even though several electricians have explained to me i can't seem to understand the purpose of the green bonding screw being used or not in a single phase 220 volt electrical panel box when followed by a sub panel. i know it's suppose to connect the neutral and equipment grd bars to the panel box there by creating equal potential of the metal box and the bars. what i can't grasp is the service neutral wire that comes from the transformer is bonded to the system neutral which is a grounding system that also has a ground rod at the pole, then that it is connected to the meter back thru it to the main panel neutral bar which has a driven ground rod connect to it, also the neutral bar is mounted directly to the box with screws as well as the equipment ground bar is screwed to the box if not mounted to the neutral bar itself . so what is the need of the green screw and the second extra coated ground wire between the boxes when they are already bonded as described?
- Thread starter gbs
- Start date
Janicholson
Well-known Member
In my opinion it is a redundancy that avoids the possibility of a ground failure. It also could be used for applications different than the current situation. Sorry, not much help. Jim
Out at the transformer the connection to ground is required to establish a center tap (neutral point) to establish a split phase from 240v to splitting it into separate and opposing 120v lines.
Any newer panel the neutral bus is isolated from ground/bond. You actually land your ground wire that goes to your ground rod/rods/plate right in a separate lug thats part of the neutral lug if its landed in a service rated panel. It is then the bonding screw or some cases strap, that bonds your panel frame to ground . Everything after that is normally called ground at your devices is technically bond. Not sure if this helps but there are way better guys in the know on here than me.
Any newer panel the neutral bus is isolated from ground/bond. You actually land your ground wire that goes to your ground rod/rods/plate right in a separate lug thats part of the neutral lug if its landed in a service rated panel. It is then the bonding screw or some cases strap, that bonds your panel frame to ground . Everything after that is normally called ground at your devices is technically bond. Not sure if this helps but there are way better guys in the know on here than me.
gbs, more good electrical questions, those and legal draw out the most opinions lay as well as professional, lots of good people here all willing to help yayyyyyyyy. Although Im longgggggggg retired as a power distribution design engineer I will try to explain how it was done when I last practiced pursuant to the then NEC guidelines.
1) Out on the pole the HV aerial Primary Neutral was bonded to driven ground rods at every 3rd (or 4th?) power pole, this was for lightning and surge suppression and to keep the grid at one common low voltage reference IE mother earth.
2) On the LV secondary out of the transformer the 240 volt winding was tapped in its center (which left two legs of 120 each with respect to the center Neutral 180 out of phase or 240 line to line) and that center tap LV Secondary Neutral was again bonded to a driven into earth ground rod.
3) Three wires L1 L2 and Neutral are brought to the homes panel.
4) At the Panel (or first disconnect) the Neutral IS ONCE MORE connected to all readily available GROUNDING ELECTRODES (such as driven rod or rods) PLUS the Neutral Buss is bonded to the Ground Buss. On some panels theres only one common NG Buss (if main panel) while others there are two Busses and a cross Tie Bar (or screw) is used to bond the two busses together (if main panel)........ Thats the Neutral Ground BOND..
5) The panels case frame also needs bonded to the Ground Buss which may be by a grounding screw that connects the frame to the buss.
BOTTOM LINES
Earth Grounding of BOTH HV Primary and LV Secondary NEUTRALS is for lightning and surge suppression and to keep the grid at one common low voltage reference. HV and LV Neutrals at power pole are earth grounded and again at the home the LV Secondary Neutral is earth grounded
At the main panel (or first disconnect) as above the Neutral is AGAIN earth grounded PLUS Neutral is Bonded to the Ground Buss (cross tie bar or screw or one buss for both N and G)
The panels case/frame ALSO needs bonded to the Ground Buss and the Neutral needs bonded to the Ground Buss and grounding screws can be used for that.
YES there is redundancy as the Neutral (HV and LV) js earth grounded at many locations,,,,,,,,, and the N needs bonded to Ground,,,,,,,,,and the case/frame needs bonded to the Ground Buss..
NOTE although Im rusty on the latest NEC and codes Im pretty sure this has NOT changed since I retired but no warranty. Where fire and life safety are concerned consider consulting current practicing professional electricians and engineers and the NEC in addition to whats offered here me certainly included
Best Wishes
John T Headed to Florida Flywheelers this morning yayyyyyyyy
1) Out on the pole the HV aerial Primary Neutral was bonded to driven ground rods at every 3rd (or 4th?) power pole, this was for lightning and surge suppression and to keep the grid at one common low voltage reference IE mother earth.
2) On the LV secondary out of the transformer the 240 volt winding was tapped in its center (which left two legs of 120 each with respect to the center Neutral 180 out of phase or 240 line to line) and that center tap LV Secondary Neutral was again bonded to a driven into earth ground rod.
3) Three wires L1 L2 and Neutral are brought to the homes panel.
4) At the Panel (or first disconnect) the Neutral IS ONCE MORE connected to all readily available GROUNDING ELECTRODES (such as driven rod or rods) PLUS the Neutral Buss is bonded to the Ground Buss. On some panels theres only one common NG Buss (if main panel) while others there are two Busses and a cross Tie Bar (or screw) is used to bond the two busses together (if main panel)........ Thats the Neutral Ground BOND..
