Requirements for grounding 3 phase system at service entrance or main panel

mull982

Senior Member
What are the requirements for grounding a 3 phase service or a seperately derived service at the main panelboard for the following:

For a 3 wire service with no neutral I would assume there is nothing to ground in the main panel and the requirement would be that the system bonding jumper between the EGC and the transformer neutral would be done at the transformer? An EGC would be run from the transformer to the ground bus in the main panel. Is there any requirement to connect the ground bus in the main panel to an grounding electrode via a GEC

For a 4 wire service including a neutral? I would assume that you would need to connect a system bonding jumper either at the transformer or at the main panel. I would assume that you would not want to connect this at both ends, because then you would set up a parallel path for unbalanced neutral current to flow on the ground. Is a EGC required between the transformer and main panel? If system bonding jumper is installed at transformer then is there no connection between ground bus and neutral bus at main panel?
 

Dennis Alwon

Moderator
Staff member
What are the requirements for grounding a 3 phase service or a seperately derived service at the main panelboard for the following:

For a 3 wire service with no neutral I would assume there is nothing to ground in the main panel and the requirement would be that the system bonding jumper between the EGC and the transformer neutral would be done at the transformer? An EGC would be run from the transformer to the ground bus in the main panel. Is there any requirement to connect the ground bus in the main panel to an grounding electrode via a GEC
Read article 250.24(C). If the ac system is grounded at any point and less than 1000V then a neutral must be brought to the service
 

lauraj

Senior Member
What are the requirements for grounding a 3 phase service or a seperately derived service at the main panelboard for the following:

For a 3 wire service with no neutral I would assume there is nothing to ground in the main panel and the requirement would be that the system bonding jumper between the EGC and the transformer neutral would be done at the transformer? An EGC would be run from the transformer to the ground bus in the main panel. Is there any requirement to connect the ground bus in the main panel to an grounding electrode via a GEC
There is no neutral, therefore, no main bonding jumper for a 3-phase, 3-wire ungrounded service. An equipment bonding jumper sized according to 250.102 would be used to bond the metal parts of the service between the transformer and the first disconnect, if needed. You still need a grounding electrode system for your service panel.

If it's a 3-wire corner grounded delta service, you will have 2 colored phase conductors and one grounded conductor. You would ground this service as if it were a typical grounded service, white or gray conductor landing on the "grounded conductor" bar and a main bonding jumper tying the grounded conductor to the equipment ground. Grounding electrode system required at the service as well.


For a 4 wire service including a neutral? I would assume that you would need to connect a system bonding jumper either at the transformer or at the main panel. I would assume that you would not want to connect this at both ends, because then you would set up a parallel path for unbalanced neutral current to flow on the ground. Is a EGC required between the transformer and main panel? If system bonding jumper is installed at transformer then is there no connection between ground bus and neutral bus at main panel?
This is still a service, just because the service is fed by a transformer from the utility, do not confuse it with a separately derived system. All bonding and grounding required as per 250.24, 250.28, 250.102, and part III, amongst other requirements.

Main bonding jumper required.
Grounding electrode system required.
 

augie47

Moderator
Staff member
What are the requirements for grounding a 3 phase service or a seperately derived service at the main panelboard for the following:

For a 3 wire service with no neutral I would assume there is nothing to ground in the main panel and the requirement would be that the system bonding jumper between the EGC and the transformer neutral would be done at the transformer? ?
If it's a true 3 phase 3 wire service there is no neutral at the transformer. If there is then the NEC requires it to be brought to the service equipment 250.24(C).
If it is an ungrounded system 250.21 requires Ground detectors.

An EGC would be run from the transformer to the ground bus in the main panel. ?
Not normally on a service
Is there any requirement to connect the ground bus in the main panel to an grounding electrode via a GEC?
Yes, although if it's to a buss it would probably be a grounding equipmemnt buss. The service equipment panel must still be attached to a grounding electrode system. 250.24(E)
For a 4 wire service including a neutral? I would assume that you would need to connect a system bonding jumper either at the transformer or at the main panel. I would assume that you would not want to connect this at both ends, because then you would set up a parallel path for unbalanced neutral current to flow on the ground.
Correct
Is a EGC required between the transformer and main panel? If system bonding jumper is installed at transformer then is there no connection between ground bus and neutral bus at main panel?
On services for a 4w 3 ph your gounded conductor serves at
you grounding conductor also. No EGC is normally run from the transformer/

On a SDS you can make the bond at the transformer or at the panel, not both.
 

iwire

Moderator
Staff member
On a SDS you can make the bond at the transformer or at the panel, not both.

250.30(A)(1) Exception No. 2: A system bonding jumper at both the
source and the first disconnecting means shall be permitted
where doing so does not establish a parallel path for the
grounded conductor. Where a grounded conductor is used
in this manner, it shall not be smaller than the size specified
for the system bonding jumper but shall not be required to
be larger than the ungrounded conductor(s). For the purposes
of this exception, connection through the earth shall
not be considered as providing a parallel path.
I am guessing this is used by facilities that own outdoor pad mount transformers and want to install them just like the power comapny would.
 

xformer

Senior Member
I am guessing this is used by facilities that own outdoor pad mount transformers and want to install them just like the power comapny would.
IMHO, I always thought this was for residential services. Let me explain. I can see where the bonding of a pole mounted transformer could get interrupted by people inadvertently messing with the ground wire running down the pole. Therefore the secondary bond or safety bond, would be at the panel in the house. This is strictly a personal theory i have.
 

iwire

Moderator
Staff member
IMHO, I always thought this was for residential services. Let me explain. I can see where the bonding of a pole mounted transformer could get interrupted by people inadvertently messing with the ground wire running down the pole. Therefore the secondary bond or safety bond, would be at the panel in the house. This is strictly a personal theory i have.
The transformer at the pole is usually not covered by the NEC so that bond is not counted.
 

mull982

Senior Member
If it's a true 3 phase 3 wire service there is no neutral at the transformer. If there is then the NEC requires it to be brought to the service equipment 250.24(C).
What if the service is fed by a wye connected secondary transformer or we are dealing with a panel on the secondary of a transformer in a plant or somewhere.
The wye transformer has a neutral but if there are no L-N loads being served anywhere is it even necessary to bring this neutral wire to the main panel or bond or bond or ground it there?
 

augie47

Moderator
Staff member
There may be a fine line.
250.20 states:
(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor


If you have a 208 v transformer with an XO tap it seems that it would be difficult to say it does not meet (1) above. That said, I have seen situations with POCO and with customer SDS systems where the neutral was not needed and the XO terminal was ignored.
 
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