Generators, MDP, and Transfer Switch

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PEL11

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I have a 300 KW generator. In the generator there is a 1200 amp CB. I have four 3 1/2 pvc. conduits, ran underground to a 1200 amp main lug emergency panel. In each of the conduits I will have four 350 kc mills, one for each of the phases and the neutral. And one 3/0 for the equipment ground. At the emergency panel the grounding electrode conductor goes to the cold water pipe then the building steel. From the emergency panel to the first ATS is a two inch PVC conduit ran underground. With four 3/0 and a number four equipment ground (the neutral is switched in the ATS) from the ATS to the critical panel, the pipe and the wire size are the same. I have a 3000 AMP MDP with a 3/0 run to the cold water pipe, and then the building steel for the grounding electrode conductor. From the MDP I have a two inch PVC conduit with four 3/0 and a number four bond wire ran underground to the critical ATS. My question is does anybody see a problem with my grounding or bonding?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Does this look right?

PEL11ATS.jpg


Is all this equipment on the same building?
 

RUWired

Senior Member
Location
Pa.
PEL11 said:
My question is does anybody see a problem with my grounding or bonding?
It sounds right providing you have the neutral and ground seperated at the generator and at the critical load panel.Your equipment bonding jumper should be at the em panel and mdp.
Rick
 

PEL11

Member
I like your drawing, and that's exactly what I have. And yes, all the equipment is in the same building. And the distance from the generator to the emergency panel is 46 ft. I have a new question. If I use the breaker in the generator as the first means of disconnect, could I then run a grounding electrode conductor from the generator, to the cold water, and to the building steel? And then remove the bonding conductor in the emergency panel?
 

Bea

Senior Member
I have a new question. If I use the breaker in the generator as the first means of disconnect, could I then run a grounding electrode conductor from the generator, to the cold water, and to the building steel? And then remove the bonding conductor in the emergency panel?[/QUOTE]


Yes

Is the ATS service rated or is the critical panel a main breaker
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
If I have the picture right, the ATS would not need to be service rated, it's on the load side of a service disconnect.

The bond for the emergency system can be made at either the generator or the 1200A MLO panel. The grounding electrode connection should take place wherever the bond is made.

I haven't checked your conductor sizes, so no comment there.
 

Bea

Senior Member
georgestolz said:
If I have the picture right, the ATS would not need to be service rated, it's on the load side of a service disconnect.

Nevermind I missed the big picture
 
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George Stolz

Moderator
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Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Gary,
Aretae said:
Should there not be a bond to ground at the genset itself?
I had said the system bonding jumper (EGCs to Neutral) may be installed at either the generator or the panel last night.

Looking at 250.30(A)(1), I might have been wrong.

... This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

For some reason, I have it in my head that the breaker on the generator can be ignored when applying this section (allowing the bonding connection in the panel if desired), but looking again, it appears that the bonding jumper and the GECs must go to the generator.

Then again, the rule I quoted seems to be aware that some SDS sources will have overcurrent protection in them, and not care. :confused:

Thoughts?
 

ramdiesel3500

Senior Member
Location
Bloomington IN
george
I have been pondering this very issue for quite a while myself. All new generators that we install have disconnecting means built into them. Based on 250.30 (A)(1). the grounded conductor must be bonded ahead of the disconnect (breaker). In my book, this means that every ATS must have a switched neutral in order to prevent stray neutral current from flowing through the bond in the other SDS (normal power) creating a parallel path. I hope someone can shed some light on this for us!

Portable generators connected to homes during power outages is a real concern for me. In my way of thinking, there is practically no way to legally connect a portable to a house short of what I have done at my home which is to install a 3-pole transfer switch and connect the generator to the grounding electrode conductor before I plug it in! My set-up is almost unheard of!
 

RUWired

Senior Member
Location
Pa.
georgestolz said:
.
For some reason, I have it in my head that the breaker on the generator can be ignored when applying this section.
Thoughts?

I would agree because the source is the load side terminals of the breaker on the generator.The grounding connection can be made anywhere between the source and the first ocpd.
Rick
 

mikehughes8

Senior Member
Location
NC
I'm not sure if I'm applying this correctly but it seems that exception #2 of article 250.30(A)1 allows us to bond at both the the source and main service disconnect for a SDS as long as doing so doesn't create a parallel path for the grounded conductor (connection through earth not being considered a path)
To accomplish this it the EGC can be eliminated much like it is from the utility to the main service disconnect.

Just a thought....and if I'm correct it solves the issue with generators with their own OCPD.
 

RUWired

Senior Member
Location
Pa.
mikehughes8 said:
I'm not sure if I'm applying this correctly but it seems that exception #2 of article 250.30(A)1 allows us to bond at both the the source and main service disconnect for a SDS as long as doing so doesn't create a parallel path for the grounded conductor (connection through earth not being considered a path)

Mike, This exception works mostly for outdoor transformers with pvc raceways. With outdoor generators, you have to watch out for the battery charger and block heater circuits. The EGC in those circuits would be a parallel path.
Rick
 

tom baker

First Chief Moderator
Staff member
ramdiesel3500 said:
george

Portable generators connected to homes during power outages is a real concern for me. In my way of thinking, there is practically no way to legally connect a portable to a house short of what I have done at my home which is to install a 3-pole transfer switch and connect the generator to the grounding electrode conductor before I plug it in! My set-up is almost unheard of!

True and the CMP understands that I was told by a former chair of CMP5.
They say its better to have a bit of objectionable current than to have a generator with no system bonding jumper.
 
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