3 phase 3 wire solidly grounded neutral MCC neutral connection

[tlona]

Good Morning. The client has a Rockwell Automation Centerline 2100 series 480v/2000A MCC that is labeled as a 3phase 3wire Solidly grounded neutral (honestly have never herd of 3p/3w/solid neutral ground). The MCC is being fed from a 480V/277 wye xmfr whose neutral point is grounded at the transformer. There is no neutral bar in the MCC and we are trying to determine where to land the neutral in the MCC (on the ground bar?). Thank-you

 

[TugboatEng]

MCC shouldn't have a neutral. No single phase loads other than control transformers. MCC cabinet should not be bonded to transformer neutral. There must be only one connection between neutral and ground for the entire system.

 

[RRaghunath]

3p/3w distribution with solidly grounded neutral at the transformer is not uncommon.
In such a distribution, Dyn transformers are installed with a 3p+n DB on secondary side for supplying single phase loads such as lighting and small power.
In MCC, control transformers connected between two phases on primary are employed to supply power for motor control schemes.

 

[tlona]

Thank-you for the response. If there is no neutral bar where/how does the neutral get bonded to the MCC? Simply attach neutral to MCC ground bar? Also, does that also mean no grounding electrode conductor from MCC ground bar to ground rod at MCC? Thank-you for your knowledge.

 

[TugboatEng]

You only have 3 wires to connect, all line voltage. There is no neutral.

 

[tlona]

So a bonding jumper needs to be installed from the MCC ground bar to the transformer neutral? I think I am getting lost in the terminology since the MCC is identified as a solidly grounded neutral I assume it needs a solidly grounded neutral run from MCC to transformer neutral.

 

[RRaghunath]

The ground bar in the MCC is for bonding the metal in the MCC including cable armour etc.
Neutral grounding is only at the transformer.
'Solidly grounding neutral' defines what is the spacing required from phase to the panel in the MCC (the spacing can be lower than that between phases).

 

[jraef]

MCC cabinet should be bonded to transformer neutral.

That may have been a typo. MCC cabinet will be (from the factory) bonded to the GROUND bar, then the ground bar must be connected to your ground in whatever fashion it exists (i.e. ground rod, building framing, ground grid etc.).

Unless you have loads that USE the neutral, which is uncommon for an MCC, then you do not need to run the neutral conductor TO the MCC. The only other time a neutral connection is needed is if the MCC Main needs to be Service Entrance listed. In that case, all you need is what’s called a “neutral landing pad” in the section with the main (or immediately adjacent to it).

The reason for stating it is a “3 wire solidly grounded neutral” feed is to state that the source is a solidly grounded Wye system, as opposed to a Delta or Resistance Grounded Wye, because that makes a difference for things like VFDs that might be in or fed from the MCC.

Note: if you have already run the neutral cable and don’t want to or can’t pull it back, then you can either not connect it at the source end and just abandon it in the conduit, or you can, I think, order a neutral landing pad retrofit kit from Rockwell. But IIRC it consumes at least (depending on size) a 1/2 Space Factor in the bottom of the MCC section with the Main or next to it, which might mean rearranging other units to accommodate it.

If you DO need to use the neutral for loads, and someone didn’t order it from the factory with the neutral bus, then you are screwed.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[tlona]

Great feed back from all. Much appreciated. Makes perfect sense now. Thank-you again, awesome forum!!

 

[che12345]

Dear Mr. tlona (Industrial)(OP)14 Nov 22 16:03
[COLOR=]"....#1. The client has a ..... 480v/2000A MCC that is labeled as a 3phase 3wire Solidly grounded neutral The MCC is being fed from a 480V/277 wye xmfr whose neutral point is grounded at the transformer."[/color]
1. It is common having a 3-phase 3-wire (Neutral solidly grounded at the transformer) but [ NOT distributed to the MCC]. Where the Neutral is NOT required/needed at the MCC.
1.1 3-phase motors do NOT need the Neutral.
1.2 For some small single-phase (e.g. lighting ) loads, a step-down transformer is added.
1.3 Attention: The MCC shall be solidly grounded.
[COLOR=]"....#2. There is no neutral bar in the MCC and we are trying to determine where to land the neutral in the MCC (on the ground bar?) ".[/color].
2.1 All 3-phase motors do NOI need the Neutral.
2.2 All 1-phse motors and 1-phase loads would be connected to the ( often known as "lighting" [single-phase transformer with L and N terminals].
2.3 All 1-phase load Neutral shall be connected to the N of this 1-phase transformer.
2.3.1 Attention: (a) All 1-phase load Neutral Shall NOT be connected to the Ground bar.
(b) All 1-phase loads/equipments conductive parts MUST be grounded (i.e. connected to the Ground bar).
Che Kuan Yau (Singapore)