only thing I know about this existing overhead line is that its 6.9KV. Here are my questions:

- Can 6.9 KV be stepped down to 208V, 3-phase?
- If so, can this only be done via pole mounted transformers, or can we provide a pad mounted transformer?
- Any rule of thumb for sizing this medium voltage transformer(s)? I know that if it were a utility transformer, the utility company would size based on "real world" loading, not code load.

Any insight on medium voltage would be much appreciated. Thanks.
Click to expand...
I suppose you can step down to any voltage, For us (UK) we'd usually use 400Vac for industrial projects and 110V LV.
 
If I said 6.9kV line I would be referring to singe phase. 12kV referring to three phase.
6.9 - 7.2 is one leg L-N off a 3-phase primary, but you'll need to go check the substation layout if its wye or delta primary distribution.
 
Elect117 said:
There are more than one way to design something like this. Some of which plays in to the site's layout and their preferences. Some sites/managements want lines UG or padmounted transformers because it can be cleaner and hidden by small trees and brush. Making things look more aesthetic. Others could care less and are more concerned about not reducing usable SQ-ft so they want everything to stay overhead.

1) What is the service entrance design for the 208V service/feeder? UG feed OH Feed? Over 1000A? Do you need busduct or is it weatherhead?
2) What size transformer would you need to feed the service? Is it too large for a single pole? Typically (3) 100kVA transformers so 300kva total is nearing the size where you might need a two pole structure (platform). The max is (3) 500kva but you are going to need a couple H5s or similar for the vertical loading.
3) Existing conditions of the 6.9kV line and how your addition impacts it. Are the OH wires large enough? What is the existing load? etc. All the normal stuff.

You could also create something known as a customer substation or a small gated off area that resembles a substation and uses 6.9kv to 208V 3ph substation type transformer. It might take more leg work on the civil side to make and I would only suggest it if a padmount doesn't work for some reason. Like you are not allowed to dig for some reason..
Click to expand...
All great info!

I'm assuming a 1600A service entrance switchboard, and would be best to go underground. I know very little about the overhead line, aside from the owners "there is plenty of capacity" spiel. I am in the very early planning stages of the design, so I just need to know if this existing overhead line is a viable source. Based on your transformer sizing parameters, it sounds like I'll need a padmount transformer.
 
pipe_bender said:
If I said 6.9kV line I would be referring to singe phase. 12kV referring to three phase.
6.9 - 7.2 is one leg L-N off a 3-phase primary, but you'll need to go check the substation layout if its wye or delta primary distribution.
Click to expand...
The existing pole we would be extending from has (3) overhead conductors attached to it; does that make it 12 KV?
 
zbang said:
Of course, and also 480/277, which might be more useful, depending on the building use and planned equipment.

One thing to consider, pad-mounts are less likely to be directly struck by lightning that pole-tops.
Click to expand...
This is a simple office building, so 120/208v is more than adequate.
 
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