???xfmr ground conductors???

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Mike01

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Location
MidWest
question 1000Kva xmfr 208Y/120V-3ph, 4w secondary the conductors from the xfmr. to the secondary OCPD are indicated to be 8-sets of 500's with a 4/0 ground in each (3/0 required), they land in a single switchboard section with a 3000A circuit breaker from there underground to the main switchboard there are 8-sets of 500's but this time with a 500kcmilm(400kmin required), my question is what is the difference between the line and load side that there is such a dramatic change in ground conductor size??
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
I may be misreading 250.30(A)(2) and 250.102(C) but it seems the required conductor to the disconnect could be as small as a 1/0 (based on the 500's) which only makes your question more interesting
 

Mike01

Senior Member
Location
MidWest
service.

service.

service yes but not a utility service the medium voltage service is a campus style system that is primary meteted 8 transformers back.
 

Volta

Senior Member
Location
Columbus, Ohio
question 1000Kva xmfr 208Y/120V-3ph, 4w secondary the conductors from the xfmr. to the secondary OCPD are indicated to be 8-sets of 500's with a 4/0 ground in each (3/0 required), they land in a single switchboard section with a 3000A circuit breaker from there underground to the main switchboard there are 8-sets of 500's but this time with a 500kcmilm(400kmin required), my question is what is the difference between the line and load side that there is such a dramatic change in ground conductor size??

So the 4/0 grounds are equipment bonding jumper(s). Section 250.30(A)(2) points us to 250.102(C), the last sentance of which allows us to size the EBJ in each of multiple paralleled raceways per the conductors in each raceway, which per 250.66, allows a 1/0. So I agree with Augie for the line side EBJs.

That last sentance is the allowance for the smaller conductors on the line side. Otherwise, the EGCs (load side) must each be large enough to carry to fault current long enough to operate the OCPD.
 

hurk27

Senior Member
What about 250.122(F)(1)? (2002)

Table 250.122 will require a 400kcmil copper in each parallel run with a 3000 OCPD?
Where is the code says it will matter if on the line or load side of the OCPD?
 
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augie47

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Location
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State Electrical Inspector (Retired)
Hurk27,
I think you have a good point and one the Code gurus can hopefully explain.
Normally Table 122 is thought to only apply to the grounding conductors on the Load side of the OCPD.
In addition, 250.30(A)(2) defines the conductor from the SDS to the first disconnecting means as an "equipment bonding jumper" and sizes it per 250.66, not 250.122.
However, if you look at the definition of a Equipment Grounding Conductor in Art 100 it states " The conductive path installed to connect normally non?current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both." which would seem to indicate back to the SDS where the grounded grounding connection is made.
To me, this is what makes Mike01's question an excellent one. I'm sure someone will explain it.
 

dicklaxt

Senior Member
Would someone help me out here,,,,,,I don't know if I'm missing something in the OP or what but I sure don't know which end is up at the moment,I'm not seeing the picture (not the ground wire size but the physical being of hardware).

Is there 8-1000 KVA transformers with 1-500 MCM per phase or 1-1000KVA transformer with 8-500MCM per phase

thanks
 

ohmhead

Senior Member
Location
ORLANDO FLA
question 1000Kva xmfr 208Y/120V-3ph, 4w secondary the conductors from the xfmr. to the secondary OCPD are indicated to be 8-sets of 500's with a 4/0 ground in each (3/0 required), they land in a single switchboard section with a 3000A circuit breaker from there underground to the main switchboard there are 8-sets of 500's but this time with a 500kcmilm(400kmin required), my question is what is the difference between the line and load side that there is such a dramatic change in ground conductor size??

Fault current primary will be less current during a fault secondary will have more fault current lower voltage higher current at kva watts make the difference.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Would someone help me out here,,,,,,I don't know if I'm missing something in the OP or what but I sure don't know which end is up at the moment,I'm not seeing the picture (not the ground wire size but the physical being of hardware).

