Transfromers

Status
Not open for further replies.
F

Flight987

Senior Member
What is minimum size THHN copper wire permitted on both the primary and secondary sides of the Tran., based on the primary protection (both primary and secondary Con. will be considered protected by the fuses on the primary side)? Transformer is a 10kva, 480/120, single-phase The primary Con. is #10. That I can figure out. Can't figure out how the answer for the Sec. Con. is a #1 copper. 83.33 amp,s x 1.25=104.16 amp's. To me that is one size
larger than needed. Please somone show me the numbers.
 
Stevareno

Stevareno

Senior Member
Location
Dallas, TX
There must be other factors involved. What's the temp rating of the wire? What is the expected ambient temperature where this transformer will be located?
 
F

Flight987

Senior Member
No Temp.

No Temp.

There is no Temp. involded in thi Ques. To me it should be a #2 rated at 115 amps.. Table 310.16, 75* C. Please
try the math for on your desk and let me know what you find.
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
curtnobel said:
There is no Temp. involded in thi Ques. To me it should be a #2 rated at 115 amps.. Table 310.16, 75* C. Please
try the math for on your desk and let me know what you find.
Click to expand...

Assuming that the primary OCPD is 30Amps (the maximum size allowed for primary only protection) then the minimum required ampacity for the secondary conductor to be considered to be protected by the primary OCPD would be 120Amps per 240.21(C)(1) (or 240.4(F)).

Secondary Ampacity = 30 * (480/120) = 120. #2 only has an ampacity of 115, so #1 would be the minimum required size.
 
F

Flight987

Senior Member
Somthing I Don't

Somthing I Don't

You see somthing that I don't. I don't get the Cal. that you worked out. Please heip me on this one. How does
(480/120) give us 120A's? Or it that 480/120=4, then 4 x 30A's=120A's. Is this the part where they talk
of the turn's ratio? Don't seem to get the math.
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
curtnobel said:
You see somthing that I don't. I don't get the Cal. that you worked out. Please heip me on this one. How does
(480/120) give us 120A's? Or it that 480/120=4, then 4 x 30A's=120A's. Is this the part where they talk
of the turn's ratio? Don't seem to get the math.
Click to expand...

Yes, 4 x 30 =120, that would be the minimum required ampacity of the secondary conductor to be protected by a 30A primary OCPD.

And yes it has to do with the turns ratio mentioned in 240.21(C)(1). Following the language in 240.21(C)(1), the secondary conductors can be permitted to be protected by the primary OCPD (30A), provided this protection does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary (120) to primary (480) turns ratio.

So if the secondary conductor was #2, with an ampacity of 115 Amps, the value determined by multiplying the secondary conductor ampacity by the secondary to primary turns ratio would be: 115 x (120/480) = 28.75. As you can see, the primary OCPD exceeds 28.75, so #2 would not be permitted.

If the secondary conductor was #1, with an ampacity of 130 Amps, the value determined by multiplying the secondary conductor ampacity by the secondary to primary turns ratio would be: 130 x (120/480) = 32.5. Here, the primary OCPD does NOT exceed the value of 32.5, so #1 is permitted. So, #1 is the minimum required size.
 
F

Flight987

Senior Member
I need

I need

I need to read that over closer. Now if I can get this thing to print #7 that would be great. Shawn and Ellain tryed
to get it sent to e=mail so I could print this fast. Thank's Curt
 
F

Flight987

Senior Member
The next Que.

The next Que.

The next part to this is and the Ques. read's What would be the minimum size THHN copper wire permitted on both the primary and secondary sides of the transformer, if the next smaller standard size fuse were used on the primary side? Test key Answer, Primary #10, Secondary # 3.
If I understand that a lower OCPD, would be a 25 amp fuse. This were my brain goes blank. I can figure out the
#10, wire size (10000/480=20.83x 1.25=26.04) Primary only Prect., Note 1. Table 450.3(B) Now what do I do?
This is the same Trans., Just an add on Ques.
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
same ratio 480/120 (4:1) 25 amp primnary fuse 100 amp secondary current.. #3 Cu
 
F

Flight987

Senior Member
New Ques.

New Ques.

New ques.
Given a single-phase Tran. with a 3-wire(center-tapped) seconddary, and a 2-wire primary(480//120/240).
If there is 14 amp on one line of the secondary, and 8 amp on the other line (Sec.) how much is the Neut.
(center-tap). This is Ques. A. Key answer. 6 amp's I came up with Number also. This is how I did it.
Correct me if this is the wrong. In= L1-l2. 14-8=6amps.

Ques. B How much current will be present on the primary of the Trans. Key Answer 5.5 Amps.

