Separately derived system (generator) and selection of correct transfer switch

[david]

So what are we left with when the SDS is located outside as far as bonding

Treat the supply just like you would a utility supply, bond the neutral at the outside source run a duel purpose natural / bond to the building and bond that conductor at the building

Bond the neutral at the outside source run what looks like a traditional feeder into the building and only bond the “ equipment ground” to the building grounding system.

Keep in mind the bonding conductor what your referring to as an equipment ground is actually the bond from the SDS system neutral to the buildings equipment grounding system / which also is bonded to the building grounding electrode system

 

[Jamesco]

Here is the problem with exception 2 SDS just being a transformer.

250.30 (A) (1) requires all outside SDS sources to be bonded at the source or at some point outside

So if left as such you could never bond the neutral at the building when the outside source is a SDS Generator. Since the last sentence of 250.30 (A) (1) mandates all SDS sources outside be bonded at the source.

Why do you have a problem with that?
Where is the main disconnecting means located for the generator? Outside as part of the generator/equipment?

Just curious what do you do when the service main disconnecting means is mounted on the outside of the building? The service neutral conductor is bonded there, Correct? You don't bond the neutral at the panel inside building again, do you? No, you don't.

In 2017 ex 2 is changed from a transformer to a separately derived source. So it was never there intention to not allow a feeder from an outside Generator to be bonded at the building and that was corrected in 2017/ 250.30 (A) (1)Exception 2

250.30 (A) (1) Exception #2

If a building or structure is supplied by a feeder from an outdoor Separately Derived Source, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor.

If the first disconnecting means is located outdoors, (part of the generator equipment), not inside the building or structure, how does Exception #2 then apply?


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[david]

Why do you have a problem with that?
Where is the main disconnecting means located for the generator? Outside as part of the generator/equipment?

Just curious what do you do when the service main disconnecting means is mounted on the outside of the building? The service neutral conductor is bonded there, Correct? You don't bond the neutral at the panel inside building again, do you? No, you don't.




250.30 (A) (1) Exception #2


If the first disconnecting means is located outdoors, (part of the generator equipment), not inside the building or structure, how does Exception #2 then apply?


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Ok I see there is some confusion,

250. 30 (A) (1) last sentience when it talks about the outside separately derived system makes a blanket statement without regard to where the location of any disconnecting means.

Under all circumstances if a building is being supplied by an SDS that is located outside 250.30 (A) (1) last sentence sends you to 250. 30 (C)

You make a bond out at the source from the source neutral (grounded Conductor)
250.30 says in addition to the source location you also comply with 250.30 (A) for grounded systems

250.30 (A) (1) exception 2 talks about not establishing a parallel path between the source and the buildings first disconnecting means
No parallel paths between the SDS source and the buildings first disconnecting means.

250.30(A) goes on to cover SDS located outside and recognizes once you earth the source grounded conductor at both the source outside location and again at the building since the earth is a conductor the (earth conductor) is not to be consider as establishing a parallel path between the two locations in applying 250.30 (A) (1) exception # 2

The first disconnecting means addressed in 250.30 (A) (1) exception #2 is the buildings disconnecting means, are you able to except that the first disconnecting means is the buildings disconnecting means?

 

[david]

Just curious what do you do when the service main disconnecting means is mounted on the outside of the building? The service neutral conductor is bonded there, Correct? You don't bond the neutral at the panel inside building again, do you? No, you don't.

every utility that i know of always bonds a grounded systems grounded conductor out at the utility source

and yes you do as i think you are aware re-bond the source grounded conductor at the building.

Edit For an outside disconnect to be considered the building disconnect it usually is mounted on the building at the point the feeder enters the building.
I know there are exceptions to that

I see the utility service bonding and a outside SDS bonding mirroring what you would expect to see at a normal utility service, when applying the exception in discussion

 

[Jamesco]

Originally Posted by Jamesco
Just curious what do you do when the service main disconnecting means is mounted on the outside of the building? The service neutral conductor is bonded there, Correct? You don't bond the neutral at the panel inside building again, do you? No, you don't.

every utility that i know of always bonds a grounded systems grounded conductor out at the utility source

Agree. The utility drives a ground rod and connects the neutral and enclosure to the grounding electrode. Pad mount. (Pole mounted transformer/s the grounding electrode may be a rod, but a least around my area the grounding electrode is a plate supported to the bottom of the buried pole)

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and yes you do as i think you are aware re-bond the source grounded conductor at the building.

Agree.

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Edit For an outside disconnect to be considered the building disconnect it usually is mounted on the building at the point the feeder enters the building.
I know there are exceptions to that

Agree.

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I see the utility service bonding and a outside SDS bonding mirroring what you would expect to see at a normal utility service, when applying the exception in discussion

Agree.

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In all the examples above there is not any type of EGC or bonding jumper conductor that connects the outdoor utility source grounded conductor to the building grounding system's/grounded conductor.
No parallel path is provided for the grounded conductor.
Would you agree?


