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Joined: Oct 2004
Posts: 2
C
Junior Member
I have a 480 3ph step down transformer feeding a 120/240v panel twenty feet away.
The feeders are 4 - 4/0 with ground. The panel is suppling electronic games and has a full 42 circuits in it with seperate nuetral for each branch circuit. The problem is the nuetral feeder is pulling 250amps and is warm to the touch. The phase conductors in the feeder are pulling 180 to 190 amps and are hot to the touch. Any ideas on why the phase conductors are hotter than the nuetral and how to fix?

Joined: Mar 2002
Posts: 582
R
Ron Offline
Member
Of course just a guess, but ..........

triplen harmonics.


Ron
Joined: Oct 2004
Posts: 2
C
Junior Member
Well I knew it had to do with harmonics but the thing about it is that I would of thought the nuetral would of showed more rise in temperature than the phase conductors. I know the code says to upsize the nuetral in this situation and that sometimes the nuetral may end up larger than the phase conductors. But I would of figured there to be a higher temperature on the nuetral since the amperage is higher.
Thanks

Joined: Feb 2004
Posts: 494
M
Member
hi,
it sounds like you are exceeding the permissible load on the branch circuits..

4/0 and a 190 A load? does that not send up a flag for you?

arent you seeing inductive heating..the more heat the higher the resistance..

i would up the size of the service..and feeders to reduce the load below 80%..
it also sounds like you have an unbalanced load..and some shared neutral issues..

-regards


mustang

Joined: Jan 2003
Posts: 4,391
I
Moderator
Quote
neutral feeder is pulling 250amps and is warm to the touch. The phase conductors in the feeder are pulling 180 to 190 amps and are hot to the touch. Any ideas on why the phase conductors are hotter than the nuetral and how to fix?

Possibly loose connections of the ungrounded conductors to the panel lugs or lugs to panel.

You did not say the panel rating or if the 4/0 is AL or CU.

To me what you described sounds like a 75 KVA transformer feeding a 225 amp panel with 4/0 copper.

If I assumed wrong disregard the following. [Linked Image]

If that is the case 180 amps is the highest allowed continuous load for this panel. The transformer would be rated 208 amps.

There is only one way to fix this and that is reduce the load.

You could run a second 4/0 neutral and try to order a 200% rated neutral bar for this panel to deal with the high current on the neutral.

After doing this you will still have an overloaded panel. The transformer will also still be dealing with 250 amps on the neutral. Ron or Dereck could say if this will shorten the transformer life. [Linked Image]

BTW, I hope these are copper conductors.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
I haven't dealt with harmonics on a neutral. Please post your findings here.

If the phase conductors feed into a main breaker in the panel, the heat could be coming from the breaker.

I had a harmonic problem that was tripping a breaker. Every year the molded case breaker had to be replaced. The customer's decision was that it was cheaper to suffer down time and replace breakers every year than to install a harmonic filter. It was on an oil well, so I assume that the harmonic correction was very expensive.

Joined: Apr 2002
Posts: 2,527
B
Moderator
cvictory — The description sounds more like it may be 208Y/120V service from the 480-primary transformer. Is that the case?

A low-end IR non-contact thermometer may be more "accurate" than fingers.

Joined: Dec 2003
Posts: 59
B
Member
did you check the grounding in the transformer?
is there any equip. that is burning up?

Joined: Oct 2003
Posts: 156
D
Member
I would like to comment, but something is not clear too me.

"I have a 480 3ph step down transformer feeding a 120/240v panel twenty feet away".

Huh? Can you clarify? Do you have a 480 delta primary with 208/120 output? Or do you have a 480 input, 240/120 output. I can understand the high neutral current on a 208/120 system, but not on a 240/120 system.

Please clarify. Need to know size, type, and "K" rating of transformer. Normally for this type of installation you would use a delta 480 input, 208/120 output, "K-13" rated step-down transformer, and a panel with 200% neutral bus bar.


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