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afscme · Dec 14, 2010, 07:06 AM

Need a recommendation, or maybe some clarification, for sizing a bonding wire.

Installing a small subpanel in an attached garage, and the feeders will be #6 copper. Per Article 250, it states bonding should be sized according to table 250.66. Based on the table, I would need #8 copper for the bonding which seems a bit large based on the #6 sized ungrounded feeder. But then again, that's why I'm asking. Any help appreciated. Thanks.

tkrussell · Dec 14, 2010, 07:29 AM

What are you bonding to what?

As Table 250.66 states, with the ungrounded conductor is #2 or smaller, the bonding conductor shall be #8 copper.

You shall not bond the neutral bar to equipment ground in a subpanel.

The neutral must be insulated from equipment grounding, so what are you bonding?

The panel's metal box will get bonded when you bolt an equipment ground bar to the box with machine thread screws and connect the feeder's equipment ground conducotr to the bar.

stanfortyman · Dec 14, 2010, 01:45 PM

250.66 is for grounding electrode conductors.
You want T250.122 for equipment grounding conductors.

A 30, 40, 50 or 60 amp feeder would require a #10cu EGC.

donf · Dec 14, 2010, 01:49 PM

Since you garage is attached to the home you are required to run a 4 wire feed from the main panel to the sub panel.

You don't necessarily need to run a separate bonding jumper since you are tied to the main panel's bonding jumper by the equipment grounding conductor.

If you were running a subpanel feed to a remote building, then you would need to set up a grounding and bonding system inside that building.

tkrussell · Dec 14, 2010, 01:59 PM

To clarify my answer to the question, I would like to bring to everyone's attention:

From the NEC 2008 Edition,

250.28 Main Bonding Jumper and System Bonding Jumper.

(1) General. Main bonding jumpers and system bonding jumpers shall not be smaller than the sizes shown in Table 250.66.

Stan, Table 250.66 is not just for sizing grounding electrode conductors.

Afscme asks about bonding in a subpanel, and is correct that Table 250.66 also is used for sizing for bonding jumpers.

I do believe he/she is confused about bonding at a subpanel, which I believe I helped eliminate the confusion.

afscme · Dec 14, 2010, 03:02 PM

Thanks for the help guys. After reading your replys it made me realize it was my error using the term bonding versus equipment ground. When we relocated the panelboard and installed a service disconnect in the original location ( to be within 5ft of the meter per local code) it was wired up as TK described. I went back and forth between the two tables, but using the wrong term led to the wrong table. Question answered, thanks again.

donf · Dec 14, 2010, 03:27 PM

TK,

I don't disagree with you in the slightest. My point was that since the garage is attached to the main structure it is already part of the bonding system.

stanfortyman · Dec 14, 2010, 06:38 PM

Originally Posted by tkrussell

From the NEC 2008 Edition,

250.28 Main Bonding Jumper and System Bonding Jumper.

(1) General. Main bonding jumpers and system bonding jumpers shall not be smaller than the sizes shown in Table 250.66.

Stan, Table 250.66 is not just for sizing grounding electrode conductors.

Afscme asks about bonding in a subpanel, and is correct that Table 250.66 also is used for sizing for bonding jumpers.

While I agree that T250.66 is not just for GEC's, the ground wire run with a feeder is not a "main bonding jumper or system bonding jumper". It is an EGC, "Equipment grounding conductor".

215.6 Feeder Equipment Grounding Conductor.
Where a feeder supplies branch circuits in which equipment grounding conductors are required, the feeder shall include or provide an equipment grounding conductor in accordance with the provisions of 250.134, to which the equipment grounding conductors of the branch circuits shall be connected. Where the feeder supplies a separate building or structure, the requirements of 250.32(B) shall apply.

In a roundabout way it does ultimately direct us to T250.122 for sizing.

tkrussell · Dec 15, 2010, 10:52 AM

Ah-hem

Originally Posted by afscme

Thanks for the help guys. After reading your replys it made me realize it was my error using the term bonding versus equipment ground. When we relocated the panelboard and installed a service disconnect in the original location ( to be withing 5ft of the meter per local code) it was wired up as TK described. I went back and forth between the two tables, but using the wrong term led to the wrong table. Question answered, thanks again.

Looks like I was right again, confusion regarding bonding and grounding.