# NCEES #101



## truong (Sep 7, 2011)

I'm really not clear about this problem:

Given current I(s) = 5A

N(p) / N(s) = 400:5

Option 1: By formular: N(p)/N(s) = I(s) / I(p)

400/5 = 5/I(p)

===&gt; I(p) = 5*5/400 This is not the right ans.

Option 2: If the given current is I(p) = 5A, then I(s) = (400/5)*5 = 400A This is the correct ans.

But the answer in NCEES is I(p) not I(s).

Would someone explain this problem for me? Thanks!!!


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## Flyer_PE (Sep 7, 2011)

You're given a CT with a turns ration of 400:5 and told that full scale reading for the measuring device is 5 amps.

N = 400/5 = 80

5*80 = 400 Amps

If the value the given maximum on the SCADA side was 4 amps instead of 5, the maximum primary current would be 320 Amps (4*80=320).

Hopefully, I didn't just make it worse for you.


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## truong (Sep 8, 2011)

Flyer_PE said:


> You're given a CT with a turns ration of 400:5 and told that full scale reading for the measuring device is 5 amps.
> N = 400/5 = 80
> 
> 5*80 = 400 Amps
> ...


Thank you but it was not my question.

My question was the 400A is primary current or secondary current?

Thanks!


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## willsee (Sep 8, 2011)

The 400A is the current in the line that you are measuring.

The CT steps it down to measurable levels. CT's are usually a something:1 or something:5 ratio.


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## Flyer_PE (Sep 8, 2011)

Ok, I'll take another swipe at this. The turns ratio of the CT is given in terms of current rather than voltage. This inverts the equation for determining N.

The "secondary" winding on the CT actually has the higher number of turns.

For your equation, then:

NP = 5

NS = 400

IP/IS = NS/NP so

IP = IS*(NS/NP) = 5(400/5) = 400 Amps


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## truong (Sep 8, 2011)

Flyer_PE said:


> Ok, I'll take another swipe at this. The turns ratio of the CT is given in terms of current rather than voltage. This inverts the equation for determining N.
> The "secondary" winding on the CT actually has the higher number of turns.
> 
> For your equation, then:
> ...


But usually the turning ratio is Np:Ns, correct? In this problem the ratio given is 400:5, that means Np= 400, Ns= 5?


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## willsee (Sep 8, 2011)

truong said:


> Flyer_PE said:
> 
> 
> > Ok, I'll take another swipe at this. The turns ratio of the CT is given in terms of current rather than voltage. This inverts the equation for determining N.
> ...


You're stepping the current down, which means the voltage is going up therefore there is 5 turns on the primary and 400 turns on the secondary.


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## truong (Sep 8, 2011)

willsee said:


> truong said:
> 
> 
> > Flyer_PE said:
> ...


If the turn in primary is 5 and in secondary is 400, should it be written as 5:400 instead of 400:5?


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## Flyer_PE (Sep 8, 2011)

truong said:


> If the turn in primary is 5 and in secondary is 400, should it be written as 5:400 instead of 400:5?


The turns ratio for a CT is given in terms of current, not voltage. It will almost always be expressed in the format given (400:5) which translates as 400 Amps primary results in 5 Amps secondary.


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## truong (Sep 8, 2011)

Flyer_PE said:


> truong said:
> 
> 
> > If the turn in primary is 5 and in secondary is 400, should it be written as 5:400 instead of 400:5?
> ...


Thanks for your explanation.


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## kris7o2 (Jun 26, 2021)

Hi everyone, does anyone know how to draw this? I understand how to get the answer, but honestly, I do not know what this looks like.


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