The bus maintenance building at a local high school was hit by lightning and burned last July. I am contracted as the electrician on the re-build. The reconstruction is by the original drawings from 1987. The original building was fed with (3) 3/0 THHN conductors in 1-1/2" conduit underground to a 3 phase ML 480V panelboard. The 480V panelboard had a 100A breaker feeding a 45kva transformer stepping it down to 208Y/120 in a 42 circuit panel. The 480V panel also had (5) other circuits feeding (3) Chromalox electric heaters and (2) 2hp OHD operators. When they demo'd out the concrete slab today we discovered the 1-1/2" RMC containing the feeders turned into 1-1/2" sched 40 PVC about 42" below grade. The conduit emerges at the school building as 1-1/2" RMC and continues above the ceiling tile in RMC all the wat to the switch gear in the electrical room. The detatched bus building had a 3/4"x10' ground rod driven outside the building and another 3/4"x10' driven inside the building through the slab. The rods were bonded together with bare #4 and connected to the ground bar in the 480V panel. Is this allowable? I ask because I have to replace the feeder conductors that are fire damaged and assumed the original contractor had gotten by with 1-1/2" conduit for the (3) 3/0 conductors by using 1-1/2" RMC as the EGC. The distance from the bus building to the school building is 160' and 150' of it is a newly paved parking lot. I thought a feeder had to include a grounded conductor or in the case of straight 3 phase an EGC. Am I mistaken? Thanks in advance.
Feeder EGC
- Thread starter amptech
- Start date
- Status
- Location
- Bremerton, Washington
Re: Feeder EGC
Previous editions of the NEC allowed a feeder to a seperate building to be treated as a service ( the neutral is "regrounded). The 2002 essentially no longer allows this. However with a three phase feed you must have an EGC, as without there is no fault current return path for a line to case fault. Note the earth can not be used as the equipment grounding path, this is a violation of Art 250.
Previous editions of the NEC allowed a feeder to a seperate building to be treated as a service ( the neutral is "regrounded). The 2002 essentially no longer allows this. However with a three phase feed you must have an EGC, as without there is no fault current return path for a line to case fault. Note the earth can not be used as the equipment grounding path, this is a violation of Art 250.
Re: Feeder EGC
hello amptech, got some good news for you.The feeder could have been run a distance of 80' more feet before voltage drop became a factor,based on 3/0, 200 amperes.But there's problem w/ the raceway. There is no continuous metalic path of the raceway as you have stated, so you'll need a EG at #6 for 200 amperes. The ridged metal conduit at 40% is 0.828 and your conductors w/EG are 0.8544.You need the minimum of a 2'conduit.
hello amptech, got some good news for you.The feeder could have been run a distance of 80' more feet before voltage drop became a factor,based on 3/0, 200 amperes.But there's problem w/ the raceway. There is no continuous metalic path of the raceway as you have stated, so you'll need a EG at #6 for 200 amperes. The ridged metal conduit at 40% is 0.828 and your conductors w/EG are 0.8544.You need the minimum of a 2'conduit.
- Location
- Illinois
- Occupation
- retired electrician
Re: Feeder EGC
amptech,
If "ML" in your first post means, "main lugs only, then there is a violation of 225.31.
Don
amptech,
If "ML" in your first post means, "main lugs only, then there is a violation of 225.31.
Don
Re: Feeder EGC
Don, there were only 6 breakers in the 480V ML. Wouldn't it have met the 6 disconnect rule? At any rate, I quoted a main breaker 480V panel board on the re-build. My dilema here is the 1-1/2" conduit feeding the building. This is an insurance job and they aren't going to pay to cut the new parking lot to install 2" conduit and the school doesn't have any money(State-wide money crunch in Indiana). I'm going to pitch them an option to go over head but I don't see them going for it. I explained to the insurance adjuster and the superintendant that I didn't care how it was before, I couldn't replace it with a code violation. The maintenance supervisor asked me if not having a ground could explain why every time they had a lightning storm the OHD operator transformers got fried. They have been replaced 17 times since 1988. Another scary thing is a gasoline/diesel dispensing station is powered from this building and located just 35' away. There was no e-stop for the pumps at this building either.
Don, there were only 6 breakers in the 480V ML. Wouldn't it have met the 6 disconnect rule? At any rate, I quoted a main breaker 480V panel board on the re-build. My dilema here is the 1-1/2" conduit feeding the building. This is an insurance job and they aren't going to pay to cut the new parking lot to install 2" conduit and the school doesn't have any money(State-wide money crunch in Indiana). I'm going to pitch them an option to go over head but I don't see them going for it. I explained to the insurance adjuster and the superintendant that I didn't care how it was before, I couldn't replace it with a code violation. The maintenance supervisor asked me if not having a ground could explain why every time they had a lightning storm the OHD operator transformers got fried. They have been replaced 17 times since 1988. Another scary thing is a gasoline/diesel dispensing station is powered from this building and located just 35' away. There was no e-stop for the pumps at this building either.
- Location
- Illinois
- Occupation
- retired electrician
Re: Feeder EGC
amptech,
I missed the six breaker part of your post. The rule does permit 6 breakers for the required building disconnect.
Don
amptech,
I missed the six breaker part of your post. The rule does permit 6 breakers for the required building disconnect.
Don
- Location
- Illinois
- Occupation
- retired electrician
Re: Feeder EGC
amptech,
Do a load calculation for the building and see if you can reduce the feeder size. It looks like that the actual load would permit conductors smaller than 3/0. The only other option that I can think of will only work if there are no other conductive paths between the two buildings. You could install a 480-480 volt isolation transformer in the first building, corner ground the secondary, pull a 3 wire feeder to the second building, and rebond the grounded conductor at the second building per 250.32(B)(2).
Don
amptech,
Do a load calculation for the building and see if you can reduce the feeder size. It looks like that the actual load would permit conductors smaller than 3/0. The only other option that I can think of will only work if there are no other conductive paths between the two buildings. You could install a 480-480 volt isolation transformer in the first building, corner ground the secondary, pull a 3 wire feeder to the second building, and rebond the grounded conductor at the second building per 250.32(B)(2).
Don
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Re: Feeder EGC
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