transformer grounding/bonding.

[wankster]

hello,

I am trying to finalyze my plan for installations of a 112.5 kva 480 primary to 240 secondary transformer and the grounding/ bonding issue is seeming a bit vague. i've found another thread with a similair question to mine, but there did not seem to be a difinitive answer.

the transformer is outdoor rated and will be outside the building. there is no building steel to be grounded as it is a concrete tilt up structure with wooden framing.

there is conductive water pipe, however it is already bonded to the system ground of the main electrical service. what i've read from the code book seems to suggest that all seperately derived systems must be bonded to the water system? is this true, or is it only the case when the water system is bonded to building steel?

the primary side is 3 wire 480 with no nuetral. all of the diagrams/pictures that i have seen do not show an equipment ground coming from the primary side, but rather, the XO terminal of the secondary side bonded to the case and the building/water system. do i not need to run a ground wire from the primary side?

The secondary side will go directly to a fused disconnect, and from there to a large kiln that uses 240 3phase with no nuetral. it does however, have a ground wire bonded to the chassis. would this ground go to the XO terminal of the transformer? or does it need to connect to a ground coming from the primary side?

hopefully this makes sense. i think my main problem is i'm having trouble finding and code regarding a secondary that does not use the nuetral.

thanks for your input.

 

[raider1]

Take a look at 250.30 for the grounding and bonding requirements for separately derived systems.

Chris

 

[wankster]

I attached this drawing (please forgive its primitive nature, whipped it up on paint just now)

the way i've been reading it this is about what it says to do.

since the shut off switch essentially becomes my "main panel" it's nuetral bus should be bonded to the chassis, yes?

but since i'm not using a nuetral wire all i have is the equipment ground from the machine, correct?

the other issue that perplexes me is the water pipe bonding. if the water pipe is already bonded at the buildings main panel will bonding the secondary XO on the transformer not become an objectional current issue?

 

[Smart $]

I attached this drawing (please forgive its primitive nature, whipped it up on paint just now)

Forgiven... this time

the way i've been reading it this is about what it says to do.

Looks good to me with one exception. You want to bond your water line GEC (grounding electrode conductor) at the same point as your SBJ (system bonding jumper; the wire from the X0 terminal to the chasis in your drawing) using a grounding busbar or a multiconductor conductor mechanical lug listed for grounding (read: preferred but technically is not explicitly required). The NEC "preferred" arrangement is to land the GEC on the X0 terminal.

since the shut off switch essentially becomes my "main panel" it's nuetral bus should be bonded to the chassis, yes?

NO! This occurs wherever the SBJ is located and no other location on the separately derived system.

but since i'm not using a nuetral wire all i have is the equipment ground from the machine, correct?

Correct! Since this is not a service, you are not required to bring the neutral/grounded conductor to the disconnecting means.

the other issue that perplexes me is the water pipe bonding. if the water pipe is already bonded at the buildings main panel will bonding the secondary XO on the transformer not become an objectional current issue?

Only if there is objectionable current... at which time remedial action should be taken, but otherwise it is what it is to be code compliant.

 

[wankster]

Thank you kindly for your input.

i spoke with the city inspector today and he told me i could run a ground wire from the main service ground bus with the supply wires from the primary side and use that as my water pipe bonding.

 

[winnie]

Keep in mind that the sizing required for the transformer secondary GEC (especially the water bond) will be different than the primary side EGC. A 'ground' conductor designed to serve both roles will have to meet the requirements of each, and will probably be larger than the normal EGC.

-Jon

 

[Smart $]

Thank you kindly for your input.

i spoke with the city inspector today and he told me i could run a ground wire from the main service ground bus with the supply wires from the primary side and use that as my water pipe bonding.

I concur with what winnie (Jon) wrote.

Your primary-EG/secondary-GE dual-purpose conductor will have to be sized to Table 250.66 based on the derived (secondary) phase conductors.

 

[raider1]

Thank you kindly for your input.

i spoke with the city inspector today and he told me i could run a ground wire from the main service ground bus with the supply wires from the primary side and use that as my water pipe bonding.

Also if you run the GEC for the transformer in the same pipe with the primary conductors you will need to comply with 250.64(E) and bond the GEC to both ends of any metallic raceway that it is run in.

Chris

 

[wankster]

Keep in mind that the sizing required for the transformer secondary GEC (especially the water bond) will be different than the primary side EGC. A 'ground' conductor designed to serve both roles will have to meet the requirements of each, and will probably be larger than the normal EGC.

-Jon

conveniently, someone has abandoned a 200a 480v 3 wire circuit that happens to terminate right about where we want to locate our new unit. they ran a #4 ground which according to table 250.66 of my 08 NEC book will be adequate for the 3/0 wires that will be used to supply the fused shut off and Kiln on the primary side.

125% of the 112.5 KVA transformer input rating comes to about 168 amps which will allow me to simply replace the 200a breaker with a 175 and the 3/0 wires will be more than adequate for our needs.

Also if you run the GEC for the transformer in the same pipe with the primary conductors you will need to comply with 250.64(E) and bond the GEC to both ends of any metallic raceway that it is run in.
Chris

there is currently a bonding bushing on the conduit leaving the OCPD enclosure, i will make sure that there is one at the other end and any Junction boxes we may encounter on the way to the transformer.

thanks again for the input.

