Bonding screw in the distribution (sub) Panel

[HI9869]

I am a home inspector trying to understand what is correct and why in the distribution (sub) panel. My understanding is that beyond the service disconnect point (main panel) (in what I call the sub panel, I believe is called a distribution panel by NEC) ground (bare copper wire) and neutral (white wire) is separated and not bonded. My question is then is the ground bus bar bonded to the panel it self with the green screw? I have asked building inspector locally and electricians locally and I can not seem to get a clear understanding. So, hopefully someone can lay this out in laymen terms so I can be a better inspector. Thank you in advance.

 

[Dennis Alwon]

First off, the main distribution panel is the panel with the main disconnect. This is the only place that the neutrals and grounds are connected together. The sub panel as you stated is the correct name for the secondary panel although the nec mentions sub panel only once.

Once you leave the main panel the ground (equipment grounding conductor) is separated and only carries current when there is a fault. The neutral carries current but not the equipment grounding conductor.

 

[infinity]

If the sub-panel has a separate ground bar then that is bonded directly to the enclosure. The neutral bar is not bonded past the service disconnect because doing so would cause neutral current to flow on the EGC and and possibly the metallic parts of the system.

 

[HI9869]

So, should the ground bus bar in the sub panel be bonded to the metal panel itself with the green screw in case the panel becomes energized?

 

[HI9869]

"If the sub-panel has a separate ground bar then that is bonded directly to the enclosure. The neutral bar is not bonded past the service disconnect because doing so would cause neutral current to flow on the EGC and and possibly the metallic parts of the system."


Thank you. That make sense.

 

[infinity]

So, should the ground bus bar in the sub panel be bonded to the metal panel itself with the green screw in case the panel becomes energized?

Sort of yes because the EGC run with the feeder to the sub-panel would be connected to the EGC bar which is connected to the enclosure.

 

[HI9869]

Sort of yes because the EGC run with the feeder to the sub-panel would be connected to the EGC bar which is connected to the enclosure.

Thank you!

 

[al hildenbrand]

My question is then is the ground bus bar bonded to the panel it self with the green screw?

The "subpanel" that has a metal enclosure, that is, the panel box is made of metal, has to be bonded to the Equipment Grounding Conductor (EGC), of which there are many types.

Very possibly, the more, if not most, confusing type of residential dwelling EGC will be a metal conduit feeder, containing only two hots and a neutral, that brings power from the main panel to the subpanel. The metal conduit is the equivalent of a bare copper wire EGC. The connection of the metal conduit to the subpanel metal enclosure establishes the EGC.

Mechanically securing a terminal bar (as you say, the ground bus bar) to the subpanel metal enclosure provides terminals to "land" the branch circuit ground wires, whether green, bare or other color (except white or gray). The ground terminal bar is usually available in a bag with mounting screws and labels. The mounting screws do not have to be any particular color, they may be green, but that is actually unusual.

Now, a different path for the EGC happens when the subpanel gets its power from the main panel through a four-wire feeder Cable, two hots, a neutral and a "ground" wire. The ground wire has to land in a terminal made for wire-type grounds, so, the feeder cable ground "lands" under one of the terminal screws of the "ground bus bar". Mounting the "ground bus bar" to the subpanel box with its mounting screws (of any color) then, in fact, "bonds" the feeder cable EGC to the subpanel metal box.

 

[HI9869]

The "subpanel" that has a metal enclosure, that is, the panel box is made of metal, has to be bonded to the Equipment Grounding Conductor (EGC), of which there are many types.

Very possibly, the more, if not most, confusing type of residential dwelling EGC will be a metal conduit feeder, containing only two hots and a neutral, that brings power from the main panel to the subpanel. The metal conduit is the equivalent of a bare copper wire EGC. The connection of the metal conduit to the subpanel metal enclosure establishes the EGC.

Mechanically securing a terminal bar (as you say, the ground bus bar) to the subpanel metal enclosure provides terminals to "land" the branch circuit ground wires, whether green, bare or other color (except white or gray). The ground terminal bar is usually available in a bag with mounting screws and labels. The mounting screws do not have to be any particular color, they may be green, but that is actually unusual.

Now, a different path for the EGC happens when the subpanel gets its power from the main panel through a four-wire feeder Cable, two hots, a neutral and a "ground" wire. The ground wire has to land in a terminal made for wire-type grounds, so, the feeder cable ground "lands" under one of the terminal screws of the "ground bus bar". Mounting the "ground bus bar" to the subpanel box with its mounting screws (of any color) then, in fact, "bonds" the feeder cable EGC to the subpanel metal box.

Is one feed more dangerous than the other? My question comes from I have been told that in a three wire feeder system it really does not matter if the neutral and ground are separate but that it does matter in a four wire feeder system. I see over and over that neutral and ground are to be separated in the sub panel and I understand that. It threw me when the person I spoke with said it only mattered in a four wire system.

 

[infinity]

Is one feed more dangerous than the other? My question comes from I have been told that in a three wire feeder system it really does not matter if the neutral and ground are separate but that it does matter in a four wire feeder system. I see over and over that neutral and ground are to be separated in the sub panel and I understand that. It threw me when the person I spoke with said it only mattered in a four wire system.

