low voltage issue

[Besoeker]

The problem would appear to be an undersized or poorly connected neutral conductor

I'm inclined to agree. The difference between phase to neutral and phase to ground would seem to infer a fairly significant voltage drop on the neutral.

 

[hilltop]

thank you all for your input, I apologize for the delay on my part for the additional info you are asking for.

Phase to Phase voltage is as follows:
A phase 71A -79A
B Phase: 71A - 79A
C Phase: 56A to 60A

Voltage phase tophase;
A to B = 456V
B to C = 4459
C to A = 458V

We do have an infrared camera...I am not picking up a lot of heat in any panel I have opened so far

 

[bob]

thank you all for your input, I apologize for the delay on my part for the additional info you are asking for.

Phase to Phase voltage is as follows:
A phase 71A -79A
B Phase: 71A - 79A
C Phase: 56A to 60A

Voltage phase tophase;
A to B = 456V
B to C = 4459
C to A = 458V

We do have an infrared camera...I am not picking up a lot of heat in any panel I have opened so far

What is the voltage at your main switch?

 

[hilltop]

Typo= B to C phase should read 445V

My concern is a floating nuetral as well.
We have had the utility check the supply, we are on the low end but within range.

I appreciate the feed back, the more info I have going in to the gear the better chance I will have to find the actual problem

 

[hilltop]

voltage at the main switchgear is:
a to nuet. - 271V
b to nuet. - 270V
c to nuet. - 272V

A to B phase - 470v
b to c phase - 470v
C to A phase - 470V

 

[hilltop]

Bear in mind that from the switchgear then feeds an load center that feeds this panel we are discussing. Total length is approx 150' between the main and the panel we are discussing.

for a visual
So we have a 4000A service(only a couple of months old) with voltages I just gave to you

4000A feeds a load center where a 1200A switch(haven't opened this one yet, no display for voltage) feeds a MCC panel where there is a 400A switch(haven't opened this one yet) that feeds the 225A panel (we have been discussing this one)

Everything downline from the 4000A gear is fair to say at least 30 years old.

 

[augie47]

I'd be curious as to whats happening in the MCC. Often these don't require a neutral and the neutral to your equipment may have been obtained by some interesting means

 

[philly]

The L-N reading on any phase should be 277V (ignoring voltage drop) between the phase conductor and the center point of the transformer if measured directly to the center point of the transformer (bypassing neutral wire) weather or not the neutral is floating or not. If the neutral if floating with respect to ground then only the N-G reading will be something other than 0V but the L-N readings should always be 277V (ignoring voltage drops and measuring directly to neutral point on transformer)

I gather that other folks here are saying that there is a high resistance or large voltage drop in the neutral that is causing these strange L-N readings.

How can we tell that all phase currents are balanced by looking at the L-G readings? If all the L-G readings are roughly the same then I guess all the phase conductor voltage drops would therefore be the same. If the L-G readings were not the same then we could assume that unbalanced phase currents are causing different voltage drops in each conductor therefore producing a different L-G voltage reading?

 

[hilltop]

Nuetral to ground I have 13.4V

 

[augie47]

have you been able to check your neutral connections back to the main switchboard yet ? how about in MCC ?

 

[hilltop]

Found a couple of strange things,
#1 the bus between the load center and the main taps from the new gear show a 20V N to Grnd difference. Only 4 feet of bus seperating the two sides of the gear...we are thinking a lose connection in the nuetral bus? We put a 250KCMIL across and received a spark and it resolved our N to Grnd voltage now both sides are at 1V.

Just as we suspected earlier, The feed to the MCC was a 3 wire, but from the MCC to the distribution panel it magically became a 4 wire...now to complicate even further an 1/1/2" conduit with no ground fromthe distribtuion panel to a sub panel was completely pulled apart about 6 inches...
We are still looking into the low voltage on each phase from froma xfrm fed from this subpanel 45KVA 480 primary 120/208V secondary..A,B, and C phase 108V, 110, and 112V. Xfrm is right above panel that it feeds....

 

[augie47]

...................................................................
Just as we suspected earlier, The feed to the MCC was a 3 wire, but from the MCC to the distribution panel it magically became a 4 wire.......

I would have put money on that. Happens all the time

Now to complicate even further an 1/1/2" conduit with no ground from the distribtuion panel to a sub panel was completely pulled apart about 6 inches...
We are still looking into the low voltage on each phase from froma xfrm fed from this subpanel 45KVA 480 primary 120/208V secondary..A,B, and C phase 108V, 110, and 112V. Xfrm is right above panel that it feeds....

Sounds like that might be rectified by changing the taps. At least the balance looks o.k. there

Thanks for reporting back.

 

[gmtt]

If the secondary is of 480V/3 phase then all phase voltage (phase to neutral) ought to be 277V for a well-regulated system. And this should be true regardless of loading condition. I suspect either the utility supply voltage is imbalanced (which is unlikely, because, then all transformers connected to this the high side would have similar problem and utility co would have fixed their end of the problem by now) or the problem is with transformer itself.