Dual service grounding

[sevlander]

Came across this situation during a troubleshoot. Customer complaint of several tenant surge disruptions causing computer outage or damages. Service consists of side by side 400 amp WP disconnects,fed from one utility transformer underground. Load side of each 400 amp disconcet feeds a trough to 5 meters with 5 125 amp breaker enclosures feeding separate tenants (5 on 1st floor on one trough and 5 on 2ndtrough for 2nd floor). A ground rod for each 400a D.S. and 1 - 4/0 aluminum to water pipe entrance. Here is where it gets funny. Each indiviual tennant unit is fed underground with a plastic 3/4 water pipe from a main water pipe undergound in the parking lot.The interior water piping then becomes copper.The 1st 4/0 is landed on one 1st floor unit water pipe. The 2nd 4/0 ran to another 1st floor unit water pipe but was never landed! SER cable feeds each unit sub panel and is properly landed.Individual #4 bare aluminum grounds also leave each sub panel and hit the cooresponding water pipe in each unit. I believe these grounds should be removed as the SER carries the ground back to the main. The problem is the water piping is not all connected or bonded together.What is the correct way to ground and bond this mess? Is this service even legal? I would apprciate any help! :wink:

 

[George Stolz]

Re: Dual service grounding

Each indiviual tennant unit is fed underground with a plastic 3/4 water pipe from a main water pipe undergound in the parking lot.The interior water piping then becomes copper.

So, starting from scratch, this interior metal water piping system must be bonded according to 250.104.

Since it sounds as though all the units are isolated from each other, you could use 250.104(A)(2) to run a #6CU (sized to 125A (not exceeding 200A)) from the panelboard of each unit to the metallic water system in each unit.

Otherwise, you'd use 250.104(A)(1) to do the job, which would mean running a 1/0AL or #2CU to each unit (by Table 250.66), which would likely be more difficult. (They were using the 4/0's for this, which made it even more difficult.)

So, looking back at your post:

The 1st 4/0 is landed on one 1st floor unit water pipe. The 2nd 4/0 ran to another 1st floor unit water pipe but was never landed!

This could be because as they were pulling these in, they saw the easier method. They might never have realized their wires were oversized in the first place.

Individual #4 bare aluminum grounds also leave each sub panel and hit the cooresponding water pipe in each unit.

Then you can see, they realized 250.104(A)(2) would work for them, so they went for it.

I believe these grounds should be removed as the SER carries the ground back to the main.

These two conductors do totally different jobs. The #4 AL is bonding the water piping, keeping it at a somewhat even potential with the earth and mostly concerned with keeping the piping at the same potential as all metallic non-current-carrying items connected to the electrical system.

The Equipment Grounding Conductor in the SER is the effective ground fault current path required in 250.4(A)(3). This conductor's primary concern is to serve as a current path in the event that there is a short from an ungrounded conductor to the metal case of an appliance, etc, so that a circuit breaker trips.

Two different agenda entirely.

The problem is the water piping is not all connected or bonded together.

As you can see, it was a godsend to the fellas who did the original installation. It's kinda funny the left option one (250.104(A)(1)) hanging like Fred Flintstone at 5:00pm, but, hey...

Edit to correct sloppy code references, and to make a final statement:

Leave it be, it's all legal.