I am looking at a pole mounted utility provided 3-can xfmr bank with a secondary delta configuration. T1 feeds T2, T2 feeds T3, T3 feeds T1. In the middle of each feed, the phase conductor taps and drops to the service disconnect. However, they also have a neutral coming to the disconnect so initially though it was a 480/277V 3-phase 4W. Voltage L-L across all phases is 480V. However, L1-N = 480V, L2-N = 480V, L3-N = 0V. On the pole, it appears they have the neutral bonded to the 3 cans but they have a jumper going from L3 to ground on the pole hence 0V L3-N. I am thinking the neutral should not have been brought to the service disconnect or it was intended to be a 4-wire system but something is wired incorrectly at the pole. Anybody here on the utility side that can help explain if this is the proper transformer wiring or what it should be? It is an industrial site so no N needed and it does not extend past the service disconnect. Something just does not seem correct.
480v 3W delta and neutral
- Thread starter mjmike
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jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
This seems like corner grounded delta. It's a fairly rare but legit system setup. Not sure if it is normal to bring an additional grounded wire to the disconnect though.
Yes, I agree, I think it is a corner grounded system as well in which case, I believe bringing the neutral down the pole I believe is incorrect.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Agreed, corner grounded delta, but strange that they have two separate conductors for 'grounded phase conductor' and 'neutral conductor'. Almost as if the utility decided to run an EGC for giggles.
That 'neutral' is of course not a neutral, but a parallel 'grounded phase' conductor. Perhaps it was intended to be connected to a transformer midpoint terminal to make a grounded 'high leg' delta.
-Jon
That 'neutral' is of course not a neutral, but a parallel 'grounded phase' conductor. Perhaps it was intended to be connected to a transformer midpoint terminal to make a grounded 'high leg' delta.
-Jon
Looking into this issue deeper and assuming it is a 480V corner grounded delta system, here are some additional questions:
The service disconnect is a fused disconnect. It is my understanding the grounded phase should be a no-blow fuse, is that correct and what is the code reference?
Next, should the grounded phase be jumpered to the ground bar in the service disconnect with only 3 wires coming down pole or should it be bonded at the pole xfmr and brought down similar to the neutral with 4-wires to the service disconnect?
From the service disconnect on, I understand that all 3-pole CB's should be straight 480V rated. If fuses are used, do all 3 phases get fused or does the grounded phase remain no-blow?
When wiring to a motor (or any other 3-phase load), does the motor need to be specifically rated for 480V delta?
How about the motor starter with overloads, does it specifically need to be delta rated? sometimes motor startes have fuses instead of CBs.
I have read that on a delta system, you only need to use 2-pole CB's. How does the grounded leg get sized and wired as it works its way from a motor back to the starter, back to a distribution panel, then back to the service?
Does anybody have a photo reference of a properly wired 480V corner grounded delta service?
Having a little difficulty with the understanding the concept of getting 3-phase power at a motor when 1 phase is grounded, does the grounded phase actually have voltage on it? Its almost like you can touch it and not get zapped similar to touching ground since at the same potential.
The service disconnect is a fused disconnect. It is my understanding the grounded phase should be a no-blow fuse, is that correct and what is the code reference?
Next, should the grounded phase be jumpered to the ground bar in the service disconnect with only 3 wires coming down pole or should it be bonded at the pole xfmr and brought down similar to the neutral with 4-wires to the service disconnect?
From the service disconnect on, I understand that all 3-pole CB's should be straight 480V rated. If fuses are used, do all 3 phases get fused or does the grounded phase remain no-blow?
When wiring to a motor (or any other 3-phase load), does the motor need to be specifically rated for 480V delta?
How about the motor starter with overloads, does it specifically need to be delta rated? sometimes motor startes have fuses instead of CBs.
I have read that on a delta system, you only need to use 2-pole CB's. How does the grounded leg get sized and wired as it works its way from a motor back to the starter, back to a distribution panel, then back to the service?
Does anybody have a photo reference of a properly wired 480V corner grounded delta service?
Having a little difficulty with the understanding the concept of getting 3-phase power at a motor when 1 phase is grounded, does the grounded phase actually have voltage on it? Its almost like you can touch it and not get zapped similar to touching ground since at the same potential.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Just to answer your last bit:
The key is that the motor doesn't care about the voltage to ground; all it cares about is the voltage between supply conductors.
The motor sees '480V' L-L. Those conductors could be 277V to ground, or one could be grounded, or the supply could be a delta with a mid point ground, or it could be ungrounded.
The only thing that cares about the voltage to ground is the insulation.
Jon
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
When the corner of delta connected transformer secondary windings are grounded you are not only establishing the voltage on that particular phase conductor but also the voltage on the other two conductors relative to ground. So it's not like your shorting it out that point, you're just shifting the point where all three voltages are referenced to. As John has said, the line-to-line voltages remain unchanged and that is all that matters for delta connected loads such as motor windings.
