250.30(a)(4)

[memyselfandI]

I posted this question in the NEC section of the forum and maybe I should have posted this here. I have 2 control transformers in a control cabinet. According the referenced article, am I required to pull a 3/0 ground to the grounding electrode. I think that this reference applies to building SDS and not internal transformer in a control cabinet. Desperate for some help on this

 

[SG-1]

I think you were answered in the other forum. To recap: so long as the energy from the control power transformers is utilized inside the control cabinet & does not exit the cabinet no additional grounding is required.

I would only worry if the cabinet does not have a proper EGC run to it.

Are the installation instructions calling for a GEC ?

Yes, the 3/0 cable is for much larger power transformers. The kind that would being supplying the feed to the control box.

Does this help ?

 

[SG-1]

Suppose your washing machine has a small internal transformer or two. Would the internal transformers require an additional 3/0 grounding electrode connection or does the appliance EGC take care of it ?

 

[memyselfandI]

I think you were answered in the other forum. To recap: so long as the energy from the control power transformers is utilized inside the control cabinet & does not exit the cabinet no additional grounding is required.

I would only worry if the cabinet does not have a proper EGC run to it.

Are the installation instructions calling for a GEC ?

Yes, the 3/0 cable is for much larger power transformers. The kind that would being supplying the feed to the control box.

Does this help ?

The installation is calling for a 3/0 CU GEC and refers to the article and Table 250.66 which deals with GEC for service entrance conductors, not this application. This is not a service, it is a control cabinet and therefore in my opinion this table does not apply. IAW the 250.122,. the EGC is sized according to this requirement. I have to install the 3/0 as per 110.3(B). to have a 3/0 CU GEC for a 1.5KVA and a 3kva transformer is ridiculous and the they are not in understanding what the 250.30(A)(4) was intended for.

 

[SG-1]

I am in total agreement with you. Is an engineer calling for the 3/0 ?

I can't imagine how you could connect it to the transformers. Exothermic welded to the case ??

 

[SG-1]

The installation is calling for a 3/0 CU GEC and refers to the article and Table 250.66 which deals with GEC for service entrance conductors, not this application. This is not a service, it is a control cabinet and therefore in my opinion this table does not apply. IAW the 250.122,. the EGC is sized according to this requirement. I have to install the 3/0 as per 110.3(B). to have a 3/0 CU GEC for a 1.5KVA and a 3kva transformer is ridiculous and the they are not in understanding what the 250.30(A)(4) was intended for.

I agree table 250.66 does not apply to a control cabinet. It does apply to SDS. It is used to size the system bonding jumper amoung other things.

 

[infinity]

The reason that they're asking for a #3/0 is that they want to create a common grounding electrode which is required to be #3/0 or larger. As you know that's in 250.30(4)(a). The tap conductors to each transformer would be sized according to 250.66. The question is do these transformers require a grounding electrode connection in the first place.

 

[memyselfandI]

The reason that they're asking for a #3/0 is that they want to create a common grounding electrode which is required to be #3/0 or larger. As you know that's in 250.30(4)(a). The tap conductors to each transformer would be sized according to 250.66. The question is do these transformers require a grounding electrode connection in the first place.

i understand what they are trying to do but is this necessary for a 1.5 and a 3kva xfmr? I mean come on, as one of the responders stated if I had a washing machine with 1 or more transformers in it would I need to run a 3/0 for that or would the EGC be sufficient?

 

[don_resqcapt19]

i understand what they are trying to do but is this necessary for a 1.5 and a 3kva xfmr? I mean come on, as one of the responders stated if I had a washing machine with 1 or more transformers in it would I need to run a 3/0 for that or would the EGC be sufficient?

I don't think a 3/0 is electrically necessary and that the primary EGC will do the job electrically, however there is a code issue that comes about when you apply the definitions of separately derived system and premises wiring. If the control conductors leave the control cabinet you have premises wiring and you trigger the SDS grounding rules.

In your example of the washing machine, that would not be a SDS as the control conductors don't leave the equipment.

Note I have never seen this rule applied to control transformers and maybe there should be an exception in the code for this type of application. There is an exception for control transformers of 1kva or less, but not for the size you have.

I'd run it by the AHJ, I expect that he will let you use the feeder EGC as the GEC for these transformers. That is common for these types of installations, but does not meet the letter of the code.

 

[cschmid]

does your installation meet these requirements?

250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.
(A) General. The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded:
(1) Electrical systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like
(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives
(3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only qualified persons service the installation.
c. Continuity of control power is required.
(4) Other systems that are not required to be grounded in accordance with the requirements of 250.20(B)
(B) Ground Detectors. Ungrounded alternating current systems as permitted in 250.21(A)(1) through (A)(4) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system.

If it does then maybe it does not need a GEC or is the question really the sizing issue?

 

[memyselfandI]

does your installation meet these requirements?

250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.
(A) General. The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded:
(1) Electrical systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like
(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives
(3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only qualified persons service the installation.
c. Continuity of control power is required.
(4) Other systems that are not required to be grounded in accordance with the requirements of 250.20(B)
(B) Ground Detectors. Ungrounded alternating current systems as permitted in 250.21(A)(1) through (A)(4) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system.

