Maintaining a 5-wire system from bonded separately derived system (generator)

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Bagram Sparky

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Nashville, TN. USA and Angeles City, Philippines c
The source is a 100 Kva generator used for permanent, spot generation. This supplies a 400 amp, 4-pole transfer switch. The transfer switch supplies the Square D, I-Line panel which is named, MDP-1.
The generator did not have an internal bond so, a main bonding jumper (MBJ) was installed. It is an oversize 500 MCM MBJ.
A 4 conductor, 500 MCM with a 1/0 EGC cable is installed from the Gen Set (10 linear feet away) to the transfer switch and from the transfer switch to the MDP-1 ( 3 linear feet away).
The generator has a 400 amp main breaker.
Each piece of equipment has the appropriate grounding electrode and GEC installed.
I have been instructed by the O&M provider to remove the EGC from the transfer switch to the MDP and add a MBJ to the MDP rather than maintain the installed 5-wire system (120/208V). According to 250.30, I am required to install a MBJ at the source or the first point of disconnect.
I find no requirement for me to change the established 5-wire system back to a 4-wire system from the transfer switch to the MDP.
I have been through article 250.28 and 250.30 and I request a clear interpretation of why this might be required.
Respectfully submitted, Bagram Sparky
 
augie47

augie47

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State Electrical Inspector (Retired)
Since you are switching the neutral with your transfer switch, the generator is a SDS, and, if I follow your description correctly, I see no problem, but to cover the bases:
Is there a utility supply to the TS also ? If so, where is the service disconnect for the utility and is there a MBJ at that point ?
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
Bagram Sparky said:
...
I have been instructed by the O&M provider to remove the EGC from the transfer switch to the MDP and add a MBJ to the MDP rather than maintain the installed 5-wire system (120/208V).
Click to expand...
An EGC is required between transfer switch and MDP-1.

With 4-pole transfer, and assuming the other source is utility, you will have an MBJ in the service disconnecting means enclosure and an SBJ for the SDS generator. With the generator having an integral 400A breaker, the SBJ has to be installed in the generator.

PS: Need clarification. Is MDP-1 on the line side or load side of the transfer switch? Where is the service disconnecting means with respect to the transfer switch?
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Bagram Sparky said:
The source is a 100 Kva generator used for permanent, spot generation. This supplies a 400 amp, 4-pole transfer switch. The transfer switch supplies the Square D, I-Line panel which is named, MDP-1.
The generator did not have an internal bond so, a main bonding jumper (MBJ) was installed. It is an oversize 500 MCM MBJ.
A 4 conductor, 500 MCM with a 1/0 EGC cable is installed from the Gen Set (10 linear feet away) to the transfer switch and from the transfer switch to the MDP-1 ( 3 linear feet away).
The generator has a 400 amp main breaker.
Each piece of equipment has the appropriate grounding electrode and GEC installed.
I have been instructed by the O&M provider to remove the EGC from the transfer switch to the MDP and add a MBJ to the MDP rather than maintain the installed 5-wire system (120/208V). According to 250.30, I am required to install a MBJ at the source or the first point of disconnect.
I find no requirement for me to change the established 5-wire system back to a 4-wire system from the transfer switch to the MDP.
I have been through article 250.28 and 250.30 and I request a clear interpretation of why this might be required.
Respectfully submitted, Bagram Sparky
Click to expand...

Based on your description and the addition of the N/G bond at the generator this appears correct to me. You can't connect a N/G bond at the MDP as this would create a parallel path and violate 250.24(A)(5). You are being asked to do something that is incorrect and a code violation.
Not sure what you mean about "each piece of equipment" for the GES. You should have one GES and a GEC from the service disconnecting means and another GEC from the genset.
 
B

Bagram Sparky

Member
Location
Nashville, TN. USA and Angeles City, Philippines c
No utility but, second gen-set to be installed

No utility but, second gen-set to be installed

augie47 said:
Since you are switching the neutral with your transfer switch, the generator is a SDS, and, if I follow your description correctly, I see no problem, but to cover the bases:
Is there a utility supply to the TS also ? If so, where is the service disconnect for the utility and is there a MBJ at that point ?
Click to expand...

The system will have a second, 100 Kva generator installed at a later date to provide redundant power. As the TS is a 4-pole, the second generator will be bonded as well.
The facilities supplied by this system will not be connected to Afghan Prime Power.
 
B

Bagram Sparky

Member
Location
Nashville, TN. USA and Angeles City, Philippines c
MDP is Load Side of TS

MDP is Load Side of TS

Smart $ said:
An EGC is required between transfer switch and MDP-1.

With 4-pole transfer, and assuming the other source is utility, you will have an MBJ in the service disconnecting means enclosure and an SBJ for the SDS generator. With the generator having an integral 400A breaker, the SBJ has to be installed in the generator.

PS: Need clarification. Is MDP-1 on the line side or load side of the transfer switch? Where is the service disconnecting means with respect to the transfer switch?
Click to expand...

The integral 400 amp breaker in the generator is being used as the service disconnecting means. The TS is service rated and is on the Line Side of the MDP. The I-line MDP has a 400 amp main breaker.
There will be a second generator installed, in the near future. The Afghanistan prime power grid will not be used.
I am looking for a definitive reference to show the need for maintaining the 5-wire configuration rather than switching from an established 5-wire at the generator to the TS and go to a 4-wire from the TS to the MDP. I might understand their request if this was a parallel feed, which would require a minimum 1/0 EGC and they will not allow a 1/0 to be run with each cable but, it is not a parallel feed.
 
augie47

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
IMO (and I stress opinion), your installation would be governed by NEC 250.20(D) which references you to 250.30(A), the 1st sentence of which states: "Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non?current-carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a separately derived system." The point of grounding for your SDS is at the generator.
That leads me to believe your installation is correct as wired.
(references are '08 NEC)
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
augie47 said:
IMO (and I stress opinion), your installation would be governed by NEC 250.20(D) which references you to 250.30(A), the 1st sentence of which states: "Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non?current-carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a separately derived system." The point of grounding for your SDS is at the generator.
That leads me to believe your installation is correct as wired.
(references are '08 NEC)
Click to expand...

I was assuming that the other side of the ATS was a service. That is why I cited 250.24(A)(5). But you are right, it also violates 250.30(A) in either case.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
My opinion aligns with Gus's (augie47).

In addition, there's 250.32 for buildings supplied by feeder, (B)(2) supplied by SDS, (a) with overcurrent, which sends us back to 250.32(B)(1)...

Supplied by a Feeder or Branch Circuit. An equipment
grounding conductor, as described in 250.118, shall
be run with the supply conductors and be connected to the
building or structure disconnecting means and to the
grounding electrode(s). The equipment grounding conductor
shall be used for grounding or bonding of equipment,
structures, or frames required to be grounded or bonded.
The equipment grounding conductor shall be sized in accordance
with 250.122. Any installed grounded conductor
shall not be connected to the equipment grounding conductor
or to the grounding electrode(s).
Click to expand...
 
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