CORNER GROUNDED DELTA SYSTEM

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fifty60

Senior Member
Location
USA
I am not very familiar corner grounded delta systems. If I have a 240V corner grounded delta system, what limitations (if any) does this put on equipment that is is 230V single phase or 230V 3-phase?
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I am not very familiar corner grounded delta systems. If I have a 240V corner grounded delta system, what limitations (if any) does this put on equipment that is is 230V single phase or 230V 3-phase?
Mainly calculating to balance the single phase loads as much as possible.
The voltage phase-to-ground on a single phase load will be 240 rather than the 120 to ground from a single phase 120/240 3-wire source.
A singe phase load between the upper two lines will have both ends hot, so leakage and phantom voltages could be worse until everything is bonded to ground.
There will be no 120 for controls or other incidental loads unless you install a separate xformer.
You cannot reduce the size of the grounded conductor even for pure three phase loads.

I must have forgotten a few things.... :)
 

Smart $

Esteemed Member
Location
Ohio
...
You cannot reduce the size of the grounded conductor even for pure three phase loads.

I must have forgotten a few things.... :)
While you cannot reduce the grounded conductor as you would a neutral, sizing a grounded feeder conductor for ampacity (when there is no series ocpd installed) is not required to include the extra 25% for continuous loads. IIRC, this applies to grounded neutral feeder conductors also, but is often overlooked.
 

fifty60

Senior Member
Location
USA
I believe I am understanding more. The line to line voltages will remain the same, but the line to ground voltage will be the L-L voltage instead of L/1.73.

So, if all of my loads are wired line to line (whether wired as single phase or three phase loads) then there will not be any performance difference. Somehow I cannot get over the fact that a motor would start the same with a phase grounded, and if this is the case I will need to think about it some more.

In summary, the loads will all perform the same as long as they are using L-L voltages. I must be sure that my equipment is not slash rated. Circuit breakers for example, if my circuit breaker is rated 480/277 then I could not use it on a delta grounded system? I would have to use a 480 to ground rated breaker? What if my power supply is 270V grounded delta, could I use the 480/277 circuit breaker then?

Does any other equipment generally have the slash ratings, or is it typically just circuit breakers that are the main concern?
 

kwired

Electron manager
Location
NE Nebraska
I believe I am understanding more. The line to line voltages will remain the same, but the line to ground voltage will be the L-L voltage instead of L/1.73.

So, if all of my loads are wired line to line (whether wired as single phase or three phase loads) then there will not be any performance difference. Somehow I cannot get over the fact that a motor would start the same with a phase grounded, and if this is the case I will need to think about it some more.

In summary, the loads will all perform the same as long as they are using L-L voltages. I must be sure that my equipment is not slash rated. Circuit breakers for example, if my circuit breaker is rated 480/277 then I could not use it on a delta grounded system? I would have to use a 480 to ground rated breaker? What if my power supply is 270V grounded delta, could I use the 480/277 circuit breaker then?

Does any other equipment generally have the slash ratings, or is it typically just circuit breakers that are the main concern?

You need to stop equating the neutral conductor (in systems that have a neutral) to grounded conductors. Though code requires the neutral to be the conductor that is grounded in almost all cases, reality is any point of the system could be grounded. If you took a typical 208/120 and grounded a phase conductor instead of the neutral point, voltages between conductors still remains the same and connected loads would still work the same. The only difference is reference to ground, and equipment grounding would have to be moved to the new grounded point otherwise if two system conductors are grounded you have a fault.
 

fifty60

Senior Member
Location
USA
So if I have a delta heater load powered by a corner grounded delta supply, I will have L-L voltages between each phase of my load. If my branch OCPD for the delta heaters are fuses, do I have to remove the fuses for the grounded leg in the branch circuit? So, instead of a branch circuit with 3 fuses I would only have 2? I am reading other posts on this topic and this is what I am understanding...
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
So if I have a delta heater load powered by a corner grounded delta supply, I will have L-L voltages between each phase of my load. If my branch OCPD for the delta heaters are fuses, do I have to remove the fuses for the grounded leg in the branch circuit? So, instead of a branch circuit with 3 fuses I would only have 2? I am reading other posts on this topic and this is what I am understanding...
Yes, as you can't have a fuse in a grounded conductor. You could use common trip 3 pole breakers but why do that when you could use 2 pole equipment with a solid neutral for the grounded conductor. Some thing to consider is that not all electrical equipment is listed for use on a corner grounded system (also known as grounded B phase to many). I think much of Square D is listed for such use. Maybe Jim Dungar could better comment on this. Also, others mentioned you can't use "slash" rated breakers.
 

jim dungar

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Staff member
Location
Wisconsin
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PE (Retired) - Power Systems
Some thing to consider is that not all electrical equipment is listed for use on a corner grounded system (also known as grounded B phase to many).

Equipment selection can definitely be an issue.
Along with the 'slash rating', Corner Grounded (grounded B phase) systems can impact the AIC ratings of breakers.
For example:
a standard QO-H breaker is only rated 5kAIC
'power' panels (e.g. I-Line) often require special breakers for 240V which are built using 3-pole housings
480V rated breakers are only available with 3-poles and special labeling.
 
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