Single Line Diagram and Grounding

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Samardas

Member
Location
Los Angeles, CA, United States
Occupation
Electrical Engineer
Hi,
I designed a 4,000A service connection to the Main Switch Board (MS - 480/277V, 3 Phase, 4W) as follows (Also please see the Single Line Diagram attached): Each phase got two 4" conduits - with each conduit having five 600 MCM cables of the same phase in it. So, there are ten 600 MCM cables per phase run through two 4" conduits.

Inspector is saying each conduit should have all three phases run through it. I don't think this is necessary. There is no voltage drop situtation here since the entire installation is very close to the service transformer and the cable size selected totals more than 4,000 AMP (420Ax10 cables per phase) as well.



Second, do we need an 'equipment ground' running between the two panels? I have shown UFER and ground rod (grid) on the Pull box side and Ground grid and Cold water pipe for the MS. (Grounding always baffles me:?)

Can you please look at the attached diagram and suggest any changes per NEC?
I would really appreciate your early response. Thanks a LOT!!
 

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Smart $

Esteemed Member
Location
Ohio
You can run parallel as isophase (short for isolated phase arrangement) per 300.3(B)(1)...
Exception: Conductors installed in nonmetallic raceways
run underground shall be permitted to be arranged as isolated
phase installations. The raceways shall be installed in
close proximity, and the conductors shall comply with the
provisions of 300.20(B).
Compliance with 300.20(B) shouldn't be a problem if you're using open bottom gear on a concrete pad.

Grounding conductors are not used on the line side of the service disconnecting means. The grounded conductor (N) serves that purpose. As such, with a 4-wire system, you only need, e.g. (8) 5#600THWN, 4"PVC [2?ISO-?]

The one thing that is puzzling me is GFI on the line side of the service disconnecting means. I've seen GFP monitoring on the line side, but never GF interrupter. See 230.82(8)
 

iwire

Moderator
Staff member
Location
Massachusetts
I agree with the issues Smart pointed out and I have to add another issue.


Hi,
I designed a 4,000A service connection to the Main Switch Board (MS - 480/277V, 3 Phase, 4W) as follows (Also please see the Single Line Diagram attached): Each phase got two 4" conduits - with each conduit having five 600 MCM cables of the same phase in it. So, there are ten 600 MCM cables per phase run through two 4" conduits.

Inspector is saying each conduit should have all three phases run through it. I don't think this is necessary. There is no voltage drop situtation here since the entire installation is very close to the service transformer and the cable size selected totals more than 4,000 AMP (420Ax10 cables per phase) as well.

The selected cables installed that way are only rated 3,800 amps. You need to change the cables or lower the trip setting.

You have put five current carrying conductors in the conduits and that requires derating per 310.15(B)(3)(a).

For the purposes of derating we can use the 90C column of 310.15(16)

So 600 kcmil @ 90C is 475 amps

475*10=4750 amps

But now we have to derate down 80% per table 310.15(B)(3)(a)

4750*.8=3,800 amps. That is a problem.
 

coop3339

Senior Member
Location
NJ
Hi Sam,

If you are using metal conduit, I don't think you can separate the phases like that.

It looks like you are bonding the neutral in 2 locations. Doing that will parallel the neutral and ground. I think you will need to isolate the neutral from ground and add a ground conductor in each feeder set to meet code.
 

Smart $

Esteemed Member
Location
Ohio
...

4750*.8=3,800 amps. That is a problem.
With the drawing as annotated, correct.

However, if main breaker were 4000AF/3800AT, it would not be a problem... as long as the calculated load, with continuous factored 125%, does not exceed 3800A.
 

Smart $

Esteemed Member
Location
Ohio
It looks like you are bonding the neutral in 2 locations. Doing that will parallel the neutral and ground. I think you will need to isolate the neutral from ground and add a ground conductor in each feeder set to meet code.
That's not a problem as long as the only paths between grounding electrodes are the neutral, parts containing and bonded to the neutral, and earth.

If the electrodes are bonded by another [compliant] path, disconnecting the GEC(s) in either enclosure will remedy the situation. Grounding conductor(s) between enclosures are not necessary.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The 10 conduits confuse me, but I've never designed an isolated phase installation. 3 phases and a neutral each with two conduits equals 8 conduits. It looks like the ground conductors are in their own conduits.
Is it correct to put the grounds in 2 separate conduits? It seems like they would be installed in each conduit with phase conductors. And do you really need a ground that is the same size as the phase conductors?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The one thing that is puzzling me is GFI on the line side of the service disconnecting means. I've seen GFP monitoring on the line side, but never GF interrupter. See 230.82(8)

Interrupter is used to say the device actually opens, versus Alarm where it only alerts like for ungrounded circuits and hospital emergency systems

GF is pretty much generic slang. In my experience, GFI or GF(A) are for trip currents >1A, while GFP or GFPE is for >15mA but < 1A. GFCI is the 'restricted' term for people protection, with Class A, 5mA, the most common.
 

nhee2

Senior Member
Location
NH
Which is the service disconnecting means? I had interpreted the service disconnect at the metering gear and the MSB feed to be a feeder.
 

