Correct primary and secondary wire for new installation (Just an exercise)

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derill03

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Location
ohio
*******See Attachment for the problem*******

Winter = 50kVA * 1.40 = 70kVA Max

Summer = 50kVA * 1.10 = 55kVA Max

Primary full load current = 70kVA / 7.2kV = 9.72A

Secondary full load current = 70kVA / 240V = 291.6A

a.) 4/0 TX for secondary

b.) 4 for primary


 

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  • Assignment 2 problem 3.pdf
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GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Did you notice that the problem provides different ampacities for each wire size in summer versus winter?
So your ampacity determination should calculate both summer and winter wire sizes and the larger of the two will be the answer.
I do not see that in your work, even if you may have gotten the right answers.

How did you interpret the transformer specification of "12.47GndY/7.2 kV primary"?
 

derill03

Member
Location
ohio
I calculated the total currents based on the summer and winter allowance percentages and since the winter allowances were the largest I chose to size the wire from the winter allowance of 140%.

I interpreted the transformer spec as 12.47kV L-L and 7.2kV L-NEU. Looking back I prolly should use L-L for primary calculation huh?

I did what you suggested in picking the largest wire based on my calculations because winter load is highest and that's what I used.

Sent from my SM-N915V using Tapatalk
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I interpreted the transformer spec as 12.47kV L-L and 7.2kV L-NEU.
That is how I would have interpreted it except for that pesky letter Y in the string. You cannot have 12.47 L-L and 7.2 L-N in a wye configuration. I suspect the test writer just goofed, but I might be misinterpreting something.
 

derill03

Member
Location
ohio
Does my work look correct? I think the only goof might be that i chose to use 7.2kV in my primary calculation instead of the L-L.
 

derill03

Member
Location
ohio
Wait it does specify single phase primary to the pole and then single phase 240V to the weatherhead, so i guess the 7.2kV i used in the primary should be correct.
 

rlundsrud

Senior Member
Location
chicago, il, USA
I know this isn't the correct way to solve the problem, but what difference does it make on the primary? No matter how you do it, you don't even come close to the max ampacity of the smallest wire listed. My point is that all paths lead to #4 as the correct one.
 
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