Single Line Diagrams

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jrml05

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Thanks for taking the time to read my question.

The incoming 12kV overhead line enters a switch. Their is one 4" conduit that has only 3 #1/0 conductors in it leading to a 1500kVA. 1500*1000/12k/1.73=72A per phase. I should be using Table 310.77, #1/0 Type MV-105 rated for 5k-35kV. That puts the #1/0 at 215A. Well over sized. Run is only 500'. 1st question, am I using the right table? Second, why not use a smaller conductor since this is rated for well over the need?

The 12kV switch has another one 4" conduit with 3 #4/0 conductors in it leading to a 2500kVA. That equates to 120A per phase. Same table, derating does not apply, #4/0 is rated for 315A. Why are they now using an even bigger rated conductor for the 2500kVA? Couldn't we just use a #1/0 for this as well? Please explain to me what I am doing wrong or not understanding. Thank you.
 
My initial thoughts would be that your conditions require derating as compared to the ambient listed in 310.77.
Maybe your configuration is not copper per fig 310.60, maybe the RHO is not expected to be 90 or the ambient temp is not 20 deg C.
On the surface, it seems that you are correct, but maybe there are other factors.
BTW, some manufacturers in the 15kV class only go as low as 1/0awg
 
ron said:
....
BTW, some manufacturers in the 15kV class only go as low as 1/0awg
Click to expand...

Haven't done a lot of medium voltage work but what I have noticed is there are relatively few wire sizes being used whether it is available or not.

#1/0 is seems to be very commonly used even if you could get by with smaller. Ron may also have the reason pegged.
 
depending on whom the manufacture is, #2 or 1/0 is usually the smallest size medium voltage cable comes in.
 
Thank you guys. However, if #1/0 does the work, why not use #1/0 for the 2500kVA as well. Why upsize to a #4/0 if #1/0 works? The single line has already been created...
 
Future load additions, reduced voltage drop, etc. Need more?

Remember, NEC wire sizing charts and calcs are minimums. It's not 'bad' to install a larger-than-required conductor.

On the other hand, economy is the only real reason to try to stick to minimum requirements in sizing stuff.

To really know why oversizing was done in this installation, you'd need to ask the decision-maker.
 
jrml05 said:
Thanks for taking the time to read my question.

The incoming 12kV overhead line enters a switch. Their is one 4" conduit that has only 3 #1/0 conductors in it leading to a 1500kVA. 1500*1000/12k/1.73=72A per phase. I should be using Table 310.77, #1/0 Type MV-105 rated for 5k-35kV. That puts the #1/0 at 215A. Well over sized. Run is only 500'. 1st question, am I using the right table? Second, why not use a smaller conductor since this is rated for well over the need?

The 12kV switch has another one 4" conduit with 3 #4/0 conductors in it leading to a 2500kVA. That equates to 120A per phase. Same table, derating does not apply, #4/0 is rated for 315A. Why are they now using an even bigger rated conductor for the 2500kVA? Couldn't we just use a #1/0 for this as well? Please explain to me what I am doing wrong or not understanding. Thank you.
Click to expand...

An oil filled 55/65C* OA/FA transformer would have a primary current rating of 162A at 12470V or 168A at 12000V. If the transformer is protected at 125%, 300% or 600%, the ratings could be 210A, 505A or 1010A and the conductor should be sized accordingly.
 
According to IEEE Std C37.04 you can only use the 105 deg C column if the terminals are silver-coated, nickel-coated, or tin-coated.

If they are bare copper, or bare copper-alloy, then the 90 deg colum applies.

You may want to verify the terminal connections prior to proceeding.
 
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