900 Amp 480/277v Y Service
- Thread starter jap
- Start date
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
It is never safe to assume anything. In this case, it would be a particularly poor assumption. The beginning of the second paragraph of 230.95 says that the rating of a fused service disconnecting means is the highest size fuse that can be installed. So your installation would exceed the 1000 amp threshold, even though you used 900 amp fuses.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Install an enclosed circuit breaker in series, and include the GFI protection on it? :lol:
- Location
- Connecticut
- Occupation
- Engineer
Just as a thought...what does "highest fuse that can be installed" mean. Does it mean that largest fuse that can be physically installed in the switch, ie, 1200A in the OP's switch...
or does it mean the largest fuse that can legally be installed to provided the code required overcurrent protection?
For instance, if the OP's installation had service conductors that were 3 sets of 350mcm (total ampacity of 930.) Installing 1000A fuses in the switch would violate 230.90(A). Would that make 900A the largest fuses that "can" be installed?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I have always found this to be a curious requirement, since it basically seems to require Wye feeders and services of 1000A or more to be CB, while nominally allowing fuses.
I have often wondered what the thinking is with this requirement. Why are grounded delta systems excluded? Why are ungrounded delta systems excluded?
why are 208/120 systems excluded?
- Location
- Illinois
- Occupation
- retired electrician
I have been taught it is because an arcing ground fault is not sustainable (does not restrike after zero crossing) on a 208/120Y system, but it is on a 480/277Y system. The arcing fault can produce enough heat to melt the switch gear down without tripping the OCPD unless it has GFP.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
In the old days, a CT would encompass the conductors, which with the help of a relay trigger a shunt trip solenoid Of course this only works with a spring loaded switch.
Last edited:
zog
Senior Member
- Location
- Charlotte, NC
Recent studies from IEEE suggest otherwise
http://ieeexplore.ieee.org/xpl/logi...iel5/28/4503404/04503412.pdf?arnumber=4503412
The Reason I ask is because the engineer designed a particular project with a 1000A 480/277v Y Service.
A 1000a 480v Enclosed 3r GFI Breaker was quoted and submitted.
The Engineer rejected the submittals indicating that the rule applied to "Above" 1000 amps.
We argued the fact that it was 1000 amps and above.
A temporary generator was coming in to power the facility while the Existing Padmount could be relocated and they needed a Disconnect to put in place where the GFI Enclosed Breaker (That got rejcted) was going to go.(The Pipes were already roughed in to this location).
They requested a 1000a Fused Disconnect. (Which is a 1200 Amp Disc with 1000A Fuses).
The Temp Gen has a GFI Output breaker Feeding the Temp Disconnect, which is ok for now.
But once the Temp Generator gets taken off line, I dont see any way around not having to replace the Fused Disconnect. The 900a Fuse Option came up but evidently the clause in the code rule would not allow this either.
A 1000a 480v Enclosed 3r GFI Breaker was quoted and submitted.
The Engineer rejected the submittals indicating that the rule applied to "Above" 1000 amps.
We argued the fact that it was 1000 amps and above.
A temporary generator was coming in to power the facility while the Existing Padmount could be relocated and they needed a Disconnect to put in place where the GFI Enclosed Breaker (That got rejcted) was going to go.(The Pipes were already roughed in to this location).
They requested a 1000a Fused Disconnect. (Which is a 1200 Amp Disc with 1000A Fuses).
The Temp Gen has a GFI Output breaker Feeding the Temp Disconnect, which is ok for now.
But once the Temp Generator gets taken off line, I dont see any way around not having to replace the Fused Disconnect. The 900a Fuse Option came up but evidently the clause in the code rule would not allow this either.
- Location
- Illinois
- Occupation
- retired electrician
From the part that I can access, it appears to be talking about 277 volt arcing faults, and I agree that those re-strike, and that is the very reason for the ground fault protection rule, but I was trying to say that the 120 arcing ground faults do not re-strike and that is why GFP is not required on 208/120Y systems.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
That engineer needs to get a lesson on "Charlie's Rule." The code says what it says, not what anyone (engineers included) thinks it says. The actual text is quoted in post 3 above, and it clearly says, "1000 amps or more."
