How to ground a 120V separately derived circuit.

[unsaint34]

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?

 

[iwire]

Re: How to ground a 120V separately derived circuit.

Originally posted by unsaint34:
[QB] 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.[QB]

Read the expetion under 250.30(A)(2)

 

[unsaint34]

Re: How to ground a 120V separately derived circuit.

thanks for the reply. I read the part and the following is why I still think that exception does not exempt a grounding requirement for a 120V control circuit.

250.30 A (2) exception applies to class 1,2 or 3 circuit. I thought those classes were power-limited circuits, and do not include a 120V control circuit. (I believe the rating of the transformer that I am using is below 1000VA, but that doesn't matter because the same exception is for the circuits of 1,2,3 classes, "AND" of a transformer not over 1000VA.)

 

[shortcircuit2]

Re: How to ground a 120V separately derived circuit.

unsaint34... Iwire pointed you in the right direction. 250.30(A)(2)(a)exception states that you are allowed to bond that control transformer to the frame of the enclosure or transformer frame and either must also be grounded according to 250.134 (According to 2002NEC)

The 2005 NEC has reorganized section 250.30(A) and describes this bonding procedure in 250.30(A)(3)exception #3

Also read article 725.21 for classification of class 1, 2, and 3 control circuits...

shortcircuit2

 

[unsaint34]

Re: How to ground a 120V separately derived circuit.

I still have to figure out whether a 120V control circuit could be a Class I circuit. But.. I found out something else that would solve all my questions.

After I carefully read the definition of a separately derived system, I realised 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.

 

[shortcircuit2]

Re: How to ground a 120V separately derived circuit.

OK...then read 250.20

and read 250.20(B)(1)

shortcircuit2

 

[don_resqcapt19]

Re: How to ground a 120V separately derived circuit.

unsaint34,

Any control circuit out of a transformer, whether isolation or auto, is NOT a separately derived system (because it is not a premise wiring).

How did you come to that conclusion?

Premises Wiring (System). That interior and exterior wiring, including power, lighting, control, and signal circuit wiring together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed, that extends from the service point or source of power, such as a battery, a solar photovoltaic system, or a generator, transformer, or converter windings, to the outlet(s). Such wiring does not include wiring internal to appliances, luminaires (fixtures), motors, controllers, motor control centers, and similar equipment.

Don

 

[dereckbc]

Re: How to ground a 120V separately derived circuit.

Is this transformer a two wire in (208) and two wire out (120)?

 

[unsaint34]

Re: How to ground a 120V separately derived circuit.

Don. I guess the definition of premise wiring is different than I thought it was. I thought premise wiring is for premises, not for control circuits within the premises. So, acording to that code definition, the 120V control circuit out of a isolated 208/120V transformer [two wire in and two wire out by the way] is a premise wiring since it extends from a power source (transformer) to outlets (the starters and pilot lights and relays). Then, back to my original question.

Since the control circuit is a separately derived system, it must be system-grounded (And I cannot apply the exception for class I, II, and III control circuits because my control circuit is 120V)

Do I need to run an extension from the X1 or X2 and to a nearest electrode?

 

[don_resqcapt19]

Re: How to ground a 120V separately derived circuit.

Why is it not a Class 1 circuit?
Don

 

[kiloamp7]

Re: How to ground a 120V separately derived circuit.

A 120V. motor control ckt. is covered by Art.430.
A 120v. control ckt. for other than motor control is covered by Art.725 Class 1 rules.

Is that essentially a true statement, don_r ?
I dont necessarily know, just asking.

 

[don_resqcapt19]

Re: How to ground a 120V separately derived circuit.

kiloamp7,
It is my opinion that Article 725 applies to motor control circuits.
Don

 

[coulter]

Re: How to ground a 120V separately derived circuit.

Originally posted by kiloamp7:
A 120V. motor control ckt. is covered by Art.430.
A 120v. control ckt. for other than motor control is covered by Art.725 Class 1 rules.

Originally posted by Don:
It is my opinion that Article 725 applies to motor control circuits.
Don

Per Art 725.3, if the control ckt is tapped off of the load side of the OCP, Art. 430 applies. Probably 99%+ of the MCC I work with are this type. Separate control power is pretty rare.

So, yes, Art 725 applies to motor control ckts, but only long enough to be immediately turned over to Art 430.

carl

 

[don_resqcapt19]

Re: How to ground a 120V separately derived circuit.

Per Art 725.3, if the control ckt is tapped off of the load side of the OCP, Art. 430 applies. Probably 99%+ of the MCC I work with are this type. Separate control power is pretty rare.

I almost never see the control circuit tapped directly off of the motor branch circuit OCPD. It is fed from a transformer where the primary is tapped off of the branch circuit OCPD. It also appears that the Article 430 section referenced in 725 only applies to the overcurrent protection of the control cirucit.
Don

 

[kiloamp7]

Re: How to ground a 120V separately derived circuit.

99% of the motor control ckts. I deal with are tapped (or more commonly, their control xfmrs are tapped), from the load side of the motor branch ckt. OCP.

I have always used Art 430 Part VI as the main reference & I believe others have as well.

For separate control of a motor control ckt. (which is not generally preferred & is not very common), Art. 725 comes in to play.

Just the fact that 725 is in Chapter 7 means something.

 

[shortcircuit2]

Re: How to ground a 120V separately derived circuit.

430 or 725...the 120 volt class 1 control circuit that unsaint34 is dealing with still must have 1 leg of the secondary grounded if the condition in 250.20(B)(1) exsists...

Motor control panels supplied from the factory with control-circuit transformers are often grounded on the secondary by the manufacturer.

If separately derived motor control circuits are field installed and wired, then grounding is done by the installer.

250.30(A)(2)(a) exception allows grounding of one of the transformers secondary separately derived conductors, 1000va or less, to the frame or enclosure with a jumper sized according to 250.30(A)(1) exception #2 and the frame or enclosure is grounded according to 250.134. Bonding connection to the grounding electrode system of one of the derived secondary conductors is not required under this exception.

shortcircuit2

 

[coulter]

Re: How to ground a 120V separately derived circuit.

Originally posted by don_resqcapt19:
I almost never see the control circuit tapped directly off of the motor branch circuit OCPD. It is fed from a transformer where the primary is tapped off of the branch circuit OCPD.

Absolutely. Good thing you pointed that out. My feeble reasoning likely considered the control xfmr (also the control xfmr primary fusing, secondary fusing, and interconnecting wiring) part of the control ckt.

carl

 

[don_resqcapt19]

Re: How to ground a 120V separately derived circuit.

Carl,

My feeble reasoning likely considered the control xfmr (also the control xfmr primary fusing, secondary fusing, and interconnecting wiring) part of the control ckt.

I didn't say that those things are not part of the control circuit. I just don't think that the conductors on the secondary of the control transformer are covered by the rule in 430.72(A), and that is the only part of 430 that is cited by 725. I think that 725 applies to the control cirucit except for the rule in 430.72.
Don