Quantcast

Bonding screw main panel/ grounded neutral

Help Support House Repair Talk:

js5518

Member
Joined
Dec 4, 2018
Messages
21
Reaction score
1
Location
60431
I will also make those changes in the subpanel connected to the main. Mark neutral, use large lug. Anything else? When we are done with this I will throw something else at you. Ha ha
 

js5518

Member
Joined
Dec 4, 2018
Messages
21
Reaction score
1
Location
60431
Yes that is correct.

--
Tom Horne
Tom, just want to be clear here. There is a meter with 200 amp breaker and ground rod. Then there are 3 wires to house with square d 200 amp breaker with it's own ground rod connected to neutral bar. So, this inside 1st panel is apparently not bonded but should? If the bond screw on that panel is top right on neutral bar it isn't there. I tried a ground screw from smaller box and it appears to be short. It would go in the smaller box but don't think in the 200 amp box. It is, I believe S series Square D QO box? Thanks
 

hornetd

Member
Joined
Nov 17, 2005
Messages
302
Reaction score
162
Yes that first panel in the house needs to be bonded so that the enclosing cabinet and all of the metallic raceways, such as EMT tubing, connected to it will be connected to the Grounded Current Carrying Conductor (Neutral). Since there is no Equipment Grounding Conductor in the Feeder which supplies that panel from the combined meter and Service Equipment the Neutral has to do two jobs. It's first job is to carry the difference of the current that is flowing on the two energized conductors back to the source of that current at the transformer's secondary winding. It's second job is to carry any current which is flowing from a ground fault back to that secondary winding so that the fault will not raise the touch potential of the normally de-energized conductive portions of the electrical system to an unsafe level for human safety while also allowing the current to cause the circuit breaker to open and thus clear the fault. There is no other way for fault current to return to it's source in the transformer secondary than threw that screw.

Use a non conductive flashlight to look into each of the holes in the top horizontal portion of the Neutral Buss bar. The one which has a threaded hole in the back wall of the enclosing cabinet right behind the hole is were the green painted screw will be installed. If memory serves that hole is usually square in SquareD panel assemblies but keep in mind that I have been inactive for a long time.

That screw, which will serve as the Building Bonding Jumper, should be tightened until it's head is in hard contact with the Neutral buss bar. DO NOT ATTEMPT TO USE ANY OTHER SCREW! The bus bar is aluminum and would be constantly at war with any ferrous metal; which is any metal that is made from Iron Ore without the addition of significant amounts of other metals so as to change it's properties. Any steel screw which has not been rendered Stainless by the addition of other substances will corrode if placed in close contact with Aluminum. The resultant Aluminum Oxide is not a good conductor and will serve as a highly resistive point in the fault current pathway.

That is why the Bonding Screw is coated with a conductive green coating to prevent direct contact between the mild steel thread in the back wall of the enclosure cabinet and the threads of the screw until the connection is made up tight. That coating subsides away; is squeezed out by the pressure of the tightened connection; from some of the contact area enough to allow direct contact while at the same time making the contact area air tight. By excluding air containing oxygen from the contact area corrosion is prevented and the connection will remain fully conductive for the life or the installation.

You may need to go to an electrical supply house to obtain the properly sized bonding screw. Write down the entire model number of the panel and take a clear and readable picture of the label with a smart phone or a digital camera so that the supply house counter staff can look up the correct screw for that panel. Save yourself some time and call ahead. If they have any trouble identifying your panel send them the picture of the label.

--
Tom Horne
 

Latest posts

Top