Separately Derived 3phase 3 wire System

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hopkins

Member
Location
New York
I am installing several 600V delta primary /480V wye secondary transformers. All transformers feed 3 phase 3 wire equipment. The NEC 250.20 B(2) requires that 3 phase 4 wire wye connected systems in which the neutral is used as a circuit conductor be grounded. It doesn't discuss 3 phase 3 wire system. Do I need to run a Grounding Electrode Conductor to building steel?

I am planning to bond the neutral point, equipment grounding conductors, and transformer case together at the transformer. I believe this will satisfy the code requirements. I am I right?
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately Derived 3phase 3 wire System

Yes you will need a GEC refer to 250.30

[ March 12, 2003, 03:39 PM: Message edited by: dereckbc ]
 

hopkins

Member
Location
New York
Re: Separately Derived 3phase 3 wire System

Article 250.30 applies to "Separetly derived systems that are grounded" per NEC. Article 250.20d states that separately derived systems as covered in 250.20a or b shall be grounded. 250.20 b specifically goes out of its way to list only the 3 phase 4 wire system with the neutral conductor used as a circuit conductor. From this a 3 phase 3 wire separately derived system isn't required to have a grounding electrode conductor.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately Derived 3phase 3 wire System

Hopkins, what about 250.30(B)(1)? :confused: Makes no difference if a neutral is used or not.
 

templdl

Senior Member
Location
Wisconsin
Re: Separately Derived 3phase 3 wire System

If I understood you correctly you really do have a 3ph, 4w supply even though you are not using the neutral which is very common with motor control centers which are made 3ph, 3w. You still have 277v to ground. The X0 is grounded and an equipment grounding conductor is used to ground the equipment. In a simple 120/240v 1ph,3w home system you could have all 240v line to line lloads with no line to neutral loads but the neutral is still grounding and the EGC still are run.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Separately Derived 3phase 3 wire System

Even if XO is left floating on the secondary of the transfomer, 250.30(B) requires that this transformer have a grounding electrode, as pointed out by Dereck.
Don
 

bennie

Esteemed Member
Re: Separately Derived 3phase 3 wire System

What is the difference in grounding (earthing) the X-O in two locations, or grounding the neutral two times after the service?
 

bennie

Esteemed Member
Re: Separately Derived 3phase 3 wire System

The separately derived system is the premises wiring, not the source.

Premises wiring that is grounded, at the service, does not require any additional earth connections.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately Derived 3phase 3 wire System

Bennie, I am probable going to hate myself for asking this, but how do you figure the Xo is being bonded twice with a delta feed? :confused:
 

gwz2

Senior Member
Location
Indiana
Re: Separately Derived 3phase 3 wire System

Don,

What is your definition of "Even if Xo is left floating" ?

hopkins noted he planned on bonding the Xo to transformer case.

I would not consider that as a floating Xo.

Since the Xo is bonded to ground at the transformer, a conductor, beit called a Grounding Conductor or a Grounded Conductor, should be taken to the remote OCPD.

If a metallic raceway system is used between the transformer and the OC enclosure ( and bonding any Flex conduit ) should be acceptable if no 277V loads on the load side of the OCPD.

Any short to ground on the load side of the OCPD should now quickly open as per 250.4(A)(5).

If the Xo is grounded at any point, a conductor should be with the circuit conductors to the OCPD, although, I don't read 250.30(A) to clearly say-so.

Is that your position ?
 

bennie

Esteemed Member
Re: Separately Derived 3phase 3 wire System

Dereck: The X-O of the transformer is solidly connected to the service ground neutral/buss and to effectively grounded building steel, or ground electrode. This comprises two earth points, creating objectionable current on the equipment ground conductor.

A separately derived system is "A premises wiring system". This does not mean a part, of a premises wiring system, it is either all the wiring system or none.

A power source to the separately derived system, can only be connected on the supply side of the main disconnect.

There can not be multiple separately derived systems within a premises. The code section concerning this, is incorrect.

A transformer is a source of power for a separately derived system.

A premises wiring system that is solidly connected to the utility MGN, can only be considered a separately derived system when the MGN is switched such as for a local generator plant.
 

bennie

Esteemed Member
Re: Separately Derived 3phase 3 wire System

Too add a bit more...The secondary X-0, of a power transformer, must be connected to the supply X-0, for the containment of a through fault.

This procedure is the same as a MGN distribution system. The MGN system produces a lot of objectionable current on the ground/neutral conductor. This current produces EMF and an unstable voltage to ground.

For outdoor installations the EMF is not a problem. Indoors the EMF is often objectionable and affects the power quality.

