Three Phase 208v system with 20 to 60 volts on the neutral buss

[GJP]

I have an older school building with a 3 phase 208v 2000 amp service a disconnect under load in the main gear feeding a Sub-Panel
in another wing of the school reads each phase to neutral 120v & each phase to ground 120v & neutral to ground around 400 mv.
The Sub-Panel under No load has the same readings, the Sub-Panel feeds 3 other panels, Kitchen, Gym & General lighting. Individually each panel under load the neutral reads a range of 7-57 v to ground depending on the panel under load and lights flicker as the load increases with motors running on and off etc.. All connections in all panels are tight.
The Sub panel is fed with (6) parallel 350 mcm aluminum conductors and (2) 350 mcm aluminum neutral conductors in (2) conduits.
I believe we might have a problem with the (2) 350 mcm neutrals originating at the main disconnect feeding the Sub-Panel.:rotflmao:

 

[Dennis Alwon]

Welcome to the forum

Is this community,city water? Could the current be coming in on the water lines?

 

[GJP]

Yes city water, building has great grounding, ground rods, building steel & main water line, and primary transformer is grounded and bonded.

 

[gar]

190206-1128 EST

GJP:

Your post is confusing because you do not give unique names to each different panel.

You have a 3 phase wye Main panel. There is no problem here.

You have at least one subpanel fed from the the Main panel, call this MS1.

Subpanel MS1 under noload has the same readings as the Main panel. These are good readings.

Subpanel MS1 has three other subpanels fed from MS1. Call these MS1K for kitchen, MSK1G for Gym, and MS1L for general lighting.

Individually each panel under load the neutral reads a range of 7-57 v to ground depending on the panel under load and lights flicker as the load increases with motors running on and off etc..

What is ground and where was the measurement point?

Is neutral at MS1 jumping around in the 7-57 V range relative to ground? If you run a long test lead, any insulated wire or extension cord will work, from Main-neutral to MS1, and measure voltage between Main-neutral and MS1-neutral what values do you read loaded and unloaded? If the readings are substantial, meaning more than possibly 1 V, then there is a problem between the Main and MS1 in the neutral wire or the terminations at either end. Also check Main-neutral to EGC voltage at MS1, loaded and unloaded.

Use similar checks at other points.

.

 

[GJP]

Checked Main-neutral to EGC voltage at MS1, loaded and unloaded. Reading was 400 mv.

 

[GJP]

Sorry I read that last post wrong I have not checked Main-neutral to EGC voltage at MS1, loaded and unloaded. (Will try this next)