Art 250.118 Types of Equipment Grounding Conductors

[Grendal]

Has anyone had problems using rigid metal conduit or intermediate metal conduit as a equipment grounding conductor? I have a project that did not have the equipment grounding conductor installed between the main switchboard on the first floor and the step down transformers on the upper floors. The contractor already pulled the feeder cables between the main switchboard and the transformers.

The simplest fix would be to use the rigid metal conduit as a equipment grounding conductor however the people in charge of the construction supervision are saying that we should install a copper ground conductor. This is a more costly solution because it would require pulling out all the feeder cables between the main switchboard and transformers and then re-installing the feeder cables along with the copper ground conductor. They are saying that the copper ground conductor is more reliable. I'm saying that if it's in the NEC it's allowable, no questions. Opinions/Comments?

 

[infinity]

It's not required if the conduit qualifies as the EGC. However if a flexible connection to the transformer was made with FMC then a conductor type EGC is required.

 

[iwire]

However if a flexible connection to the transformer was made with FMC then a conductor type EGC is required.

What am I missing?:-?

250.118 Types of Equipment Grounding Conductors.
The equipment grounding conductor run with or enclosing the circuit conductors shall be one or more or a combination of the following:

(5)Flexible metal conduit where both the conduit and fittings are listed for grounding.

 

[tryinghard]

If the equipment grounding conductor is a wire it insures an excellent (low impedance) effective ground fault path to open the source in the event of a ground fault, this method is preferred for this reason, and it protects the shorting in equipment excellently. The conduit can certainly be this equipment grounding conductor, this is often referred to a mechanically grounded, but it must be installed correctly!

The use of ground bushings may be required, tight lock nut connections, tight coupling and connector connections, correct flex and flex length at minimum these are factored to create an effective ground fault path to source (low impedance). This conduit system still must be installed correctly even with an EGC because a ground fault can likely take place anywhere in the conduit system and the goal is to have a low impedance path to source to quickly open the source, if it is poor (high impedance) it may not open the source. Some electricians, with a poor knowledge of EGS and their purpose, will install the EGC wire and not give correct attention to the conduit system causing a great hazard. Check out 250-4(A)(3) through (5), these explain the purpose of the EGC.

If this project is engineered using mechanical EGC?s then the contractor is correct and must only meet the criteria of 250 part VI. In my humble opinion if this were great concern now would be a good time to present a change to their contract and offer both types, this is safest.

 

[infinity]

What am I missing?:-?

Take a look at 250.118(5). FMC needs to meet all 4 of those requirements. Most transformers that we install have circuits rated over 20 amps and require flexibility. Therefore they wouldn't satisfy either b. or d.

 

[iwire]

Take a look at 250.118(5). FMC needs to meet all 4 of those requirements.

No that is not exactly the case if it is listed for grounding it does not have to meet any of those requirments.

 

[infinity]

No that is not exactly the case if it is listed for grounding it does not have to meet any of those requirments.

I don't see how. The words say "Listed flexible metal conduit meeting all of the following conditions:"

 

[stickboy1375]

I don't see how. The words say "Listed flexible metal conduit meeting all of the following conditions:"

You need to reread (5) & (6) again... # 5 is for listed grounding, and # 6 is for NON- listed grounding...

 

[iwire]

I don't see how. The words say "Listed flexible metal conduit meeting all of the following conditions:"

Your right and I apologize, I had looked it up on my 2002 and goofed.

Stick, 250.118(6) is for liquidtight, but thanks for trying to help me out.

 

[stickboy1375]

Your right and I apologize, I had looked it up on my 2002 and goofed.

Stick, 250.118(6) is for liquidtight, but thanks for trying to help me out.

I was also using my 2002 handbook... 5 & 6 are both FMC, I guess that changed in 2005?

 

[Grendal]

Thank you all for your words of wisdom.

The rigid conduit would work as an equipment grounding conductor but it has to be installed properly. However when we transition from the rigid metal conduit to the flexible metal conduit then i run into trouble. It seems that I cannot meet 250.118(5)b.

My next question is:

If you look at the NEC 2005 Handbook, Exhibit 250.13 and 250.14 both exhibits are treated as a separately derived system and are connected to the nearest grounding electrode. There is an equipment bonding jumper on the transformer enclosure that is grounded.

Why does the primary side of the transformer show an equipment grounding conductor(next to 480V delta connection)?

 

[Grendal]

Thanks for the quick response. Good feedback.

 

[don_resqcapt19]

Why does the primary side of the transformer show an equipment grounding conductor

That is the fault clearing path for any fault on the primary side of the transformer and is required by the code.
Don