5) The panels case frame also needs bonded to the Ground Buss which may be by a grounding screw that connects the frame to the buss.
BOTTOM LINES
Earth Grounding of BOTH HV Primary and LV Secondary NEUTRALS is for lightning and surge suppression and to keep the grid at one common low voltage reference. HV and LV Neutrals at power pole are earth grounded and again at the home the LV Secondary Neutral is earth grounded
At the main panel (or first disconnect) as above the Neutral is AGAIN earth grounded PLUS Neutral is Bonded to the Ground Buss (cross tie bar or screw or one buss for both N and G)
The panels case/frame ALSO needs bonded to the Ground Buss and the Neutral needs bonded to the Ground Buss and grounding screws can be used for that.
YES there is redundancy as the Neutral (HV and LV) js earth grounded at many locations,,,,,,,,, and the N needs bonded to Ground,,,,,,,,,and the case/frame needs bonded to the Ground Buss..
NOTE although Im rusty on the latest NEC and codes Im pretty sure this has NOT changed since I retired but no warranty. Where fire and life safety are concerned consider consulting current practicing professional electricians and engineers and the NEC in addition to whats offered here me certainly included
Best Wishes
John T Headed to Florida Flywheelers this morning yayyyyyyyy
MarkB_MI
Well-known Member
- Location
- Motown USA
You go over a bunch of stuff, then you ask: 'so what is the need of the green screw and the second extra coated ground wire between the boxes when they are already bonded as described?'
I think what you may be asking is 'why are two panels bonded together with a ground wire when they're already connected via conduit?' Although it's not mandatory to add a ground wire between two boxes that are connected with conduit, many electricians will do so to provide a redundant ground in case there's a poor conduit connection somewhere. That's true for any electrical box, not just circuit breaker panels. This wire is usually insulated rather than bare, so that it won't arc against a box or conduit if there is a bad conduit connection.
I think what you may be asking is 'why are two panels bonded together with a ground wire when they're already connected via conduit?' Although it's not mandatory to add a ground wire between two boxes that are connected with conduit, many electricians will do so to provide a redundant ground in case there's a poor conduit connection somewhere. That's true for any electrical box, not just circuit breaker panels. This wire is usually insulated rather than bare, so that it won't arc against a box or conduit if there is a bad conduit connection.
Good point JMOR, Yep his exact question was a bit hard to understand....
As you know but he or others may not, in a Sub Panel the Equipment Ground Buss and Neutral Buss are insulated and isolated from each other HOWEVER the panels case/frame still needs connected to the Ground Buss which a screw may be used to attach the two......
If he's concerned about grounding a sub panel ?? You carry an Equipment GroundING Conductor FROM the Main TO the Sub (NEVER NEVER NEVER used conduit for that, an actual conductor ran in same conduit as the Hots and Neutral) at the Sub the EGC connects to the Ground Buss PLUS the subs case/frame connects to the Ground Buss perhaps using a screw..
If he reads this and my detailed post below he should be able to figure it out yayyyyyyyy
gbs, got it ?? If not post back more questions
John T At Florida Flywheelers
As you know but he or others may not, in a Sub Panel the Equipment Ground Buss and Neutral Buss are insulated and isolated from each other HOWEVER the panels case/frame still needs connected to the Ground Buss which a screw may be used to attach the two......
If he's concerned about grounding a sub panel ?? You carry an Equipment GroundING Conductor FROM the Main TO the Sub (NEVER NEVER NEVER used conduit for that, an actual conductor ran in same conduit as the Hots and Neutral) at the Sub the EGC connects to the Ground Buss PLUS the subs case/frame connects to the Ground Buss perhaps using a screw..
If he reads this and my detailed post below he should be able to figure it out yayyyyyyyy
gbs, got it ?? If not post back more questions
John T At Florida Flywheelers
buickanddeere
Well-known Member
The plan is to eliminate neutral current from the ground system . And to hold the neutral at earth potential .
Stephen Newell
Well-known Member
Your neutral is grounded to the utility at the pole but you should also have a separate ground rod, sometimes 2 rods which is driven into the ground next to your house. Without seeing the box I would guess the green screw is for that separate ground. The ground rod is a copper plated steel rod about 8' long which has a 6 gauge bare copper wire bonded to it.
The sub-panel should have it's own earth ground and you keep the ground wiring separate than the neutral.
If you live in a place that is particularly dry the ground rod may fail to work. It needs to be into damp soil to be effective. Sometimes it takes a second rod in a different location to have a good ground. You would need a ground tester to insure the rod is making a good earth ground.
The sub-panel should have it's own earth ground and you keep the ground wiring separate than the neutral.
If you live in a place that is particularly dry the ground rod may fail to work. It needs to be into damp soil to be effective. Sometimes it takes a second rod in a different location to have a good ground. You would need a ground tester to insure the rod is making a good earth ground.
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