math help

[Cliff B]

I am studying for my supervisors license and I have two math problems I would like help with.
1) Max primary overcurrent protection allowed for 2.8% impendance 500Kva 480 x 208/120 3phase transformer how do I slove for this?
2) The maximum ampacity rating of 4in x .5 in copper bus bar 4ft long installed in an auxiliary gutter I know it's 1000amp/inches squared but how do I set up the problem

 

[kingpb]

I am studying for my supervisors license and I have two math problems I would like help with.
1) Max primary overcurrent protection allowed for 2.8% impendance 500Kva 480 x 208/120 3phase transformer how do I slove for this?
2) The maximum ampacity rating of 4in x .5 in copper bus bar 4ft long installed in an auxiliary gutter I know it's 1000amp/inches squared but how do I set up the problem

#1 - Transformer is under 600V, so Table 450.3(B) applies. Now, since the transformer current is 9A or more, you have 2 choices, Primary only, and primary and secondary protection. The one that achieves the higher setting is both primary and secondary protection, 250%. So, to calculate the primary OCP just determine the current on the 480V side, then multiply by 250%. The 2.8% impedance for this question is to try and confuse you, regarding Note 3.

#2 - This is a cross sectional area units problem, in. x in. - sqin, so 4 x 0.5 = 2 sqin that is the cross sectional area. If the value of 1000A/sqin is correct, then simply multiply 2 x 1000 = 2000A (the sqin cancel). The fact it is 4ft long has no barring on the cross sectional area. It could be 5000ft long and it still can handle 2000A.

 

[Dennis Alwon]

I am studying for my supervisors license and I have two math problems I would like help with.
1) Max primary overcurrent protection allowed for 2.8% impendance 500Kva 480 x 208/120 3phase transformer how do I slove for this?

I will leave this for the smart guys.

2) The maximum ampacity rating of 4in x .5 in copper bus bar 4ft long installed in an auxiliary gutter I know it's 1000amp/inches squared but how do I set up the problem

I probably should leave this also but here goes.. Figure the sq. in of the bus-- 4x.5= 2 sq. in. I don't see the 4' coming into play

I guess I was too slow- got distracted and should have checked.

 

[Lady Engineer]

I am studying for my supervisors license and I have two math problems I would like help with.
1) Max primary overcurrent protection allowed for 2.8% impendance 500Kva 480 x 208/120 3phase transformer how do I slove for this?

If the Xfmer has overload, that's when you worry about impedence. I would just do: 500/.831 x 2.5 or (500,000/(1.732 x 480V)) x 2.50 and that should give you the answer for the primary. Like kingpb said, that is just do confuse you. Most standard transformers don't go over 6% impedence when rated 600V or less. Usually larger oil filled xfmrs might be over 6%, depending on the size.

 

[kwired]

I am studying for my supervisors license and I have two math problems I would like help with.
1) Max primary overcurrent protection allowed for 2.8% impendance 500Kva 480 x 208/120 3phase transformer how do I slove for this?
2) The maximum ampacity rating of 4in x .5 in copper bus bar 4ft long installed in an auxiliary gutter I know it's 1000amp/inches squared but how do I set up the problem

Pretty stiff questions even for contractor/masters exams don't you all think?

First question maybe not so bad, but why throw in the impedance to confuse people?

I never understood why you would use trickery to fool people on these exams. Ask simple questions. Don't throw in a bunch of crap in the question that has nothing to do with the subject of the question.

 

[Little Bill]

Pretty stiff questions even for contractor/masters exams don't you all think?

First question maybe not so bad, but why throw in the impedance to confuse people?

I never understood why you would use trickery to fool people on these exams. Ask simple questions. Don't throw in a bunch of crap in the question that has nothing to do with the subject of the question.

I remember a question about lightning arrestors (well most of it) that asked how many were needed for a 3 ph system when lighting storms frequent the area?