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[david]

Originally Posted by Jamesco
Just curious what do you do when the service main disconnecting means is mounted on the outside of the building? The service neutral conductor is bonded there, Correct? You don't bond the neutral at the panel inside building again, do you? No, you don't.




Agree. The utility drives a ground rod and connects the neutral and enclosure to the grounding electrode. Pad mount. (Pole mounted transformer/s the grounding electrode may be a rod, but a least around my area the grounding electrode is a plate supported to the bottom of the buried pole)

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Agree.

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Agree.

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Agree.

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In all the examples above there is not any type of EGC or bonding jumper conductor that connects the outdoor utility source grounded conductor to the building grounding system's/grounded conductor.
No parallel path is provided for the grounded conductor.
Would you agree?


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other than the earth and that does not count,

I agree

In all of those examples the source is bonded in two locations

 

[Jamesco]

Ok I see there is some confusion,

250. 30 (A) (1) last sentience when it talks about the outside separately derived system makes a blanket statement without regard to where the location of any disconnecting means.

Under all circumstances if a building is being supplied by an SDS that is located outside 250.30 (A) (1) last sentence sends you to 250. 30 (C)

You make a bond out at the source from the source neutral (grounded Conductor)
250.30 says in addition to the source location you also comply with 250.30 (A) for grounded systems

250.30 (A) (1) exception 2 talks about not establishing a parallel path between the source and the buildings first disconnecting means
No parallel paths between the SDS source and the buildings first disconnecting means.

250.30(A) goes on to cover SDS located outside and recognizes once you earth the source grounded conductor at both the source outside location and again at the building since the earth is a conductor the (earth conductor) is not to be consider as establishing a parallel path between the two locations in applying 250.30 (A) (1) exception # 2

The first disconnecting means addressed in 250.30 (A) (1) exception #2 is the buildings disconnecting means, are you able to except that the first disconnecting means is the buildings disconnecting means?

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First.

250.30 (A) (1) Exception #2
If a building or structure is supplied by a feeder from an outdoor Separately Derived Source, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor.

"shall be permitted if doing so does not establish a parallel path for the grounded conductor."

Shall be permitted, your choice, not required.

You want to do it though.

So to meet the requirements of Exception #2 you drive a ground rod at the outdoor generator and bond the neutral to the generator/frame/wire makeup enclosure and connect a grounding electrode conductor from the common bonding point of the neutral conductor to the grounding electrode, ground rod.
(Just a guess if the generator is designed to be installed outdoors the manufacture may have already installed the required bonding at the generator and provided a lug for the grounding electrode conductor to connect.)

You're all set to bond the feeder neutral again inside the building at the distribution panel board.

Edit:
And you are still required to switch the neutral at the TS if a neutral is required.


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[david]

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First.


"shall be permitted if doing so does not establish a parallel path for the grounded conductor."

Shall be permitted, your choice, not required.

You want to do it though.

So to meet the requirements of Exception #2 you drive a ground rod at the outdoor generator and bond the neutral to the generator/frame/wire makeup enclosure and connect a grounding electrode conductor from the common bonding point of the neutral conductor to the grounding electrode, ground rod.
(Just a guess if the generator is designed to be installed outdoors the manufacture may have already installed the required bonding at the generator and provided a lug for the grounding electrode conductor to connect.)

You're all set to bond the feeder neutral again inside the building at the distribution panel board.

Edit:
And you are still required to switch the neutral at the TS if a neutral is required.


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ok but you seem to be implying the permission is to bond at the GEN, if that is what your implying that is not correct, the permission to bond is at the building.

The mandate is to bond always at the outside source.

 

[Jamesco]

ok but you seem to be implying the permission is to bond at the GEN, if that is what your implying that is not correct, the permission to bond is at the building.

The mandate is to bond always at the outside source.

but you seem to be implying the permission is to bond at the GEN, if that is what your implying that is not correct, the permission to bond is at the building.

Really??
Please reread my post again and supply that part of my post where you got the idea I implied any such thing. Please.....


To be crystal clear:
Exception #2 gives you the option to bond the generator neutral again at the building. You gonna ground the neutral outdoors, PERIOD!

Question is, why do you want to bond it again inside the building?
Other than Exception #2 says you can.
(Again, Provided you follow the requirements stated in Exception #2)

Best regards,
Jim


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[kwired]

Ill reread it but i don't think i need to, it is a feeder so if you bond the neutral as required at all outside sources of a separately derived system and run an equipment ground from the out side source bonded neutral, you would not bond the neutral at the building ,you would bond the equipment ground to the grounding electrode system at the building but the neutral would be insulated (float)

what the exception permits is a neutral / bond as a single conductor the same as you would see at a typical utility transformer supplying a service. here you have a SDS (GEN) with a feeder supplying a building

you keep saying re-read what am i reading wrong?

Edit: in the exception the neutral serves a duel purpose in addition to the Neutral it serves as a bonding jumper from the source to the equipment grounding system at the building

But the neutral at the building is already bonded if supplied by a service, that is why you need transfer switch to switch the neutral to isolate that bonding from the feeder neutral conductor(s) so that non current carrying components won't be carrying neutral current.