Smart $

Esteemed Member
Location
Ohio
Interrupter is used to say the device actually opens, versus Alarm where it only alerts like for ungrounded circuits and hospital emergency systems

GF is pretty much generic slang. In my experience, GFI or GF(A) are for trip currents >1A, while GFP or GFPE is for >15mA but < 1A. GFCI is the 'restricted' term for people protection, with Class A, 5mA, the most common.
Yeah, I got that. One-line shows GFI on the line side of the switchgear main disconnect. AFAIK, that's a violation.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
As others pointed out, something like the position of the GFP confuses me. My understanding is that if a UFER exists you do not need any ground rods.


Further, my understanding is that the water bonds should be coming from the service disconnect, not switchgear?
 

sandsnow

Senior Member
Seems ok to me. GFP on 4000amp OCPD in its own enclosure is the service. Then a feeder to a 4000amp switchboard with a 4000amp main. I suspect service is outside per POCO rules so they don't need to get inside to access their equipment.

So the neutral cannot be bonded inside the 4000amp switchboard per 250.24(A)(5) and 250.32
And the 4000amp switchboard does not need any connections to earth if service is immediately outside the building. All bonding to water, steel, etc can be done in the service.

4000amp switchboard inside needs to be supplied with an EGC. It's supplied by a feeder so EGC is automatic. In looking at 300.3(B)(2) and the sections referenced there, it looks like you have to run an EGC in each raceway. Maybe I missed something.

You're Isophase installation looks possible, but if you really want to do it, you're going to have present you're Code backup by section number to the inspector. The inspector is making the call based on the general rule and your using an exception that in my opinion is not used much. Of course my experience is limited to what is commonly done in my area and I would guess his too.
 

nhee2

Senior Member
Location
NH
Seems ok to me. GFP on 4000amp OCPD in its own enclosure is the service. Then a feeder to a 4000amp switchboard with a 4000amp main. I suspect service is outside per POCO rules so they don't need to get inside to access their equipment.

So the neutral cannot be bonded inside the 4000amp switchboard per 250.24(A)(5) and 250.32
And the 4000amp switchboard does not need any connections to earth if service is immediately outside the building. All bonding to water, steel, etc can be done in the service.

4000amp switchboard inside needs to be supplied with an EGC. It's supplied by a feeder so EGC is automatic. In looking at 300.3(B)(2) and the sections referenced there, it looks like you have to run an EGC in each raceway. Maybe I missed something.
.

I agree with this - If the OCPD at the metering gear is the service disconnect then an EGC would be required and N-G bond at MSB would be removed, a single N-G bond at the service.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I may be overlooking something you guys covered but it seems to me that we need clarification from the OP on a couple of issues to form a conclusion:
Are the conduits non-metallic and are the enclosures open bottom ?
What is the purpose of the 4000 amp breaker at the metering equipment (is this the service disconnect ?)
 

coop3339

Senior Member
Location
NJ
That's not a problem as long as the only paths between grounding electrodes are the neutral, parts containing and bonded to the neutral, and earth.

If the electrodes are bonded by another [compliant] path, disconnecting the GEC(s) in either enclosure will remedy the situation. Grounding conductor(s) between enclosures are not necessary.

I don't think that is correct. If the 4000A GFI is the main the neutral should be bonded there only. The panel should be a subpanbel with the neutral and grounds separated. This would also require grounding conductors to be run with the feeders between the main and the panel.
 

Samardas

Member
Location
Los Angeles, CA, United States
Occupation
Electrical Engineer
Single Line Diagram and Grounding

I may be overlooking something you guys covered but it seems to me that we need clarification from the OP on a couple of issues to form a conclusion:
Are the conduits non-metallic and are the enclosures open bottom ?
What is the purpose of the 4000 amp breaker at the metering equipment (is this the service disconnect ?)

Thank you ALL for your quick response. Some more clarifications. Conduits are non-metallic, encased in concrete and the enclosures are open bottom. (In the mean time, I have asked the contractor to run all three phases in one conduit with eight of such runs to cover the 4,000 Circuit Breaker AMPS, instead of each conduit having the same phase conductors).

Now as for grounding, I am a little bit confused (my fault) seeing different suggestions from different angles by the readers. I am sure all of you are correct because you did not know the type of conduits being used here. Now, my question is
a) Do I need to run a ground conductor (#4/0) between the Service Pull Box (SPB) and MS?
b) If so, do I need to run one #4/0 through every conduit where the phase conductors are run through (eight of them)? In other words, if 8 conduits have phase conductors and a neutral running through them, do I also have to include one #4/0 through every conduit? Or, one #4/0 ground conductor run through one of the eight conduits or one of the empty conduits is enough and connect between the ground bus in SPB enclosure and the ground bus in MS
c) Is it okay to connect the ground bus in SPB to the SPB enclosure?
d) Otherwise, if I do not run any ground conductor between the SPB and the MS, how do I ground the MS. In the drawing a separate grounding rod and a cold-water pipe or another ground rod and bonding between N & G and to the enclosure is shown
e) Is the SPB grounding as shown okay?
f) Lastly, do I need to change the GFI to GFP in the Pull Box as some of you suggested?

Thanks again for putting up with me.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Lastly, do I need to change the GFI to GFP in the Pull Box as some of you suggested?

No.
Some of the debate has been on the descriptor (GFI, GFP, or simply GF) to use not its function.

The service entrance main device is required to have ground fault protection. Ground fault function on downstream series devices, like a remote main in a second building, is optional. Ground fault protection of feeder circuits is addressed by NEC sections.
 
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