N-G Bond in Medium Voltage Switchgear

[32Lateralus]

Typically I see commercial buildings with 480V services, fed from utility transformer. The 480V service has N-G bond.

There is a building with owner medium voltage switchgear in the main elec room that then feeds substations with transformers from 12.47k-480V in the same room.

In this case since the main feed comes into the medium voltage switchgear, would the N-G bond be made at the medium voltage?

The situation is we want to add an ATS after the substation transformer, and 'typically' this would be service rated. In this case would we just have a main breaker since it's on the transformer secondary and NOT have a N-G bond if that occurs at the medium voltage switchgear?

 

[zbang]

You would most definitely have a bond somewhere on the secondary side of the transformer; it creates an SDS.

 

[32Lateralus]

You would most definitely have a bond somewhere on the secondary side of the transformer; it creates an SDS.

So the ATS would be 'service rated' since it is a SDS, not necessarily because it's the building's service entrance.

And I assume the same N-G bond takes place at 480-208V transformer secondary as it's another SDS. I just never really see that shown on one-lines the way it is shown on the service with the |N|-----|G| sort of symbols.

 

[kwired]

Is there any line to neutral loads? that would determine if you needed a separate neutral and EGC or if all the grounded conductors beyond the service disconnecting means are simply for bonding/grounding purposes.

 

[32Lateralus]

Is there any line to neutral loads? that would determine if you needed a separate neutral and EGC or if all the grounded conductors beyond the service disconnecting means are simply for bonding/grounding purposes.

Yes, for this, I would assume we have L-N loads. Im moreso running through this scenario in my head as I was curious, still learning

 

[zbang]

Pretty much downstream of every transformer there will be a bond somewhere (often inside the transformer case). Read up on SDS's and transformer bonding, that should clarify things.

 

[32Lateralus]

Pretty much downstream of every transformer there will be a bond somewhere (often inside the transformer case). Read up on SDS's and transformer bonding, that should clarify things.

Awesome, will do.

And any ATS that occurs directly after a transformer would be 'service rated' meaning it would have a N-G bond and can have a main disconnect since it's coming off a SDS? Also those ungrounded secondary conductors have a limited distance they can run before an OCPD? So your service rated ATS would have to be very close to your transformer?

Am I on the right track?

 

[zbang]

Beware of calling something "service rated". Ofttimes people mean that the device contains a N-G bond, but that's not necessarily true. Better wording is "suitable for use as service equipment" ("SUSE"), which means you're allowed to use if for a service. There's also "only suitable for service use" or words to that effect, which means there's a non-removable N-G bond.

Either way, an SDS is not a service so it doesn't need anything "service" rated, and lots of ATSs aren't.

Another point- since a generator is involved, be careful of where that N-G bond is located (there are a lot of threads on MH about this). If you have bonds in both the transformer and the generator, you usually need to also switch the neutral in the ATS.

 

[32Lateralus]

Beware of calling something "service rated". Ofttimes people mean that the device contains a N-G bond, but that's not necessarily true. Better wording is "suitable for use as service equipment" ("SUSE"), which means you're allowed to use if for a service. There's also "only suitable for service use" or words to that effect, which means there's a non-removable N-G bond.

Either way, an SDS is not a service so it doesn't need anything "service" rated, and lots of ATSs aren't.

Another point- since a generator is involved, be careful of where that N-G bond is located (there are a lot of threads on MH about this). If you have bonds in both the transformer and the generator, you usually need to also switch the neutral in the ATS.

Ah, that's interesting. So with 'SUSE' the N-G bong could be field installed/removed as need based on the system? One last question lol

So when you say SDS is not a service, this brings me back to my initial question of MV inside the building and a unit substation for 12.47k-480V. The building that made me start wondering about all this has MV switchgear inside the building, Im assuming the N-G would be bonded here and this would be the 'service'. Then it goes to the unit substation. If the owner wants to put an ATS after that unit substation transformer, before the main on the 480V side. That ATS would need to be 'SUSE' and have a N-G bond?

Cause it is a SDS so the ATS needs to be 'SUSE' in order to have N-G bond... but it is not a service? Am I understanding that right?

 

[kwired]

Ah, that's interesting. So with 'SUSE' the N-G bong could be field installed/removed as need based on the system? One last question lol

So when you say SDS is not a service, this brings me back to my initial question of MV inside the building and a unit substation for 12.47k-480V. The building that made me start wondering about all this has MV switchgear inside the building, Im assuming the N-G would be bonded here and this would be the 'service'. Then it goes to the unit substation. If the owner wants to put an ATS after that unit substation transformer, before the main on the 480V side. That ATS would need to be 'SUSE' and have a N-G bond?