You can if the ATS switches the neutral and the N-G bond at the building is on the POCO side of the ATS. You have to, I believe.

That is how I see it.

 

[david]

Really??
Please reread my post again and supply that part of my post where you got the idea I implied any such thing. Please.....


To be crystal clear:
Exception #2 gives you the option to bond the generator neutral again at the building. You gonna ground the neutral outdoors, PERIOD!

Question is, why do you want to bond it again inside the building?
Other than Exception #2 says you can.
(Again, Provided you follow the requirements stated in Exception #2)

Best regards,
Jim


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First I did not mean to imply anything offensive

We both seem to be on the same page now, I would not be doing the design, but would however have to except either design.

Cost is usually the reason behind doing things when given a choice.

 

[Jennifer K.]

Thank you- I appreciate everyone's input. Sorry to bring this in a different direction, for this situation I was mistaken on the generator location - it is actually going to be located inside of the building. It is a 750KW, 480V, 3PH, 4W diesel emergency generator with a 1200A main disconnect at the generator (downstream equipment is main lug generator distribution board). Transfer switches (ATS's) served from the GDP (generator distribution panel) would all be 3-pole since no neutral is presently required. The building this is serving will be a commercial high rise. So the thought is to specify the generator as separately derived and bring an EGC to the GDP and terminate there in case any 277V loads are ever required for the emergency system (understood that 4-pole transfer switch would then need to be introduced and neutral would have to be switched). I am not seeing how the generator can be classified as separately derived still though for the case I describe.

 

[ggunn]

Thank you- I appreciate everyone's input. Sorry to bring this in a different direction, for this situation I was mistaken on the generator location - it is actually going to be located inside of the building. It is a 750KW, 480V, 3PH, 4W diesel emergency generator with a 1200A main disconnect at the generator (downstream equipment is main lug generator distribution board). Transfer switches (ATS's) served from the GDP (generator distribution panel) would all be 3-pole since no neutral is presently required. The building this is serving will be a commercial high rise. So the thought is to specify the generator as separately derived and bring an EGC to the GDP and terminate there in case any 277V loads are ever required for the emergency system (understood that 4-pole transfer switch would then need to be introduced and neutral would have to be switched). I am not seeing how the generator can be classified as separately derived still though for the case I describe.

Is there a neutral in the service from the utility? Is there a neutral in the generator? If yes to both, why not just tie them all together, have a neutral in your panel(s) in case you ever need it, and keep the 3 pole ATS? It wouldn't be separately derived so you'd leave the N and G isolated in the genny. It seems to me that this is a simple solution; am I missing something?

 

[kwired]

Thank you- I appreciate everyone's input. Sorry to bring this in a different direction, for this situation I was mistaken on the generator location - it is actually going to be located inside of the building. It is a 750KW, 480V, 3PH, 4W diesel emergency generator with a 1200A main disconnect at the generator (downstream equipment is main lug generator distribution board). Transfer switches (ATS's) served from the GDP (generator distribution panel) would all be 3-pole since no neutral is presently required. The building this is serving will be a commercial high rise. So the thought is to specify the generator as separately derived and bring an EGC to the GDP and terminate there in case any 277V loads are ever required for the emergency system (understood that 4-pole transfer switch would then need to be introduced and neutral would have to be switched). I am not seeing how the generator can be classified as separately derived still though for the case I describe.

My thoughts are if this is 480/277 wye source and over 1000 amps then ground fault protection is likely required. I believe others have already mentioned one reason to switch the neutral is because of ground fault protection, without isolation you can get "false tripping" of the GFP.

 

[Jamesco]

Thank you- I appreciate everyone's input. Sorry to bring this in a different direction, for this situation I was mistaken on the generator location - it is actually going to be located inside of the building. It is a 750KW, 480V, 3PH, 4W diesel emergency generator with a 1200A main disconnect at the generator (downstream equipment is main lug generator distribution board). Transfer switches (ATS's) served from the GDP (generator distribution panel) would all be 3-pole since no neutral is presently required. The building this is serving will be a commercial high rise. So the thought is to specify the generator as separately derived and bring an EGC to the GDP and terminate there in case any 277V loads are ever required for the emergency system (understood that 4-pole transfer switch would then need to be introduced and neutral would have to be switched). I am not seeing how the generator can be classified as separately derived still though for the case I describe.

- Generator neutral conductor will bonded at the generator.

- A grounding jumper, sized per code, will be installed from the same neutral conductor bond connection to the building electrical service main grounding system, per code.

- I assume parallel feeders will be installed from generator to generator distribution MLO panel board.

- Each conduit will have 3 ungrounded conductors + neutral grounded conductor + EGC.

- Generator Distribution Panel.
A separate bonded to enclosure ground bar will be installed in panel.
All EGCs will connect to this bar.

The supplied neutral bar shall remain isolated, insulated, from the grounded panel enclosure. Generator feeder neutral conductors terminate here as well as any future neutrals for feeders fed from the panel.