We are installing a new 2500 kVA, 13,800-480Y/277 V transformer (solidly grounded) that will feed a new 480 V, 3000 A lineup of switchgear (3 phase, 4 wire). The transformer is located outside the building and will be cable connected to the new switchgear inside the building via underground conduit. The secondary main breaker is in the switchgear and it is about a 50? run from the transformer secondary to the switchgear. We are going to run 8 parallel sets of 500 kcmil cables (3 phase and 1 neutral). The question I have is about where the system should be connected to ground. Should the neutral be connected to the system ground at the transformer X0 bushing or in the switchgear? Do grounds need to be run from the transformer to the switchgear along with the phase and neutral conductors?
Question on grounding a new transformer/switchgear
- Thread starter cbark
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(Since you didn't reply, let jme ask Samrt$ question differently....)
Where is the service point ?
(Is the power company responsible for the primary & transformer and your connection point is at the transformer secondary as opposed to it being primary metered.. it effects the answer..?)
Where is the service point ?
(Is the power company responsible for the primary & transformer and your connection point is at the transformer secondary as opposed to it being primary metered.. it effects the answer..?)
That makes this a Separately Derived System and brings it under NEC 250.30 as to grounding.
There have been some changes in the last three Codes. Can you advise the Code cycle that will be applicable ?
There have been some changes in the last three Codes. Can you advise the Code cycle that will be applicable ?
Lets hope I get this right as I am more familiar with earlier Codes... I;m sure others will critique..
Assuming for a moment that establishing a ground at both the transformer and the 1st disconnect will not create a path (other than earth), Exception 2 of 250.30(A) applies and you can have a system bonding jumper from the transformer XO to a grounding electrode and from your 1st disconcerting means to a grounding electrode. In addition you will have a supply side bonding jumper between the transformer and 1st disconnecting means (along with you neutral and phase conductors). 250.30 will provide the details.
Assuming for a moment that establishing a ground at both the transformer and the 1st disconnect will not create a path (other than earth), Exception 2 of 250.30(A) applies and you can have a system bonding jumper from the transformer XO to a grounding electrode and from your 1st disconcerting means to a grounding electrode. In addition you will have a supply side bonding jumper between the transformer and 1st disconnecting means (along with you neutral and phase conductors). 250.30 will provide the details.
Connecting X0 to electrode(s) at transformer is not a problem... but if you have an SBJ at both the transformer and disconnecting means along with an SSBJ, you have a parallel path for grounded conductor current on the SSBJ.
Install SBJ in switchgear or transformer, but not both. My preference with an outdoor transformer is in the switchgear.
Assuming the grounding electrode is connected at the source (connected at the transformer and not the switchgear):
Is this correct?
- The size of the grounding electrode conductor (i.e. the conductor from the transformer X0 to the grounding electrode) would be a minimum of #3/0 AWG copper [250.66].
- The neutral bus and ground bus in the switchgear will need to be separated.
- Each of the 8 parallel feeds from the transformer to the switchgear will need (3-500 kcmil copper phase conductors, 1-500 kcmil copper neutral conductor and 1-#1/0 copper bonding jumper [250.102(C)(1)]. The bonding jumpers will connected the ground bus in the transformer to the ground bus in the switchgear.
Is this correct?
petersonra
Senior Member
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- Northern illinois
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- engineer
Depends on what the grounding electrode is. If is is a ground rod it is not required to be > #6.
I don't know what you mean by separated.
Where is your grounded conductor?
north star
Senior Member
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cbark,
Wouldn't the interior switchgear [ as matter of practicality &
convenience ] be more readily accessible than the exterior
transformer ?...........Article 230.205(A); from the `08 NEC,
allows the Service Disconnecting Means to be installed in a
location that is not Readily Accessible. ......EX: It's raining cats
& dogs outside, and Ready Access to the disconnecting
means is needed.......Would it be more practical to have the
Disconnecting Means inside or or outside ?
Just saying...
= & =
cbark,
Wouldn't the interior switchgear [ as matter of practicality &
convenience ] be more readily accessible than the exterior
transformer ?...........Article 230.205(A); from the `08 NEC,
allows the Service Disconnecting Means to be installed in a
location that is not Readily Accessible. ......EX: It's raining cats
& dogs outside, and Ready Access to the disconnecting
means is needed.......Would it be more practical to have the
Disconnecting Means inside or or outside ?
Just saying...
= & =
You are correct. My error. That was the problem when the system bonding jumper requirement was introduced in '11. When '14 came out it added an exception to 250.30(A)(2) stating that the system bond jumper was not needed when 250.30(A)(1) exception 2 was employed.
So correcting my earlier post, if it does not create a parallel path you can install a system bonding jumper to a grounding electrode at both the transformer and 1st disconnecting means and NOT install the system bond jumper.
The alternative would be to install the system bond jumper between the transformer and 1st disconnect in which case ti would have to be sized by 250.102(C) based on the size of the service conductors 250.102(C) allows you to size the system bond jumper based on the phase conductor in each raceway, in this case a 1.0 in each conduit.
I believe you are confusing terminologies: (system bonding jumper) SBJ <--> SSBJ (supply-side bonding jumper).
SBJ never leaves the enclosure, connecting grounded conductor to grounding bus. GEC can be connected to either end.
SSBJ is essentially a line-side equipment grounding conductor between transformer and disconnect.
If the Grounding Electrode Conductor (GEC) is connected to the switchgear, then a Supply-Side Bonding Jumper (SSBJ) needs be installed between the transformer and the switchgear. Since there are 8 sets of parallel cables (in individual conduits), does a SSBJ need to be run in each conduit?
Would a SSBJ need to be run in each conduit if the GEC is connected a the transformer instead of the switchgear?
Would a SSBJ need to be run in each conduit if the GEC is connected a the transformer instead of the switchgear?
The SSBJ(s) is(are) required regardless of where the GEC/SBJ connected connections are made. The SSBJ in each cable between transformer and disconnect is sized per Table 250.102(C)(1) based on the size of the ungrounded conductors in each cable: 500kcmil requires 1/0 SSBJ per cable [250.102(C)(2)].
** did the '14 Code not add this exception to 250.30(A)(2) ?
a supply side bonding jumper shall not be required between enclosures for installations made in
compliance with 250.30(A)(1) Exception No. 2.
Without knowing the particulars of the install it seems possible that the exception would apply.
(outside transformer, non-metallic conduit (?) )
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
just out of curiousity, how is a SSBJ any different than an EGC.
You are correct!!!
I keep forgetting 2014 added that exception... :slaphead: ...as it only applies to outdoor SDS, which most are not.
The main difference is an SSBJ is on the line side of the source disconnecting means. Other consequential differences ensue.
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