Cause it is a SDS so the ATS needs to be 'SUSE'... but it is not a service? Am I understanding that right?

I don't think so, SUSE marked equipment certainly can be acceptable though. With any SDS you have options of bonding it at the source or at the first disconnecting means, but most the time can not be both.

Yes your MV is the service voltage. Beyond the service disconnect you still need separate grounded and EGC's just like for lower voltages. If there is no loads utilizing the neutral then you only need an EGC beyond the service disconnect. Appears all our MV loads are transformer primaries, if they are truly line to line connected then you don't need a neutral, if neutral is connected as a part of the working circuit, then you need separate EGC beyond service disconnect. That is general rules, could possibly be some exceptions with specific details.

 

[32Lateralus]

I don't think so, SUSE marked equipment certainly can be acceptable though. With any SDS you have options of bonding it at the source or at the first disconnecting means, but most the time can not be both.

Yes your MV is the service voltage. Beyond the service disconnect you still need separate grounded and EGC's just like for lower voltages. If there is no loads utilizing the neutral then you only need an EGC beyond the service disconnect. Appears all our MV loads are transformer primaries, if they are truly line to line connected then you don't need a neutral, if neutral is connected as a part of the working circuit, then you need separate EGC beyond service disconnect. That is general rules, could possibly be some exceptions with specific details.

So at the substation transformer, let's say the N-G is bonded at the source, inside the transformer. This means the ATS right after doesnt need to be SUSE and no N G bond.

SUSE is acceptable because if we dont know, contractor has the ability to install or remove that N-G bond, based on the actual site conditions of the system?

Thank you for your explanations btw

 

[electrofelon]

The bond, if present, on the MV service, has nothing to do with the bonding arrangement on the secondary of the transformer. For the secondary, you either Bond at the transformer or in the first disconnect. Since this is an existing installation this will probably already be determined for you. You will probably want a "service rated" ATS as you will need a main breaker for your 450.3(A) transformer protection and your 240.21(C) secondary conductor protection. Typically a non-service rated ATS does not have a breaker in it. Of course you could use a non-service rate ATS and put a separate disconnect and ocpd in front of it.

 

[zbang]

Part of what I was trying to get at is that past the service point, SUSE isn't relevant since it's no longer a service.

I'm being a bit lose with the language here, but look at it this way-
From the utility to the first over-current device is the service, and that device/enclosure must be SUSE. Nothing after that point is "service".

The secondary of every transformer forms an SDS with it's own bonding/grounding rules.

Every transformer has rules for both primary and secondary over-current protection.

When there are two sources feeding one device (e,g. a transfer switch), there can only be one N/G bond in the circuit. If both sources have that bond, then the neutral must be switched if it passes through the TS.

 

[32Lateralus]

Part of what I was trying to get at is that past the service point, SUSE isn't relevant since it's no longer a service.

I'm being a bit lose with the language here, but look at it this way-
From the utility to the first over-current device is the service, and that device/enclosure must be SUSE. Nothing after that point is "service".

The secondary of every transformer forms an SDS with it's own bonding/grounding rules.

Every transformer has rules for both primary and secondary over-current protection.

When there are two sources feeding one device (e,g. a transfer switch), there can only be one N/G bond in the circuit. If both sources have that bond, then the neutral must be switched if it passes through the TS.

Yes, makes sense, I think @electrofelon hit that point as well.

The manufacturers call it 'Service Rated' based on the components it has like a main breaker or means of bonding, but like you said, past the MV service, calling it 'service rated' is really not accurate. I think the fact that Im looking at manufacturer drawings of ATSs and them labelling as 'service rated' causes some issues in language.

Based on design, it has nothing to do with a service, but based on manufacturer language, they deem the components I need as 'service rated'.

Overall I think I understand the requirements of what that sort of design would look like.

 

[kwired]

Yes, makes sense, I think @electrofelon hit that point as well.

The manufacturers call it 'Service Rated' based on the components it has like a main breaker or means of bonding, but like you said, past the MV service, calling it 'service rated' is really not accurate. I think the fact that Im looking at manufacturer drawings of ATSs and them labelling as 'service rated' causes some issues in language.

Based on design, it has nothing to do with a service, but based on manufacturer language, they deem the components I need as 'service rated'.

Overall I think I understand the requirements of what that sort of design would look like.

SUSE or "service rated" should have removable bonding jumper or one that is not pre installed and must be installed where needed.

"Suitable only for use as service equipment" usually has the grounded conductor bus bolted directly to the enclosure and isn't as simple to add/remove the bonding means.