GEC Options for Multiple SDSs

[charlie b]

This is a spin-off from another thread ( here ). I didn't want to confuse that discussion by bringing up an issue on a separate installation.

I am designing a new ?light industrial? facility that will be about 160 x 250 feet. It will have electric rooms in three of the four corners. Each electric room, including the main, will have an SDS (i.e., transformer to step-down from 480 to 120/208).

My original plan was to bury a ground ring around the building, with ground rods ?every so often,? and with the Main Panel?s GEC and the GEC for each SDS connected to this ring in two places. I think this is an adequate Grounding Electrode System, though I admit it may be overkill. What I am being told by a reviewer is that it is absolutely overkill. So what is my best options for ?down-sizing??

There is no ?building steel,? as it is a single story building with ?tilt-up? concrete walls. Water enters the building at one place, and the point of entry of the pipes is at least 40 feet from the nearest electric room (and is much farther from the other two electric rooms). The entire length of water pipes is not exposed, so using them as a Grounding Electrode is not an option.

My questions:
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Can I just call for two ground rods outside each of the three electric rooms, and call it a day?</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Or if that is a ?No,? then
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Do I also need to bond the ground bars in the three electric rooms to each other? That is, do the main and the SDSs need to share a common Grounding Electrode System?</font>

<font size="2" face="Verdana, Helvetica, sans-serif">And if that is a ?yes,? then
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Can I run bonding conductors overhead across the building, using bare copper inside pvc conduit (for physical protection)?</font>

<font size="2" face="Verdana, Helvetica, sans-serif">And finally,
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Is there a ?significant difference? (i.e., more than 25% difference) between the installation cost of my original plan (i.e., ground ring) and the installation cost of the ?down-sized plan??</font>

<font size="2" face="Verdana, Helvetica, sans-serif">

 

[iwire]

Re: GEC Options for Multiple SDSs

Charlie IMO if the building has a water service that complies with 250.52(A)(1) it is 'available' even if it is inconveniently located.

IMO, absent building steel, this water service must be used you do not get other options.

I think the least costly option in regards to the SDS GECs would be underground PVC run from water service and daisy chaining through each electric room using the rules in 250.30(A)(3) Grounding Electrode Conductor Taps.

 

[charlie b]

Re: GEC Options for Multiple SDSs

If I were to connect the Main and each SDS to the water pipe, using underground PVC as you suggest, I believe that I would still have to use a supplementary electrode for each SDS. I could either put two ground rods inside each electric room, or run GECs from each room outside to a pair of ground rods (I don?t plan to measure ground resistance, so I?ll just use two rods). This cost would have to be taken into account as well. Or am I not reading 250.53(D)(2) correctly?

 

[petersonra]

Re: GEC Options for Multiple SDSs

The way I read this is if you have a ground rod, you only need one other GE or you have to meet the 25 Ohm rule. Since you have another GE, I don't see why you need two ground rods.

 

[mpd]

Re: GEC Options for Multiple SDSs

I would say just use a common grounding electrode, as per 250.30 (2) (b) & 250.30 (A) (3)
you do not have much choice as per 250.30 (A) (4)
and also look at 250.104 (4)

 

[charlie b]

Re: GEC Options for Multiple SDSs

Originally posted by petersonra: The way I read this is if you have a ground rod, you only need one other GE or you have to meet the 25 Ohm rule.

The way I read it, if you use a water pipe, then you have to use a supplementary electrode. If you choose to use a rod as your supplementary electrode, then you are back to the "2 rods or 25 ohms" rule.

 

[pierre]

Re: GEC Options for Multiple SDSs

CharlieB

You have one service to the building.
That service has to be grounded to the cold water serving the building provided the cold water had at least 10 ft of direct contact with the earth.
The other two electric rooms that have separately derived systems need to fulfil the requirements of 250.30


250.30
(4) 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) An effectively grounded structural metal member of the structure
(2) An effectively grounded metal water pipe within 1.5 m (5 ft) from the point of entrance into the building
Exception: In industrial and commercial buildings where conditions of maintenance and supervision ensure that only qualified persons service the installation and the entire length of the interior metal water pipe that is being used for
the grounding electrode is exposed, the connection shall be permitted at any point on the water pipe system.
(3) Other electrodes as specified by 250.52 where the electrodes specified by 250.30(A)(4)(1) or (A)(4)(2) are not available
Exception to (1), (2), and (3):Where a separately derived system originates in listed equipment suitable for use as service equipment, the grounding electrode used for the service or feeder shall be permitted as the grounding electrode for the separately derived system, provided the grounding electrode conductor from the service or feeder to the grounding electrode is of suffıcient size for the separately
derived system. Where the equipment ground bus
internal to the service equipment is not smaller than the required grounding electrode conductor, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.

 

[shortcircuit2]

Re: GEC Options for Multiple SDSs

Charlie b, answers to your questions...

1) You can not just drive 2 ground rods near each SDS and call it a day...

2) All available electrodes at a building must be bonded together to establish a common electrode system...

3) You can run your bonding jumpers overhead in PVC to each SDS or loop from room to room...

4) You are only required to supplement the metal water pipe service with 2 ground rods maximum...

What year code are you required to follow for your design?

Is steel rebar being set in the footing and foundation? It would be required to bond to this also. If so, you won't need the ground rods.

I don't see the need for the ground ring...

shortcircuit2

 

[charlie b]

Re: GEC Options for Multiple SDSs

Originally posted by shortcircuit2: What year code are you required to follow for your design?

2002.

Many thanks for all the replies.

 

[petersonra]

Re: GEC Options for Multiple SDSs

Originally posted by charlie b:

Originally posted by petersonra: The way I read this is if you have a ground rod, you only need one other GE or you have to meet the 25 Ohm rule.

The way I read it, if you use a water pipe, then you have to use a supplementary electrode. If you choose to use a rod as your supplementary electrode, then you are back to the "2 rods or 25 ohms" rule.

but since you have to conenct them all together anyway, you have 4 ground rods. Why would you need 2 at each spot, rather than 1?