3-Phase 3-Wire Service Entrance

[mull982]

Typically I see 3-Phase service entrances as 3-Phase 4-Wire services with the service entrance equipment having phase, ground, and neutral buses with the neutral bus being bonded to the ground bus.

Can there be a situation where a service entrance is only a 3-Phase, 3-Wire service entrance with the service entrance equipment only having the three phase busses and ground bus? I am thinking of facilities that have a 480V service entrance but have all three phase loads and do not require any 277V L-N loads or the 277 L-N voltage is derived downstream of the service entrance with isolation transformers.

Is a neutral always required with the service entrance or can there be a service entrance that is only 3-phase 3-wire?

 

[jumper]

Ungrounded Delta or a Corner Grounded Delta. Neither has a neutral.

Code requires that a neutral for wye and single phase services be brought to the service equipment. 250.24 something.

 

[Steve.Evans]

3P3W service entrance

3P3W service entrance

Yes, 3 phase/3 wire is used sometimes. Industrial mostly, and irrigation pumps. Commercial buildings require 3P4W and ground fault (up to 1000A service).

 

[Smart $]

Ungrounded Delta or a Corner Grounded Delta. Neither has a neutral. ...

Also 3Ø 3W ungrounded. A ground fault detector is required.

 

[kwired]

Ungrounded Delta or a Corner Grounded Delta. Neither has a neutral.

Code requires that a neutral for wye and single phase services be brought to the service equipment. 250.24 something.

250.24(C).

It basically says if the system is grounded that the grounded conductor must be brought to the service equipment. If you only have a three wire system then one of those conductors is the grounded conductor or you have an ungrounded system. It is possible to have center taps in the supply that are not used, in that case the system doesn't really have a neutral.

 

[mull982]

250.24(C).

It basically says if the system is grounded that the grounded conductor must be brought to the service equipment. If you only have a three wire system then one of those conductors is the grounded conductor or you have an ungrounded system. It is possible to have center taps in the supply that are not used, in that case the system doesn't really have a neutral.

Ok so it looks like per the NEC that the service entrance equipment must have a neutral bus if the system is grounded?

What about equipment (panels, switchboards, etc..) located downstream of the service entrance equipment? Do they need to have neutral buses or neutrals pulled to them if there are no L-N loads being served from them, or located anywhere else in the system?

Also 3Ø 3W ungrounded. A ground fault detector is required.

So it appears if a system is ungrounded (ungrounded delta) then a neutral isn't required to be brought to the service entrance equipment, and the service entrance equipment does not have to have a neutral bus? What about a corner grounded delta? This is still a grounded system but does not have a "neutral" per say. Does service entrance equipment still have to have neutral bus in this case?

Yes, 3 phase/3 wire is used sometimes. Industrial mostly, and irrigation pumps. Commercial buildings require 3P4W and ground fault (up to 1000A service).

Yes I see 3-Phase/3-wire in a lot of industrial systems. Usually these are separately derived systems located downstream in the plant distribution system. Can an industrial facility have a 3-phase/3-wire system but still require a neutral or "grounded conductor" and bus in the service entrance equipment?

What about at Medium voltage levels such as 4.16kV? Most of these systems are 3-Phase/3-Wire. Do the service entrance rules apply here as well?

 

[mull982]

Also what about equipment located downstream of an emergency generator? For example does a switchboard connected to a generator have to have the generator neutral brought to is and have a neutral bus if the system has no L-N loads?

What is this switchboard then feeds an ATS which has the normal side of it connected to the service entrance equipment?

 

[kwired]

Ok so it looks like per the NEC that the service entrance equipment must have a neutral bus if the system is grounded?

Not necessarily a neutral bus. Any service supplied system that is grounded the grounded conductor must be brought to the service equipment though and must be bonded to the service equipment enclosure. Grounded phase conductor of a corner ground system is a grounded conductor. Systems with a neutral conductor almost always are required to have the neutral grounded - therefore the neutral is the grounded conductor of those systems.

What about equipment (panels, switchboards, etc..) located downstream of the service entrance equipment? Do they need to have neutral buses or neutrals pulled to them if there are no L-N loads being served from them, or located anywhere else in the system?

Beyond service equipment you don't have to run a grounded conductor if you are not feeding any loads that utilize the grounded conductor. You must still run equipment grounding conductors - even with ungrounded systems.

So it appears if a system is ungrounded (ungrounded delta) then a neutral isn't required to be brought to the service entrance equipment, and the service entrance equipment does not have to have a neutral bus? What about a corner grounded delta? This is still a grounded system but does not have a "neutral" per say. Does service entrance equipment still have to have neutral bus in this case?

An ungrounded system has no grounded conductor to bring to the service equipment. If it has a neutral it very likely is required to be grounded though.

Neutral conductor and grounded conductors are often the same conductor but they each have separate definitions and different function sometimes. Not all systems have a neutral. All grounded systems do have a grounded conductor though.

Yes I see 3-Phase/3-wire in a lot of industrial systems. Usually these are separately derived systems located downstream in the plant distribution system. Can an industrial facility have a 3-phase/3-wire system but still require a neutral or "grounded conductor" and bus in the service entrance equipment?

I feel I have already answered this above.

What about at Medium voltage levels such as 4.16kV? Most of these systems are 3-Phase/3-Wire. Do the service entrance rules apply here as well?