Equivalently what happens when the corner of the delta is grounded is that the voltage at each phase is offset by the same AC voltage (i.e., a zero sequence voltage). If we started off with 277V symmetrically to ground on phases A, B, C with phase A at 0°, then grounding phase A effectively adds 277V at 180° to each of the three phases so that the voltage relative to ground is nulled out on phase A. If we ignore usually minor effects like parasitic capacitance, then there's no current through the corner connection to ground unless there's a ground fault on another phase. And so the current through the phase A conductor is unaffected by it being grounded.
- Location
- Illinois
- Occupation
- retired electrician
240.22 prohibits the use of a fuse in the grounded conductor
The grounded service conductor must be connected to a main bonding jumper and to a grounding electrode conductor at the service disconnect.
240.22 does permit the use of 3 pole breakers for this system, and you are correct that they must be straight rated at 480 volts. 480/277 volt breakers are not permitted. If you are using a fuse for motor overload protection, you would be permitted to put a fuse in the grounded circuit conductor, also part of the rule in 240.22.
The motor only cares about the line to line voltage. No special motor is required.
Nothing special is required for the starter.
For this system the grounded conductor is really a phase conductor and will be the same size as the ungrounded conductors. You can, in some case use single phase panels and breakers for this system. In that case the two ungrounded conductors are connected to the two pole breaker and the grounded conductor would be connected to the neutral bar. The installation would look just like a 120/240 volt system, but of course with 480 volt rated panels and breakers. The motor still needs overload protection for all 3 phases so it would still be a standard starter.
The motor is only looking at the voltages between the 3 conductors, it does not even see the voltage to ground. The motor does not know and does not care what type of system it is connected to. For the motor, an ungrounded system, a corner grounded system, and a wye system are all the same as long as you have the correct voltage.
Don't forget that the grounded conductor is a grounded conductor covered by Article 200 and it must be identified exactly the same as a neutral, that is the conductor must be white or gray.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
A single conductor does not have a voltage on it. Voltage is measured between two points or conductors.
Don't get confused by the slang of using Phase to mean a single 'line' or 'hot' conductor. Typically phase means the voltage between two line conductors, such as A-B, B-C, and C-A and not between a line and neutral conductor.
When we build them we bring it out on the overhead drop bare conductor.
Don’t care if you use it or not..
James L
Senior Member
- Location
- Kansas Cty, Mo, USA
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- Electrician
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Many are confused when something other than a neutral is grounded. On systems with a neutral the neutral point isn't automatically something you can touch and not get zapped. You can touch it because it has been grounded. You can ground any point on that or any other system. Just so happens when using NEC if there is a neutral conductor it almost always must be the conductor that gets grounded if you are going to ground that system.
Two wire 120 volt system is no different. There is no neutral here, you can ground either system conductor. Once you do so that is the "grounded conductor".
Corner ground delta same way. You can ground any one of the three lines. That will be the one that is at ground potential and reason you read zero to ground. The three lines are still 480 to each other and still 120 degree phase difference to each other and the reason a 480 volt load still operates as intended when connected.
To the service or wherever the system bonding of a separately derived system is located, there is no EGC. Beyond that point you still must run separate "grounded conductor" and "equipment grounding conductor" just like you do with systems with a neutral as the grounded conductor, that current carrying conductor will impose undesired current on EGC's and non current carrying components if you don't keep them separated past the main bonding jumper in either corner ground or grounded neutral applications.
Thanks for all the input. Still working with POCO to verify it is corner grounded. So far, I see the following violations: Neutral and grounded phase brought to service disconnect instead of being jumpered in disconnect, grounded phase is fused, the grounded phase is identified with yellow marking and not as a white or gray.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you have 0V to ground on one phase and 480 to both other phases, it about has to be a grounded phase.
I agree those other things you mention are code violations.
- Location
- Illinois
- Occupation
- retired electrician
There is no power system that has both a neutral and a grounded phase.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
I would endeavor to find out whether the grounded phase was grounded intentionally.
It should be colored as a "neutral" and should not be fused.
It should be colored as a "neutral" and should not be fused.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
No there isn't. I'd guess whoever didn't know what they were doing pulled a 4th wire, landed it on a neutral bus, pulled the grounded phase conductor landed it on one of the fused poles, and when connecting the supply end probably either landed them both at same location or possibly even landed the grounded phase conductor on whichever line was grounded and the "neutral conductor" on any ground bus that may be there or even just bolted a lug to case or something.
I kind of have no issue with separate grounded and grounding conductors between the source and the service, or even the first disco for separately derived, but make sure there is no additional bonding between them, but at same time isn't necessary either and maybe if being really picky is a code violation especially on the service application. And by all means don't put a fuse in the grounded conductor, common trip breakers are ok though.
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