By only grounding the X-0 to the single point at the service, the objectionable current does not have a completed circuit.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Separately Derived 3phase 3 wire System

gwz,
I didn't mean to imply that XO is floating in this case. I just want to point out that even it you install the transformer as an ungrounded system by not making any connection to XO, a grounding electrode is still required by the code.
Don
 

hopkins

Member
Location
New York
Re: Separately Derived 3phase 3 wire System

dereckbc, My point is the the code says 250.30 only applies to separately derived systems required to be grounded by 250.20D. A three phase three wire system is not one of them. Not that its gospel but in a Square D transformer application guide they state "Grounding of transformer secondaries without neutals is not necessary. In such cases phase to ground voltage measurements are unpredictable and normally insignificant". The only thing required is the the equipment grounding conductors (primary and secondary) and the transformer case be bonded to the X0 terminal. This will ensure any ground faults on the load side will operate the OCPD on the load side.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Separately Derived 3phase 3 wire System

Hopkins,
My point is the code says 250.30 only applies to separately derived systems required to be grounded by 250.20D.
What section of the code says this? 250.30(A) says it applies to any SDS that is grounded, not just systems that are required to be grounded.
The Square D information is not correct, it that is what they say. While it is correct to say that the code does not always require a conductor on the secondary side of the transformer to be grounded, it always requires a grounding electrode bonded to the transformer enclosure and the secondary equipment grounding conductors.
You have to look at 250.30(B) for this installation if it has an ungrounded secondary.
If any conductor on the secondary side of this transformer is bonded, then it is a grounded system and 250.30(A) applies. If none of the secondary conductors are bonded, then 250.30(B) applies. You have indicated that the secondary XO terminal is bonded, so 250.30(A) applies.
Don
 

hopkins

Member
Location
New York
Re: Separately Derived 3phase 3 wire System

DON,

250.20 Alternating Current Circuits And Systems to be grounded Section D deals with Separately derived systems. It states Separately derived systems as covered in 250.20 A or b shall be grounded as specified in 250.30. In sections 250.20 A and B only require 3 phase 4 wire with the neutral used as a conductor to be grounded. 250.30 Applies only to sds systems required by 250.20 a or b to be grounded. 3 phase 3 wire isn't one of them.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately Derived 3phase 3 wire System

Hopkins, ?250.20(D) Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30?. I can?t really add anything that Don and I have already stated. 250.30(A) details the grounded conductor (4-wire, neutral and 3-P) systems, and 250.30(B) details the ungrounded system (3-wire, 3-P). It does not matter if you have a Xo terminal, or whether or not you ground the Xo terminal. It is still required by code to provide a GEC.

If you have a Xo terminal and ground it, you will measure 277 VAC to ground. You are just not going to use the neutral. In the event you have a phase to ground fault anywhere along the system the GEC will provide the fault-clearing path to operate the OCPD. The GEC has to be rated to withstand the fault without damaging the insulation.

In the event the secondary Xo is not grounded or Delta configured, a phase to ground fault would still allow the system to operate, but you would have to provide ground fault detection. But you will still need to provide a GEC to minimize touch potential, and provide a fault clearing path in the event of a second phase to ground fault to safely operate the OCPD.

Hope this helps.

Dereck
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Separately Derived 3phase 3 wire System

Hopkins,
The wording in 250.20 applies to systems that are required to be grounded. 250.20(D) is just saying that if the output of your SDS falls under the rules in 250.20(A) and (B),then the SDS is required to be grounded. There is nothing in 250.20 that says you don't have to comply with 250.30(A) when you ground a SDS that is not required to be grounded. Your system is not required to be grounded, but you bonded XO on the secondary side of the transformer creating a ground system. 250.30(A) applies to all grounded systems, not just those that are required to be grounded by 250.20. That being said, it still doesn't make any difference, grounded or ungrounded, both require a grounding electrode. See 250.30(A) for grounded systems and 250.30(B) for ungrounded systems. They both require grounding electrodes. Your installation requires a grounding electrode.
Don

[ March 13, 2003, 01:28 PM: Message edited by: don_resqcapt19 ]
 

jmc

Member
Location
Massachusetts
Re: Separately Derived 3phase 3 wire System

Hi Hopkins, try this approach.
There are specific voltage systems that are required to be grounde. Then there are other systems that are permitted to be grounded if so desired. 250.20(B) lists specific systems that are required to be grounded. A 480 volt, 3-phase, WYE connected transformer secondary is not required to be a grounded system, however if so desired, the X-0 terminal is allowed to be grounded. If the X-0 terminal is connected to ground, it becomes a grounded system and the enclosure must be bonded to it as well as all noncurrent-carrying metal parts of the installation in accordance with 250.30(A).

If the X-0 of the WYE secondary is not connected to a grounding electrode system, then all of the noncurrent-carrying metal parts of the system are required to be connected to the equipment grounding bus and to the grounding electrode system in accordance with 250.30(B).
Regards, John M. Caloggero
 

bennie

Esteemed Member
Re: Separately Derived 3phase 3 wire System

The X-O of the transformer is connected to a ground electrode system through the equipment ground conductor. The equipment ground conductor is for containing a through fault.

A power transformer is not a separately derived system.

Connecting the X-O to two different ground points is in violation of the objectionable current code section.

There is no reference to a transformer in the code articles quoted.

A transformer may be the source for a separately derived system but is not the system. The premises wiring is the separately derived system.
 
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