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#129469 - 03/30/05 05:22 PM How to ground a 120V separately derived circuit.  
unsaint32  Offline
Member
Joined: Aug 2004
Posts: 15
eden prairie, MN, USA
I need to hook up a 208v motor. The control panel will contain 120V
control circuit and a 208/120V transformer.

I looked through the code book to find what I should do as far as grounding the 120V control circuit. Should I just jumper one leg of
the secondary to the control box enclosure?

Before you say yes, let me tell you two reasons why I think that might not be legal.

1) The system is 120V, therefore it has be grounded, meaning, one of
the legs (either X1 or X2) shall be connected to a grounding electrode.

2) Now, about the grounding electrode... The code 250.30 (A) (4) states that the electrode of a separately derived system has to be as
near as practicable to the same area as the grounding electrode conductor conenction to the system. So, I was thinking I might have to run a wire (grounding electrode conductor) through the control panel enclosure, then run it to a nearest metal structure.

Am I right? Am I wrong?


Tools for Electricians:

#129470 - 03/30/05 10:21 PM Re: How to ground a 120V separately derived circuit.  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
This question just raises more questions than can be answered without more information. And without more to go on there is no right or wrong. And you dont nessesarily have to have a grounded conductor to have 120...

This is some commentary from 250.20(D)
Quote
Two of the most common sources of separately derived systems in premises wiring are transformers and generators. An autotransformer or step-down transformer that is part of electrical equipment and that does not supply premises wiring is not the source of a separately derived system. See the definition of premises wiring in Article 100.

A read through 250.21 is appropriate too, as this control panel may fall under this code as well.

Now, a question: Is the Control Panel part of "Listed" equipment, or made in the field?

If made, why not use a different voltage.


Mark Heller
"Well - I oughta....." -Jackie Gleason

#129471 - 03/31/05 06:07 PM Re: How to ground a 120V separately derived circuit.  
unsaint32  Offline
Member
Joined: Aug 2004
Posts: 15
eden prairie, MN, USA
About the comment you pointed me out... I've never saw that in the code book. It could very well be because I am very new to the elctrical code, or that I am using a newer code book than yours?

However, the comment you have listed was:
An autotransformer or step-down transformer that is part of electrical equipment and that does not supply premises wiring is not the source of a separately derived system.

Is that actually in the code book? I could not find any such reference in the code. The article 100 doesn't say anything about a separately derived system. And no part of 250.20 mentions about a transformer that is a part of an electrical equipment.

Can you tell me where in the code 2004 or 2005 I can find that information?


#129472 - 03/31/05 10:15 PM Re: How to ground a 120V separately derived circuit.  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
That was from commentary in the 2002 NEC Handbookversion of the Code.

Every code cycle (3 years) the NFPA who publishes the NEC also publishes a Handbook version that contains the Code, plus commentary from various Code making panel members and other people and explain changes, mis-conseptions, intent of various codes. Athough the commentary is not CODE itself, it does represent a viable no nonsense explainations and intent from the publishers of it.

The one I have is on disk, and searchable. So in this instance, I seached "Seperately derived", and I get every hit with those words. All 108 of them! (in code and commentary) One of the first ones is the definition in Art 100:
Quote
Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.


Now, I will point out, that what I quoted was you may, or may not have to ground it. As there are exceptions. What needs to be done by you is find out where, and how the code will apply to your install. And, what exceptions there may be. There is no right or wrong without more info to go on.

You also mention that the motor is 208, is a neutral available on this branch panel, why not use it for this 120? And, that on average, most people will opt to use a lower control voltage that would fall under class 2, or 3 wiring methods. What is this 120 for?


Mark Heller
"Well - I oughta....." -Jackie Gleason

#129473 - 04/01/05 03:13 PM Re: How to ground a 120V separately derived circuit.  
unsaint32  Offline
Member
Joined: Aug 2004
Posts: 15
eden prairie, MN, USA
Well, I should've told you. I am working on a tech school project. I'll have to ask my teacher why we dont' just use one of the hots and a neutral bus (or equipment grounding bus) for 120V.

