transformer neutral

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toireht

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Location
port allen, la, usa
looking at existing installation of a transformer. transformer is located in building with 480 volt system. transformer is 37.5 kva. 2 hots off 480 system and ground ran to primary side. h2 and h3 connected together. hots to h1 and h4. the secondary side actually runs to detached trailer. secondary is 4 wire, hots on x1 and x4, x2 and x3 connected together with neutral. the ground from the primary side and secondary side are connected to transformer frame and to ground rod near transformer. trailer has a meter with circuit breaker disconnect. hots connect to line lugs and neutral connects to neutral bus that is bonded to ground. ground wire connects to ground bus. Is this proper set up? should the bonding of the neutral be done at transformer or is it ok at meter panel on trailer? only one bounding of neutral to ground is allowed? trailer is 25 feet from building with transformer. about 30-35 feet of wiring. trying to determine if we need to redo this installation.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Actually bonding at the trailer can lower the impedance for ground fault current if the EGC is smaller then the neutral between the trailer and transformer, but the EGC between these two points is now considered a main bonding conductor and needs to be sized per table 250.66 (250.28(D)(1) and 250.30(B)(1) will allow the grounding electrode to be connected at the transformer.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
ActionDave said:
Neutral and ground bonded at the trailer is fine if that is the first disconnect, but the Grounding Electrode Conductor needs to land there also.
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Even with a service you can always connect the GEC ahead of the first disconnect means, just never after it. see 250.24(A)(1)
This is why we can land the GEC at the meter or at the service panel or both, I have even seen a GEC run up a riser pipe and connected to the service drop neutral, kind of an over kill but allowed.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
toireht said:
looking at existing installation of a transformer. transformer is located in building with 480 volt system. transformer is 37.5 kva. 2 hots off 480 system and ground ran to primary side. h2 and h3 connected together. hots to h1 and h4. the secondary side actually runs to detached trailer. secondary is 4 wire, hots on x1 and x4, x2 and x3 connected together with neutral. the ground from the primary side and secondary side are connected to transformer frame and to ground rod near transformer. trailer has a meter with circuit breaker disconnect. hots connect to line lugs and neutral connects to neutral bus that is bonded to ground. ground wire connects to ground bus. Is this proper set up? should the bonding of the neutral be done at transformer or is it ok at meter panel on trailer? only one bounding of neutral to ground is allowed? trailer is 25 feet from building with transformer. about 30-35 feet of wiring. trying to determine if we need to redo this installation.
Click to expand...

with a 37.5kva transformer, and a 150 amp main you should have 1/0 ungrounded conductors, and at least a #6 EGC or main bonding jumper between the transformer and trailer which ironically is the same size egc sized for table 250.122 150 amps is one of those in-between sizes that tables 122 and 66 give the same results:happyyes:
 
ActionDave

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
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Licensed Electrician
250.30
(5) Grounding Electrode Conductor, Single Separately
Derived System. A grounding electrode conductor for a
single separately derived system shall be sized in accordance
with 250.66 for the derived ungrounded conductors. It shall
be used to connect the grounded conductor of the derived
system to the grounding electrode as specified in 250.30(A)
(4). This connection shall be made at the same point on the
separately derived system where the system bonding jumper
is connected.

Hurk, you made me look but this is what I see and I don't see any exceptions in the exceptions for putting the GEC at the transformer.
 
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iwire

iwire

Moderator
Staff member
Location
Massachusetts
hurk27 said:
Actually bonding at the trailer can lower the impedance for ground fault current if the EGC is smaller then the neutral between the trailer and transformer,
Click to expand...

I find that to be reaching, I am not saying it is untrue, just question if it has any value.

There is almost no circuit we could not install another way to lower impedance.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
ActionDave said:
250.30
(5) Grounding Electrode Conductor, Single Separately
Derived System. A grounding electrode conductor for a
single separately derived system shall be sized in accordance
with 250.66 for the derived ungrounded conductors. It shall
be used to connect the grounded conductor of the derived
system to the grounding electrode as specified in 250.30(A)
(4). This connection shall be made at the same point on the
separately derived system where the system bonding jumper
is connected.


Hurk, you made me look but this is what I see and I don't see any exceptions in the exceptions for putting the GEC at the transformer.
Click to expand...

Yep I need to slow down and read:ashamed:
I posted the requirement for the system bonding jumper:lol: 250.30(A)(1)

ActionDave said:
You and me, we're a team now.:)
Click to expand...

Oh now ya teaming up on me, gona have to break out the big guns:lol:

iwire said:
I find that to be reaching, I am not saying it is untrue, just question if it has any value.

There is almost no circuit we could not install another way to lower impedance.
Click to expand...

I agree the amount of impedance gained will depend upon the length of the run, but I would say putting the 1/0 neutral path in place of the #6 would gain a few amps, considering that the ground fault current could be fairly high?
The down side of this is the ability of loosing the neutral between the panel and transformer will result in voltage on the grounding after the panel. might not be a better idea looking at it with this in mind
 
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