Unless you're dealing with a separate structure and prior to the 2008 NEC you will almost always need a "4 wire" feeder. IMO there wouldn't have been a code change in 2008 if the CMP felt that a 3-wire feeder were just as safe as a 4-wire feeder.

 

[HI9869]

Unless you're dealing with a separate structure and prior to the 2008 NEC you will almost always need a "4 wire" feeder. IMO there wouldn't have been a code change in 2008 if the CMP felt that a 3-wire feeder were just as safe as a 4-wire feeder.

So from my prospective as a home inspector I am usually inspecting homes that are 10-20 years old and I see and point out that grounds and neutrals should be separated in the sub panel. How important (as a safety issue) should this be corrected. Had a building inspector tell me it is not required to correct unless they are adding/upgrading the sub panel.

 

[al hildenbrand]

Is one feed more dangerous than the other? My question comes from I have been told that in a three wire feeder system it really does not matter if the neutral and ground are separate but that it does matter in a four wire feeder system. I see over and over that neutral and ground are to be separated in the sub panel and I understand that. It threw me when the person I spoke with said it only mattered in a four wire system.

Careful, here. I think you may be mixing two very different ideas.

BOTH of my post's feeders are FOUR conductor. In the first, the metal conduit with three conductors, the metal conduit itself is the fourth conductor.

Your original post question is about "bonding" the grounded terminal bar (EGC) to the metal subpanel enclosure. Your question defines a four conductor feeder situation.

Actual three conductor feeders exist, but in special situations.

 

[infinity]

So from my prospective as a home inspector I am usually inspecting homes that are 10-20 years old and I see and point out that grounds and neutrals should be separated in the sub panel. How important (as a safety issue) should this be corrected. Had a building inspector tell me it is not required to correct unless they are adding/upgrading the sub panel.

IMO if the sub-panel is within the same structure it needs to be corrected because it has never been allowed.

 

[Little Bill]

Just to clear up what might be confusing to the OP. The "green" screw is usually found in the neutral bar. It is only to be used at the main panel. Since the neutral bar is mounted on a non-conductive mount, the screw is tightened in so that it contacts the metal enclosure. That bonds the neutral to the metal enclosure and to the ground (EGC) bar.

If the panel has a "green" screw and it's not the main panel, then the screw should NOT be tightened in. Now sometimes the neutral bar and ground bar are both mounted on a non-conductive mount, neither of which "bond" or contact the metal enclosure. These also have been isolated from each. Technically these are both neutral bars but one is being used as a ground bar. If it is to be used as the ground bar then it has to be bonded to the enclosure. If the "green" screw happens to be in the bar to be used as the ground and there is no connection to the neutral conductor, then the screw can be tightened in. This is unusual but does happen.

To sum up, if the "green" screw is tightened in then it either should be the main panel or, if in a subpanel, it should not have any connection to the neutral. The "green" screw is only to bond the bar to the enclosure.

 

[HI9869]

Careful, here. I think you may be mixing two very different ideas.

BOTH of my post's feeders are FOUR conductor. In the first, the metal conduit with three conductors, the metal conduit itself is the fourth conductor.

Your original post question is about "bonding" the grounded terminal bar (EGC) to the metal subpanel enclosure. Your question defines a four conductor feeder situation.

Actual three conductor feeders exist, but in special situations.

I understand..... a 3 phase system with 4 wires. Ok, I'm with you.

 

[HI9869]

Just to clear up what might be confusing to the OP. The "green" screw is usually found in the neutral bar. It is only to be used at the main panel. Since the neutral bar is mounted on a non-conductive mount, the screw is tightened in so that it contacts the metal enclosure. That bonds the neutral to the metal enclosure and to the ground (EGC) bar.

If the panel has a "green" screw and it's not the main panel, then the screw should NOT be tightened in. Now sometimes the neutral bar and ground bar are both mounted on a non-conductive mount, neither of which "bond" or contact the metal enclosure. These also have been isolated from each. Technically these are both neutral bars but one is being used as a ground bar. If it is to be used as the ground bar then it has to be bonded to the enclosure. If the "green" screw happens to be in the bar to be used as the ground and there is no connection to the neutral conductor, then the screw can be tightened in. This is unusual but does happen.

To sum up, if the "green" screw is tightened in then it either should be the main panel or, if in a subpanel, it should not have any connection to the neutral. The "green" screw is only to bond the bar to the enclosure.

Awesome! This really clears it up for me along with the other responses. Love the Zig Ziglar quote! It is so true........

 

[HI9869]

Thank you all that have responded. You have cleared much up for me. Enjoy your weekend!

 

[al hildenbrand]

I understand..... a 3 phase system with 4 wires. Ok, I'm with you.

Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.

 

[HI9869]

Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.

So what does a 3 phase have? 3 hots, neutral, and ground? Thank you for clarifying.

 

[infinity]

Sorry, no. Single phase 240 / 120 Volt. The feeder from the main electrical panel to the subpanel you are asking about is four wires in a single phase 240 / 120 Volt system. There are two hots, one neutral and one "ground" -- four wires.

I agree with you and we all seem to call this a 4-wire feeder but we should really call it a 4-conductor feeder because the EGConductor (4th "wire") can be a suitable cable armor or a metal raceway, etc listed in 250.118.