I will go home and read the handbook.

If a 120V control circuit is a class I circuit, then it solves all my questions (I can just jumper x1 or x2 to the control panel as allowed by exception of 250.30 (A)(2)). But I believe, although not sure, a class I circuit is limited to be 30V, as indicated by a website contect I found at http://www.electrician.com/eltrain/lpage103.html
"Class 1 circuits are divided into two types (1) power-limited, or (2) remote-control and signaling circuits. Power limited circuits are limited to 30 volts and 1000 volt-amperes. Class 1 remote-control and signaling circuits are limited to 600 volts, but there aren't any limitations on the power output of the source.
Class 2 and Class 3 circuits are defined by two tables, one for alternating current and one for direct current. In general, a class 2 circuit using 24 volts with a power supply durably marked 'Class 2" and not exceeding 100 volt-amperes is the one most commonly used."


#129474 - 04/01/05 09:28 PM Re: How to ground a 120V separately derived circuit.  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
"one of the hots and a neutral bus (or equipment grounding bus) for 120V"

Now: It's a 208 fed piece of equipment? A 208 voltage would suggest a Wye system, and the high likelihood of there being a neutral available. (Not always the case for the 'available' part.) So there would be no need for a transformer at all...

So now lets say for S&G there is no neutral available... your options for 120 are autotransformer, which would never be grounded!

Or, transformer which may or may not need to be grounded to an electrode, or feeder ground with some exceptions. The idea here is to make the installation simple, and avoid the hassle of adding an electrode, or other methods of bringing this ground to the main ground buss or earth close to it. If I where teaching this class, I would be trying to get you to salve issues by design, not time and material. So, if this control circuit does not leave the confines of the contol cabinet, by conduit cable or receptical, no need to ground it. If it does for any reason, I would seek to modify the design to accomadate less labor and material. Thats if you need a transformer at all. If you had said you were using 240 delta, I would have been worried that there was no neutral available.

And: "Power limited circuits are limited to 30 volts and 1000 volt-amperes"

Power limited circuits are practically a class themselves, as thier use and scope can be very, well, 'limited'.

Anyway, I wish you luck on this....


Mark Heller
"Well - I oughta....." -Jackie Gleason

#129475 - 04/04/05 12:53 PM Re: How to ground a 120V separately derived circuit.  
unsaint32  Offline
Member
Joined: Aug 2004
Posts: 15
eden prairie, MN, USA
I think I found out something that would solve my question (grouding a certain control circuits).

What I thought was, since my secondary side was not anyway electrically connected to the primary side, the secondary must be a separately derived system, so I thought I needed to system ground it and equipment ground it like a separate system.


But, after I carefully read the definition of a separately derived system, I realized that a separately derived system only refers to a premise wiring.

Any control circuit out of a transformer, whether isolation or auto, is NOT a separately derived system (because it is not a premise wiring).
If I have some equipments supplied only by generators, that circuit is not a separately derived system.


#129476 - 04/05/05 05:20 PM Re: How to ground a 120V separately derived circuit.  
unsaint32  Offline
Member
Joined: Aug 2004
Posts: 15
eden prairie, MN, USA
Disregard my last post.

According to the code definition, my 120V control circuit out of a isolation transformer is a premise wiring, therefore a separately derived system. Therefore, I need to ground it separately. I will run a wire from X2 to nearest suitable electrode.


#129477 - 04/12/05 10:45 AM Re: How to ground a 120V separately derived circuit.  
Creighton  Offline
Member
Joined: Jan 2002
Posts: 59
Hayward, CA
The separately derived system (control circuit) can be left ungrounded if ground detectors are installed. 250.21(3).
OR, if the control transformer is not over 1000 va, bond the grounded conductor to the transformer frame. 250.30(A)(3)Exc.No.3.
Creighton



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