If it does then maybe it does not need a GEC or is the question really the sizing issue?

It does meet requirement 3abc, and yes it is a sizing issue. The transformer is feed with a #12 AWG and the secondary ids the same. However i had to install this as per 110.3(B). You should see this thing. This huge wire connected right next to a piece of #8AWG

 

[infinity]

Since you have two transformers can you just run two small GEC's based on the secondary conductors and forget the #3/0?

 

[memyselfandI]

Since you have two transformers can you just run two small GEC's based on the secondary conductors and forget the #3/0?

That's what I wanted to do, but they said I had to run a 3/0 CU otherwise it would void the warranty. 110.3(B)

 

[pfalcon]

That's what I wanted to do, but they said I had to run a 3/0 CU otherwise it would void the warranty. 110.3(B)

How did you come up with 3/0 CU as the appropriate size from Table 250.66? What size are your ungrounded conductors?

 

[augie47]

It comes from 250.30(A)(4) Exception 2, if applicable in this situation.
One possible "out" in this particular situation might be, from your earlier post one of the secondaries is 24v and therefore doesn't meet the 2540.21 grounding requirement so you can actually ground only one transformer taking you back to 250.66...

 

[pfalcon]

It comes from 250.30(A)(4) Exception 2, if applicable in this situation.
One possible "out" in this particular situation might be, from your earlier post one of the secondaries is 24v and therefore doesn't meet the 2540.21 grounding requirement so you can actually ground only one transformer taking you back to 250.66...

What does the control panel supply? Machine control wiring is not premises wiring no matter where on the machine it goes. Researching back into this thread I see (I think):

480VAC 3ph 50A service supply to a machine control panel disconnect. Presumably something like motors, pumps, and heaters that directly use the 3ph. A 120VAC 1.5kVA transformer for some controls, auxiliary lighting, and peripherals. A 24VDC 3kVA transformer for sensors and pushbuttons.

What am I missing? What did I describe wrong?

 

[augie47]

What does the control panel supply? Machine control wiring is not premises wiring no matter where on the machine it goes. Researching back into this thread I see (I think):

480VAC 3ph 50A service supply to a machine control panel disconnect. Presumably something like motors, pumps, and heaters that directly use the 3ph. A 120VAC 1.5kVA transformer for some controls, auxiliary lighting, and peripherals. A 24VDC 3kVA transformer for sensors and pushbuttons.

What am I missing? What did I describe wrong?

I don't know that you described anything wrong. I think there is legitimate question as to the need for a GEC which IMHO would depend on if the control wiring leaves the cabinet.
All I was stating was that IF one concedes a GEC is needed, then I see no need for a 3/0 as only the 1.5 kva transformer requires grounding.

 

[pfalcon]

I don't know that you described anything wrong. I think there is legitimate question as to the need for a GEC which IMHO would depend on if the control wiring leaves the cabinet.
All I was stating was that IF one concedes a GEC is needed, then I see no need for a 3/0 as only the 1.5 kva transformer requires grounding.

If everything is I described then there is no premises wiring in the system except for the original 480VAC power drop to the disconnect. Machine control wiring is never premises wiring. Check the "does not include" under Premises Wiring (System). Despite the apparent exposure of cables on a machine, they are considered internal to the equipment. We just don't wrap machine tools in fancy black, plastic boxes with TV remotes. Makes service tough. 250.30 itself does not normally apply to industrial equipment.

Yes, the machine needs to be bonded. But bonded using a conductor properly sized for the ungrounded conductors; not 3/0.

 

[augie47]

we don't happen to read it the same way.
I see that premises is not wiring that is "internal to motor control centers.....etc"
To me you leave that motor control center, say to a pushbutton on the othed side of the building, that is premises wiring (again, to me). In order to reduce a possibilty of potential from that wiring to building steel I would think we would want a electrode system

 

[pfalcon]

we don't happen to read it the same way.
I see that premises is not wiring that is "internal to motor control centers.....etc"
To me you leave that motor control center, say to a pushbutton on the othed side of the building, that is premises wiring (again, to me). In order to reduce a possibilty of potential from that wiring to building steel I would think we would want a electrode system

Well, first of all, if the pushbutton is on the other side of the building then it's not a part of the machine or it's control panel. It has to be local or it must be a part of a separate system that might be talking to the original machine. The local cabling from the control panel to the local machine are not premises wiring.

As to the requirement for bonding see:

NFPA79: 2007: Chapter 8 Grounding
NFPA79: 2007: Table 8.2.2.3 Minimum Size of Equipment Grounding Conductors and Bonding Jumpers

Typically at an industrial site the building steel is the ground mat. Properly done the steel is tested for continuity and ground. Three phase is brought from the facility and the protective ground is brought from the building steel. For an industrial machine the protective bonding is mandatory. Just not at the size of 250.30 . Rather it is a sensible sized connection based on the above referenced table.

Based on the "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of the Equipment (Not Exceeding Amperes)"; 50A requires a minimum 10AWG Copper Conductor Size. ...but shall not be required to be larger than the circuit conductors supplying the equipment.