We're working to provide a small subpanel in a building with no convenient 208Y120. I want to use a 7.5kVA trans-panel unit(panel with built-in transformer). This would be connected to 480 volts from an existing subpanel. I would use a 2-pole CB for single phase primary of trans-panel. There is no ground lug or bar at this panel, but there is a neutral- it feeds HVAC units and has plenty of capacity. I am looking for thoughts about grounding the secondary side(120/240). Would it be protected properly with a cabinet ground at the 480 subpanel? This is all metal pipe installation back to main distribution, with grounded service. So total info- 7.5kVA xfmr(62A@120, rated 16A@480) with a load of 2) 2200 VA UPS units for computers(120 V). What do u think?

edit: The neutral in the 480 volt panel is not bonded, there is potential to 480V cabinet, cabinet is grounded through raceway at distribution 60 ft below. The 480V neutral will not be used in the trans panel- there is a neutral tap on 7.5 kVa secondary. Also made mistake and put 5 kVA instead of 7.5kVA-brain gas- sorry.

[This message has been edited by chi spark (edited 04-05-2004).]

[This message has been edited by chi spark (edited 04-05-2004).]

[This message has been edited by chi spark (edited 04-06-2004).]

Metal conduit is an acceptable equipment grounding conductor. Your transformer will constitute a separately derived source, and will need to be bonded on it's secondary neutral to the enclosure and to building steel and to any metal water pipes in the area served by the transformer. It might be a good idea to drive a couple of ground rods too.
The neutral in the 480 volt panel should not be bonded.
I would run an insulated green equipment grounding conductor in Greenfield (Along with the circuit conductors) to and from the transformer, attached to bonding bushings at the four ends.
In the transformer, I would continue the grounds to the neutral lug(s) or buss. The building steel, water pipe, ground rod grounds I would also connect to the transformer neutral lug(s) or buss. In the 240/120 panel I would have separate neutral and grounding busses. The ground from the bushing would extend to the ground buss. That may be the only wire terminated to the ground buss. The insulated neutral buss would terminate the neutral from the transformer and all branch circuit neutral conductors.

Thanks for the input, Earl. That's what I ended up with this AM- I was just concerned about multiple paths to ground across 2 species(480, 120/240). This application is on the 5th floor, and we found a good 3" copper water riser close by. BUT the real whammy is that the facility manager who insisted, then confirmed that there was no available 208Y120 around, was wrong as we located a panel he had "checked" with 20 out of 30 spaces as spares, with CB's installed already- so...120' of emt & wire and we walk on water again(until next time)

I do not believe that the 3" water riser can act as the grounding electrode conductor (unless it was connected 5' from the entry to the building). Check 250.

I believe that in industrial and commercial establishments (where the entire pipe is visible and is maintained by qualified people) any section of the water pipe can be used.

--250.52(A)(1) exception

iwould think blding steel would probably be your best grounding source. since your on the 5th floor ground rods would probably be unfeasible. as far as grounding the xfmr. the xo would be a good place to run your neutral,case ground and the main bonding electode. if you run a grounding conductor to a ground bar and have a separate neutral run to a neutral bar which is bonded you create 2 paths to ground. i would not use pipe as a ground. as far as the water pipe is concerned its probably difficult to confirm that its a copper line all the way to the meter and that its been jumpered.

You are required to go to closest available electrode with this grounding electrode conductor

250.30 Grounding Separately Derived Alternating-Current Systems.

You are also required to bond this SDS to the water pipe in the area it serves.

250.104 Bonding of Piping Systems and Exposed Structural Steel.

i wire, doesn't 250.30(A)(4) contradict the next code you listed by stating that "one of the following" ? Grounding in the code book always seems to lead me in circles and often seems to give me more than one answer which at times confuses the issue im looking for an answer to.

Rizer 250 is tough no doubt about it.

The quick answer is right now grounding and bonding are two separate items that are both addressed in article 250.

Grounding electric systems to metal water pipes must be most times within 5' of the point of entrance.

Bonding metal water pipes to a electric system can be done anywhere.


The first code reference I posted 250.30(A)(4)has only to do with Grounding the SDS.

The second code reference I posted 250.104(A)(4)has only to do with Bonding metal water piping in the area served to this SDS.

These rules mirror the rules for a typical house service.

The service must be grounded to the metal water pipe (if there is one) within 5' of the entrance, if this is done the interior water piping is now also bonded.

But what if this house has a plastic water main and metal interior piping.

In that case no service ground will be needed but bonding is still required, the bonding can be done anywhere on the piping system.

thanks bob