 

[winnie]

conveniently, someone has abandoned a 200a 480v 3 wire circuit that happens to terminate right about where we want to locate our new unit. they ran a #4 ground which according to table 250.66 of my 08 NEC book will be adequate for the 3/0 wires that will be used to supply the fused shut off and Kiln on the primary side.

Please note that the GEC has to be sized for the _secondary_ side of the transformer.

-Jon

 

[augie47]

There are a couple of points where I disagree. I may not be correct, but thats why we discuss these, right.
First, you state: "the primary side is 3 wire 480 with no neutral. all of the diagrams/pictures that i have seen do not show an equipment ground coming from the primary side" . The transformer must have an equipment ground on the primary side. This may be a conductor or the conduit if it complies with 250.118. Be careful if flex is involved.
Secondly, Depending on your layout, I disagree with the inspector's advice on using the primary equipment ground as your route to the water. 250.30(A)(7) states:" (7) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1) "
Third: You mention the equipment does not need a neutral, but you also mention the XO terminal of the transformer. 250.20(B) states that:"(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts "
To me, this indicates, if there is a XO terminal, you should have a grounded-grounding bond. If the transformer is delta secondary (or if a decision is made that you don't need XO) I would remind you that 250.21(b) required ground fault detection on ungrounded systems"

whew! I will now step off my soapbox

 

[wankster]

Please note that the GEC has to be sized for the _secondary_ side of the transformer.

-Jon

i guess it was late last night, thought i included that.

the secondary side will be a single 175a circuit fed by 3/0 wires, thus making a #4 ground adequate, correct?

 

[Smart $]

i guess it was late last night, thought i included that.

the secondary side will be a single 175a circuit fed by 3/0 wires, thus making a #4 ground adequate, correct?

Yes, a #4 Cu GEC is code compliant for 3/0 Cu SDS transformer secondary taps.

 

[augie47]

Clarification, please. Assuming you don't have the electrodes specified in 250.30(A)(7) {water pipe or structural metal), is it permissible to connected your transformer secondary XO to the primary equipment grounding conductor provided the EGC is of adequate size and the raceway is bonded as required for a grounding electrode conductor ?

 

[raider1]

is it permissible to connected your transformer secondary XO to the primary equipment grounding conductor provided the EGC is of adequate size and the raceway is bonded as required for a grounding electrode conductor ?

Right now it would be permissible to use the primary EGC as the GEC for the transformer provided it is sized correctly and meets the requirements of 250.64.

There is a code proposal that has been accepted by the CMP for the 2011 NEC that would prohibit using an EGC as a GEC. Check out ROP 5-259, 5-185.

Chris

 

[Smart $]

Clarification, please. Assuming you don't have the electrodes specified in 250.30(A)(7) {water pipe or structural metal), is it permissible to connected your transformer secondary XO to the primary equipment grounding conductor provided the EGC is of adequate size and the raceway is bonded as required for a grounding electrode conductor ?

Why limit the question to the electrodes not being present. Say they are. If the primary is properly grounded, is it not connected to both those electrodes if they are present? Which is better, a GEC connected to one of the electrodes or a GEC connected to both?

 

[augie47]

Why limit the question to the electrodes not being present. Say they are. If the primary is properly grounded, is it not connected to both those electrodes if they are present? Which is better, a GEC connected to one of the electrodes or a GEC connected to both?

I may be off base, but from 250.30:
(7) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1)
(2) Structural metal grounding electrode as specified in 250.52(A)(2)
Exception No. 1: Any of the other electrodes identified in 250.52(A) shall be used where the electrodes specified by 250.30(A)(7) are not available.
Exception No. 2 to (1) and (2): Where a separately derived system originates in listed equipment suitable for use as service equipment, the grounding electrode used for the service or feeder equipment shall be permitted as the grounding electrode for the separately derived system.
FPN: See 250.104(D) for bonding requirements of interior metal water piping in the area served by separately derived systems. (emphasis mine)

The use of the word "nearest", the word "same area", added to the 250.104(D) requirement, leads me to think that if the water or steel electrodes were in the vicinity of the transformer, they would be the "required" electrode.

 

[Smart $]

I may be off base, but from 250.30:

(7) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1)
(2) Structural metal grounding electrode as specified in 250.52(A)(2)
Exception No. 1: Any of the other electrodes identified in 250.52(A) shall be used where the electrodes specified by 250.30(A)(7) are not available.
Exception No. 2 to (1) and (2): Where a separately derived system originates in listed equipment suitable for use as service equipment, the grounding electrode used for the service or feeder equipment shall be permitted as the grounding electrode for the separately derived system.
FPN: See 250.104(D) for bonding requirements of interior metal water piping in the area served by separately derived systems. (emphasis mine)​

The use of the word "nearest", the word "same area", added to the 250.104(D) requirement, leads me to think that if the water or steel electrodes were in the vicinity of the transformer, they would be the "required" electrode.

Apparently, by "if they are present" you mean one is present locally... and by locally, I mean it is nearer to the transformer than the primary's GEC. Near as practicable and preferably in the same area terminologies are ambiguous. Just how close is practicable? What if the required nearest of two, if present, is on the other side of the primary feeder's source?

Granted, if one of the electrodes is present and more practicable to not use the primary supply's GEC, because the electrode is nearer than the connection to the primary's GEC, then this shoud be the course used.

PS: Any chance you could not use "Size 1" text? It is readable, but not comfortably as it appears quite small in my browser.