If it is POCO system NEC doesn't apply. If it is not POCO then yes the base rules are the same unless you find something that says otherwise in the over 600 or over 1000 volt sections in the code.

4160 is usually line to line voltage of a 1460/2400 wye system. Most of the time however there is no neutral load so the 4th wire is just an equipment grounding conductor beyond service equipment. Should there be neutral loads then one would need separate neutral and EGC just like for lower voltage systems.

 

[Smart $]

Ok so it looks like per the NEC that the service entrance equipment must have a neutral bus if the system is grounded?

Not necessarily. There are many grounded neutral systems in use in which the grounded neutral is brought to the ground bus. There is no requirement for a neutral or grounded conductor bus to be included in the service equipment.

What about equipment (panels, switchboards, etc..) located downstream of the service entrance equipment? Do they need to have neutral buses or neutrals pulled to them if there are no L-N loads being served from them, or located anywhere else in the system?

No.

So it appears if a system is ungrounded (ungrounded delta) then a neutral isn't required to be brought to the service entrance equipment, and the service entrance equipment does not have to have a neutral bus? ...

Correct.

...What about a corner grounded delta? This is still a grounded system but does not have a "neutral" per say. Does service entrance equipment still have to have neutral bus in this case?

In this case, the service is either wired 3-pole with switched grounded conductor (but grounded/bonded on line side; not my preference), or 2-pole with the grounded corner conductor wired the same as if it were a neutral (it even has to be white).

Yes I see 3-Phase/3-wire in a lot of industrial systems. Usually these are separately derived systems located downstream in the plant distribution system. Can an industrial facility have a 3-phase/3-wire system but still require a neutral or "grounded conductor" and bus in the service entrance equipment?

See above answer regarding corner grounded service.

What about at Medium voltage levels such as 4.16kV? Most of these systems are 3-Phase/3-Wire. Do the service entrance rules apply here as well?

I can't think of anything different regarding grounded vs. ungrounded...

 

[mull982]

Not necessarily. There are many grounded neutral systems in use in which the grounded neutral is brought to the ground bus. There is no requirement for a neutral or grounded conductor bus to be included in the service equipment.

Ok so if its is a 3P-3W system then service entrance has "grounded conductor" (wye point) brought to the service entrance equipment and it is bonded to that equipment. The service entrance equipment in this case would need to have a "ground bus" but would not necessarily have to have a "neutral bus" The ground bus in this case would be used for bonding grounded conductor to enclosure as well as connecting EGC's for downstream feeders? This is of course as long as no L-N loads are being fed downstream.

If there are L-N loads in the system then the service entrance equipment would likely have a ground bus as well as a neutral bus which would be bonded in the SE equipment and the neutral bus would be used to serve any L-N loads downstream?

Lets say you have a 3P-3W system and for some reason the SE equipment does have a neutral bus. Is it ok to connect nothing to this neutral bus and just use the ground bus for connecting grounded conductor, bonding SE enclosure, and connecting EGC's for downstream loads?

 

[Smart $]

Ok so if its is a 3P-3W system then service entrance has "grounded conductor" (wye point) brought to the service entrance equipment and it is bonded to that equipment. The service entrance equipment in this case would need to have a "ground bus" but would not necessarily have to have a "neutral bus" The ground bus in this case would be used for bonding grounded conductor to enclosure as well as connecting EGC's for downstream feeders? This is of course as long as no L-N loads are being fed downstream.

If there are L-N loads in the system then the service entrance equipment would likely have a ground bus as well as a neutral bus which would be bonded in the SE equipment and the neutral bus would be used to serve any L-N loads downstream?

Correct

Lets say you have a 3P-3W system and for some reason the SE equipment does have a neutral bus. Is it ok to connect nothing to this neutral bus and just use the ground bus for connecting grounded conductor, bonding SE enclosure, and connecting EGC's for downstream loads?

The only 3Ø 3W system that you can have with a grounded conductor is a corner-grounded delta. Except for a switched neutral setup, the neutral bus would be used for the grounded-corner conductor.

 

[kwired]

Ok so if its is a 3P-3W system then service entrance has "grounded conductor" (wye point) brought to the service entrance equipment and it is bonded to that equipment. The service entrance equipment in this case would need to have a "ground bus" but would not necessarily have to have a "neutral bus" The ground bus in this case would be used for bonding grounded conductor to enclosure as well as connecting EGC's for downstream feeders? This is of course as long as no L-N loads are being fed downstream.

If there are L-N loads in the system then the service entrance equipment would likely have a ground bus as well as a neutral bus which would be bonded in the SE equipment and the neutral bus would be used to serve any L-N loads downstream?

Lets say you have a 3P-3W system and for some reason the SE equipment does have a neutral bus. Is it ok to connect nothing to this neutral bus and just use the ground bus for connecting grounded conductor, bonding SE enclosure, and connecting EGC's for downstream loads?

Not all systems have a neutral conductor. That is why NEC uses the term grounded conductor. If there is a neutral it will normally be required to be the conductor that is grounded. Three phase three wire does not have a neutral. It is either ungrounded or one of the phase conductors is grounded. Either case you have an equipment grounding conductor beyond the service equipment or source/first disconnect of a separately derived system. The grounded conductor and equipment grounding conductor are tied together at the service equipment so one bus can have both on it.