Which Ground sizing chart for sub panel feeders

[sevlander]

I have had conflicting answers depending on whether it came from an AHJ or an electrical engineer. Table 250.66 is used for system grounding size and 250.122 is used for
equipment grounding. Which of these charts is used for grounding sub panels (or any panel downstream from the main not from a separately derived system)? Is a sub panel treated as a service or equipment. I believe it is equipment. Where is the wording that distiguishes this in the NEC? Thanks

 

[iwire]

250.66 is never used to size equipment grounding conductors.

250.66 is used to size grounding electrode conductors and bonding jumpers when there is no overcurrent protection ahead of it such as services and seperatly derived systems


250.122 is used to size equipment grounding conductors.

 

[infinity]

EGC's are as Bob stated sized according to 250.122(A) which references Table 250.122 or 250.122(B) when the conductors have been increased in size.

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case shall
they be required to be larger than the circuit conductors sup-
plying the equipment.

 

[charlie b]

Bob gave the correct information, but I am not certain he answered the question you were trying to ask.

Which of these charts is used for grounding sub panels (or any panel downstream from the main not from a separately derived system)? Is a sub panel treated as a service or equipment. I believe it is equipment.

My problem with your question is that I don't understand what you mean by grounding a panel (or sub-panel). The feeder supplying power to the sub-panel must carry with it a green insulated or bare copper wire that connects to the bground bar in the sub-panel and the ground bar in the upstream panel. That is the EGC that Bob mentioned, and it is sized per 250.122. If the sub-panel is within the same building as the upstream panel, that is the end of the story. But if the sub-panel is located in a detached building, let us say the garage, then you also need to "ground the panel" by running a wire from its ground bar to planet Earth (e.g., to a ground rod or two). That wire would be the GEC that Bob mentioned, and it is sized per 250.66.

 

[sevlander]

I agree, but I have an engineer saying the sub panel feeder ground must be sized the same as the system ground per 250.66. The AHJ says 250.122 and that a sub panel is "equipment". I believe as you do, but where do you determine a sub panel is equipment by the NEC?

 

[infinity]

When in doubt check Article 100:

Equipment. A general term, including fittings, devices, appli-
ances, luminaires, apparatus, machinery, and the like used as a
part of, or in connection with, an electrical installation.

What is a system ground? Trying to properly answer the question but as Charlie stated some of the terms are causing confusion. Did you mean the GEC or main bonding jumper?

 

[tom baker]

We get in troulbe when we don't use code terms. Ground means nothing - what color is the wire and what does it do.
Same with subpanel, you are referring to a feeder that goes to another panel.
If its in the same building, then the EGC is sized according to its OCP.
If the feeder goes to a seperate building, than its still sized according the OCP, but now the panel must be connected to a grounding electrode system

 

[LEO2854]

I agree, but I have an engineer saying the sub panel feeder ground must be sized the same as the system ground per 250.66. The AHJ says 250.122 and that a sub panel is "equipment". I believe as you do, but where do you determine a sub panel is equipment by the NEC?

Your sub panel is simply a load like a receptacle , so 250.122 applies.


You should have the engineer read both sections,But if he's paying the bill do it his way and take the profit :thumbsup:

 

[sevlander]

They only answer I don't seem to get is where in the code it says that a panel after the main breaker is considered equipment, Is there a specific code article that clears this up? Article 100 equipment definition does not mention panels. The PE says in his training panels are considered part of the AC system. You have a lot of good answers, but I see nothing definite in the NEC.

 

[charlie b]

I believe you will find what you need in two places.

First of all, the opening text of 250.66 speaks of the size of the Grounding Electrode Conductor (and here is the key part) ". . . at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system . . . ." Nothing in that paragraph speaks of panels. So you can conclude that 250.66 does not apply to the "ground wire" that comes with the feeder to the panel.

Secondly, 408.40 explicitly states, in no uncertain terms, that panelboards ". . . shall be connected to an equipment grounding conductor."

The basic concept that your engineer seems to be missing is that the GEC involves a connection to planet Earth, whereas the EGC exists to provide a low impedance path for current to flow back to the source, in the event of a ground fault, so as to cause the breaker to trip and thereby terminate the event. If the wire goes to the dirt, then use 250.66. If the wire goes back to the main panel, then use 250.122.

 

[sevlander]

I will read up more based on you reply. Thank you!

 

[infinity]

As stated earlier EGC's are found in T250.122 and when used in concert with 408.40, as Charlie mentioned, it's pretty clear that the engineer is incorrect.