how to size transformer neutral and ground conductor.

Status
Not open for further replies.
G

gmtt

Member
Scenario: A transformer with a secondary voltage of 240/120V is supplying a load of 200 A per phase.
Question: What should be size of the 1) neutral and 2) ground wire?
My thoughts are these.
1) The neutral wire of the transformer should be rated to carry sum of the two phase current that is 400A and the size should be 600 kcmil. is common for both phases then the neutral will carry sum of the current of 2 phases, that is 400 amp ( but I don?t recall seeing any neutral that double the size of phase)
2) For ground wire, I choose the size this way: the current carrying conductor size for 200A is 3/0. From table 250.66 the ground conductor size is #4 AWG
I think I am wrong in both, buy why?
 
I am closing this thread, in accordance with the Forum rules. This Forum is intended to assist professional electricians, inspectors, engineers, and other members of the electrical industry in the performance of their job-related tasks. However, if you are not an electrician or an electrical contractor, then we are not permitted to help you perform your own electrical installation work.


If I have misjudged the situation, if for example this project is related to your work, then send me a Private Message. If you can show me that I am wrong, and that you are a licensed electrician (or at least a licensed apprentice), then I will reopen your post, and offer an apology for the delay and inconvenience.

A reminder to all members, if you see a questionable post, feel free to report it by clicking the
report.gif
button at the top right corner of the post.

Chris
 
I am re-opening this thread.

After talking with the original poster they confirmed that they are in fact an new electrical engineer.

I apologize for the delay.

Chris
 
gmtt said:
I think I am wrong in both, buy why?
Click to expand...
You are right in one and wrong in the other. But welcome to the forum. You?ll find plenty of learning opportunities.

Here is what you are missing on the neutral current. The current leaving on one phase conductor, and the current leaving on the other phase conductor, are 180 degrees out of phase with each other. The practical result is that current leaving on Phase A returns on Phase B, not on the neutral. Also, current leaving on Phase B returns on Phase A, again not on the neutral. Therefore, if the loads are perfectly balanced, there is no current on the neutral at all.

Another practical result is that the current in Phase A is the same current as in Phase B. That is, you don?t add the 200 amps on Phase A to the 200 amps on Phase B and get a ?total? of 400 amps. Indeed, the notion of ?total current? is meaningless.

As to the size of the neutral, I almost always make it the same size as the phase conductors. In this case, a 200 amp load would call for 3/0 phase conductors. If a large portion of the load is non-linear (i.e., a harmonics problem), I might consider up-sizing the neutral a size or two. If you have a formal load calculation, including a calculation of the neutral load, you may be able to justify a reduction in the size of the neutral.

As to the size of the ground conductor, I agree with your choice of #4.
 
gmtt said:
Scenario: A transformer with a secondary voltage of 240/120V is supplying a load of 200 A per phase.
Question: What should be size of the 1) neutral and 2) ground wire?
Click to expand...

Looks good.
For reference, here is a calc for Neutral Currents, 3 phase.

Ineutral =
SQRT (
I?A + I?B + I?C
- (IA x IB) - (IB x IC) - (IC x IA)
)

Welcome back into the forum.
:smile:
 
Status
Not open for further replies.
Top