I have to agree with you. But you need to get the engineer to wake up to the fact that your original submittal correctly included the GFI feature.
zog
Senior Member
- Location
- Charlotte, NC
You need to read the entire study, this was done to conduct research specifically on 208/120V systems to settle that arguement from IEEE 1584 ommiting 208/120 systems from arc flash analysis requirements, which was then removed from 70E after this research showed sustainable arcing faults to ground on 208/120V systems.
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
zog
Senior Member
- Location
- Charlotte, NC
SOB, I posted wrong link. The 2012 70E removed the 208/120V <125kVA exception based on ongoing IEEE research, but now I can't find that link. I know the IEEE 1584 commitee has some new data on this and those studies were based off a 2004 PG&E study that suggested a 120V arc can self sustain for >1 second so they backed off that exception. I have a super busy day ahead of me but will try and find new study link Friday.
- Location
- Illinois
- Occupation
- retired electrician
I don't have any access to the entire study. It does not appear to be available to me without cost.
There was some discussion of this on one of the recent AFCI threads saying that at sustainable series arc on a 120/240 system is not possible.
If there really is the possibility of an "acring burndown" on a 208/120Y system like there is for a 480/277Y system, why does the code only require GFP protection for the 480/277Y system?
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
I don't know that 208Y is particularly in danger. Here is an excerpt from the latest rev of IEEE 1584:
It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm? 508 mm ? 508 mm box. In all other tests with that box and the 305 mm ? 368 mm ? 191 mm box, the arc blew itself out as soon as the fuse wire vaporized. An arc was sustained several times at 215 V in a device box (100 mm ? 100 mm ? 50 mm size). It appeared from the arc-flash photos from the
305 mm ? 368 mm ? 191 mm box that testing arcs usually jumped from the electrodes to the box wall and
from another point on the box wall back to another electrode. The magnetic forces created by these arc
currents forced them away from each other and into the box wall.
Arc faults can be sustained at 208 V and have caused severe injuries with very high short-circuit current
applications in meter enclosures. A meter enclosure is small and tends to confine an arc more than laboratory test boxes with no door. Used equipment at 208 V was not tested, but it is recognized that many types of equipment have relatively small open spaces between components, such as the space in a panelboard between the circuit breakers and the wall of the enclosure.
While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it
will work and will yield conservative results. The arc-flash hazard need only be considered for large 208 V
systems: systems fed by transformers smaller than 125 kVA should not be a concern.
305 mm ? 368 mm ? 191 mm box that testing arcs usually jumped from the electrodes to the box wall and
from another point on the box wall back to another electrode. The magnetic forces created by these arc
currents forced them away from each other and into the box wall.
Arc faults can be sustained at 208 V and have caused severe injuries with very high short-circuit current
applications in meter enclosures. A meter enclosure is small and tends to confine an arc more than laboratory test boxes with no door. Used equipment at 208 V was not tested, but it is recognized that many types of equipment have relatively small open spaces between components, such as the space in a panelboard between the circuit breakers and the wall of the enclosure.
While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it
will work and will yield conservative results. The arc-flash hazard need only be considered for large 208 V
systems: systems fed by transformers smaller than 125 kVA should not be a concern.
As to why the NEC doesn't recognize 208 systems, probably because the danger is with small, high SCC, meter boxes - not particularly with normal sized switchgear boxes.
And, of course, 90.1.A, "practical safeguarding", not perfect safeguarding.
ice
cmcj1975
New User
- Location
- Houston, TX
So installing a 1200 amp main lug only switchboard and dividing up the feeders to not exceed 1000 amps (staying 6 switches or less) is ok? I guess as long as it would be physically impossible to put anything 1000 amps or larger in it?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Doing this is very commonly used to get around the GFPE requirement. A 1000 amp MLO panelboard with 6 breakers is about 5K, twice that to add a main, and that's 208 gear without GFP.
There is no requirement that it be set up such that a 1000 amp or more breaker cannot be installed.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Use a Bolted Pressure Switch (Pringle switch) as the disconnect. They can be fitted with a shunt trip coil, then tie that to a GF relay connected to a GFCT.
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