Is there 8-1000 KVA transformers with 1-500 MCM per phase or 1-1000KVA transformer with 8-500MCM per phase

thanks
Well one transformer at 480 volts primary and 120 /208 volts secondary one .


He is going to connect it with lugs inside the transformer these are going to be 8 mechanical type lugs 8 on each of the phases 8 A PHASE - 8 B PHASE -8 C PHASE- 8 NEUTRAL- 8 GROUNDS one 400 mcm ground in each conduit total of 8 runs on that secondary side to the switchboard when he hits the swb its the same it will have 8 terminations inside section one of that switch bd .
 

dicklaxt

Senior Member
Thanks,thats the way I read it the first time but in my mind I was having trouble terminating 24 phase conductors and 8 grounds in the secondary chamber of that transformer.I sure would like to see the bushing / bus arrangement and just how it was accomplished,,,,,,,,,8 conductors per bushing has got to be a tight fit:-? I have never worked a 208/120 that large so never had ro deal with ampacities in the range,I think 480 was the smallest we went to in that transformer size.If we did it was bus duct ,,,,,,,,Thanks again

dick
 

ohmhead

Senior Member
Location
ORLANDO FLA
Thanks,thats the way I read it the first time but in my mind I was having trouble terminating 24 phase conductors and 8 grounds in the secondary chamber of that transformer.I sure would like to see the bushing / bus arrangement and just how it was accomplished,,,,,,,,,8 conductors per bushing has got to be a tight fit:-? I have never worked a 208/120 that large so never had ro deal with ampacities in the range,I think 480 was the smallest we went to in that transformer size.If we did it was bus duct ,,,,,,,,Thanks again

dick


Well hows this ?

434.jpg



435-1.jpg
8 RUNS 750 MCM
 
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dicklaxt

Senior Member
Very nice, neat installation,believe it or not I thought about back to back but thought also clearance might be a problem.The staggered bushing's helped put that to bed.

dick
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
What about 250.122(F)(1)? (2002)

Table 250.122 will require a 400kcmil copper in each parallel run with a 3000 OCPD?
Where is the code says it will matter if on the line or load side of the OCPD?
On the line side of the OCPD, it is not an EGC and is not sized by 250.122. It is a line side bonding jumper that is sized by 250.30(A)(2) which sends you to 250.102(C) and that in turn sends you to T250.66.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
On the line side of the OCPD, it is not an EGC and is not sized by 250.122. It is a line side bonding jumper that is sized by 250.30(A)(2) which sends you to 250.102(C) and that in turn sends you to T250.66.


does seem a bit odd thought, doesn't it. To operate the OCP on a ground fault the current has to return to the grounded conductor and this will entail a path using the bonding jumper which is smaller than the EGC.
At least it's normally short :)
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Gus,
That is why my first post said that the only reason for the difference is code rules. I was trying to imply that there is no technical reason.
 

Mike01

Senior Member
Location
MidWest
Gus,
That is why my first post said that the only reason for the difference is code rules. I was trying to imply that there is no technical reason.

Don thanks and to everyone else thanks for your responses however in trying to uncover the "technical reason" any suggestions?
 

hurk27

Senior Member
Don thanks and to everyone else thanks for your responses however in trying to uncover the "technical reason" any suggestions?

I know the NEC tries to treat the conductors ahead of the OCPD different, when only one set of conductors are used, but it seems that they have over looked the fact that the conductors ahead of the OCPD will be subject to even a higher level of fault current that could present a failure point in the system of protection when this method is applied to parallel installations, I see 250.102 at the point it address the parallel installation (last sentence) as a contradiction the the requirements of 250.122(F)(1), basing the size upon each cable in each conduit without thinking of the fault current of the circuit as a whole is nuts?:confused:
My opinion is they should be sized to the ampicity of all the parallel conductors using table 250.122 for the final size

The fact that this conductor not only will be hit with the full fault current of the service/SDS, but it will be even closer to the supply which will make it a higher level of current.

Wonder if anyone ever tried to change this in the code, I would love to see the reason this was rejected:roll:

I have to agree with Don, it is what it is:confused:
 
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