NEC250.24(c): is a grounded conductor needed if it will not be used?

Status
Not open for further replies.
G

George O'Brien

Member
An industrial plant has new equipment going in. Power to it It will be supplied from a 4160VAC-480VAC Delta to Wye transformer. The neutral terminal on the secondary windings of the transformer will be tied to the plant ground grid to provide a zero reference and keep the system voltages from "floating". There will be a grounding conductor run from the transformer to the Switchboard even though both the transformer and Switchboard are tied to the plant ground grid.

But there are no 277VAC loads on the equipment. All motors etc. are 3phase 480VAC, and only three current carrying conductors are required to operate the equipment. The Switchboard has accomodations for grounding but does not have a neutral bus. My question is: Does NEC250.24(c) apply? That is, must a 4th current carrying conductor be run as the grounded conductor? if it must be run, what purpose does it serve? and where should it be terminated in the switchboard?
 
Welcome to the Forum.
If I understand correctly it will make a difference if this is a "service situation" where the transformer is utility supplied or if the transformer is part of the plant system and a SDS.

If it is a service supplied system, IMO, you would have to meet the requirements of 250.24(C) and bring a grounded conductor to the building.
It's a bit of a semantics question as you would not need any additional grounding conductor between the transformer and the building as that grounded conductor will be the ground return path needed.

If the transformer is not service but part of a plant SDS system we can approach that if needed.
 
And I think the distribution panel has to be identified as not having a grounded conductor, this reminds me of when I was a rookie and my boss and an inspector ended up looking more like an umpire and coach after a bad call ! The big argument was why pull a neutral if it wasnt needed.
 
My understanding was that a neutral or grounded conductor must be supplied to the building. Assuming somewhere you have lighting etc being supplied with a grounded conductor then IMO, this panel, that only supplies 3 phase loads with no grounded conductor needed, then you do not need to run a grounded conductor.
 
Seems pretty straight forward to me.

250.24(C) Grounded Conductor Brought to Service Equipment.

Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be connected to each disconnecting means grounded conductor (s) terminal or bus. A main bonding jumper shall connect the grounded conductor(s) to each service disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.24(C)(1) through (C)(3).
Click to expand...

So if the power company grounded the system at their transformer then you must bring the grounded conductor to the service disconnecting means. :)
 
Bob,
How does this change if the situation is one where the "service" is the primary, the transformer is outside the building and there is no neutral load ?

Now the building does not have a "service supplied system", but we have a SDS. Am I correct that we simply have a terminology change and that we bond XO to the transformer and a ground rod at the transformer and bring a system bonding jumper to the building gear along with the phases ?
 
Gus in that case I think we would just treat it like any SDS it would just happen that the disconnecting means is further from the transformer then we are used too.:)
 
That particular rule applies only to a service and not a separately derived system. Look at the definition for a service in article 100. It state's:

The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Since you are using transformers the system is therefore separately derived and the rules of article 250.30 would apply.
 
augie47 said:
Now the building does not have a "service supplied system", but we have a SDS. Am I correct that we simply have a terminology change and that we bond XO to the transformer and a ground rod at the transformer and bring a system bonding jumper to the building gear along with the phases ?
Click to expand...

No, I'd say we have an SDS, then bonding to a ground rod at the transformer would be improper, unless the grounding electrodes listed in 250.30 (metal structure grounding electrode or metal water pipe grounding electrode) are not available.
 
david luchini said:
No, I'd say we have an SDS, then bonding to a ground rod at the transformer would be improper, unless the grounding electrodes listed in 250.30 (metal structure grounding electrode or metal water pipe grounding electrode) are not available.
Click to expand...

But, if the transformer is separate from the building, there are normally no electrodes present.
If a system bonding jumper is brought to the building, it could then be bonded to the building electrodes.
 
augie47 said:
But, if the transformer is separate from the building, there are normally no electrodes present.
If a system bonding jumper is brought to the building, it could then be bonded to the building electrodes.
Click to expand...

I don't think the code says the transformer has to be in the building for the grounding requirements of 250.30 to apply, or at least I don't read it that way.

250.30 says the GE shall be the nearest of the the water pipe or structural metal electrodes. Only if they don't exist can you use a different electrode. So it the if the water pipe of structural metal GE existing in the building that is being fed by the SDS, I'd say that they must be used.

Of course the code lets you make the bond at the disconnecting means rather than at the transformer. So you don't need to run a GEC out to the transformer, you can bond the grounded conductor and GEC inside the building at the disconnect.
 
The grounding electrode would have to be connected at the same point where the system bonding jumper is connected in accordance with NEC 250.30(A)(3).
 
Status
Not open for further replies.
Top