I need help on this. Mike doesn't have example. Also, when they say (center-tap)., how does that apply?
Single phase are center-tap, to XO. Is that right. I thinking that if I had some Kva number I could get it.
 
F

Flight987

Senior Member
Ques. # 8

Ques. # 8

What is the Max Fuse on a 90Kva 3/0 480//120/208 Trans. Got that Prim. 250Amp, Sec. 350 Amp..
Part B of question. What is the Min. size THHN Cop. wire. Key Answer Prim. 4/0, Sec. 500Kcm.
I got the Sec. size, But 250Kcm, 75c is 255, 4/0 is 230 75c, 4/0 90c is 260,. We are putting this
on 75c termal's. Why did they use 90c wire? We not doing Adj. or Corr. Fact. are we. What is going on this time.
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
curtnobel said:
New ques.
Given a single-phase Tran. with a 3-wire(center-tapped) seconddary, and a 2-wire primary(480//120/240).
If there is 14 amp on one line of the secondary, and 8 amp on the other line (Sec.) ...

Ques. B How much current will be present on the primary of the Trans. Key Answer 5.5 Amps.

I need help on this. Mike doesn't have example.
Click to expand...

You have 14A x 120V=1680VA on one leg, and 8A x 120V = 960VA on the other leg, for a total load of 2640VA.

On the primary, 2640VA / 480V = 5.5Amps.

curtnobel said:
What is the Max Fuse on a 90Kva 3/0 480//120/208 Trans. Got that Prim. 250Amp, Sec. 350 Amp..
Part B of question. What is the Min. size THHN Cop. wire. Key Answer Prim. 4/0, Sec. 500Kcm.
I got the Sec. size, But 250Kcm, 75c is 255, 4/0 is 230 75c, 4/0 90c is 260,. We are putting this
on 75c termal's. Why did they use 90c wire? We not doing Adj. or Corr. Fact. are we. What is going on this time.
Click to expand...

They are using the 75c wire, not the 90c wire. 240.4(B) allows you to protect the wire at the "next higher standard overcurrent device rating." So, #4/0 with an ampacity of 230 can be protected by a 250A OCPD. 240.21(C) prohibits using the next standard size up for the secondary conductors, so you need 500mcm instead of 400mcm on the secondary.
 
F

Flight987

Senior Member
Ques. 9

Ques. 9

150Kva,3/0,480//120/240 volt Protected on both Prim. and SEC. Prim. OCD 450 Amp's, Sec. OCD 500 Amp's!
I got that right, having trouble sizing Cond. These are THW Cond. Key Answ. Prim., 1-600 Kcm or 3-2/0 or
4-1/0 Per Phase, Sec., 1-900 Kcm, or 2-250kcm or 3-2/0. I understand Pall. them, But 600 kcm is only 420 amp's!
or 3-2/0 come to 525, that work's for me , cause it is over 450 Amp's. Don't get it!

Oh, Sec. 1-900Kcm or 2-250Kcm or 3-2/0 and so on.( This is key Answ.) The numbers don't add up right.
Some are udder and other's or over. To me they should over the OCD ratting
 
F

Flight987

Senior Member
New Ques.

New Ques.

1kva,480/120,single phase, Prot. on primary only. Answ. key 3.48
My calculator 1000/(480x1.732=831.36) 831.36=1.2028x3.00(450.3B)=3.60
My calculator seem's to think different,Batt. are good. Or does it have to do with note 3?
Try your Cal. and see.
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
curtnobel said:
1kva,480/120,single phase, Prot. on primary only. Answ. key 3.48
My calculator 1000/(480x1.732=831.36) 831.36=1.2028x3.00(450.3B)=3.60
My calculator seem's to think different,Batt. are good. Or does it have to do with note 3?
Try your Cal. and see.
Click to expand...

Per T450.3(B), primary only protection would not be greater than 167% of the primary current.

The primary current is 2.083Amps.

2.083Amps x 1.67 = 3.48 Amps. That would be the maximum OCP rating/setting allowed on the primary.
 
F

Flight987

Senior Member
On Page 2

On Page 2

On page under Ques. you helped me work out a answer. Thank's. I made the same mistake on a Ques.
below that one. When I se the 480, 1.732 comes to me to use. I had another 480/120/240 Trans.
Ques.. Did fine on the Neu. Amp's., but went and took for Prim. Amp's 7200VA/480x1.372=8.6.
That was wrong on my part. The correct way is 7200/480=15 Amps. I understand that it simple
olm's law, the right way to do it. Is that 480 in this type of problem not considered to be a 3 phase
formula. By looking through Mikes book, it appears that if it is 3 phase on Pri. and Sec. then we use
1.732. If so, that is a easy mistake to make.
 
Status
Not open for further replies.
Top