Question about the green bonding screw sub panel

[600SL]

When I sold my house in CT. The home inspector made me put a green bonding screw in the sub panel in my garage. Which I thought was wrong.

When I came here to NC an electrician told me the bonding screw was being debated at that time ~ 3 years ago for NC. Weather you put it in or leave it out on a sub panel was at that time under reconsideration for North Carolina.

So OK I now have a steel building this its own meter to a 200 amp main CB panel. The green screw is in place in the main panel. I am now putting in a 100 amp sub panel. Both the sub panel and main CB box are attached to the steel building by being physically mounted to the steel building and connected to each other by both EMT with compression fittings and a #6 ground wire. What difference can this green bonding screw possibly make on this setup.

Can anyone give me a scenario of a possible fault that could occur if I were to leave the green screw in the sub panel.

Can anyone give me a scenario of a possible fault that could occur if I were to not put the green screw in the sub panel.

Would there be any difference if this was a wood frame building.

Would a voltage potential be expected between neutral and ground at the sub-panel if the green screw is out.

Does anyone know where NC currently stands on the green screw.

 

[wyliesdiesels]

The one thing missing here is which bar youre putting the bonding screw on. Are we talking about a neutral bar in a subpanel or a grounding bar in a subpanel?

If the neutral bar, then it should be isolated which means NO bonding screw IF its fed by a 4-wire feeder. If its fed by a 3-wire feeder, then the neutral bar SHOULD have a bonding screw.

With that being said, subpanels in the same structure as the main were ALWAYS suppose to be 4-wire fed with an isolated neutral bar(no bonding screw). This is your situation so u should leave the bonding screw out.

I cant imagine NC allowing 3-wire subpanels in the same structure as the main up til now. Perhaps youre talking about subpanels in DETACHED structures.

If thats the case, bonded neutral bars and 3-wire fed subs in detached structures were allowed up until the 2008 NEC code cycle. Perhaps this is what youre thinking of...

Now to your question of what could happen.

If u leave the screw in, u setup the potential for neutral return current to flow on grounded metal pathways. If the neutral connection between the main and sub is lost or becomes high resistant, then neutral return current WILL flow on grounded metal on its way back to the main service neutral bar. U dont want that!!

 

[Mustang51js]

To make it easy on yourself,3 wire use screw,4 wire dont.

 

[600SL]

The one thing missing here is which bar youre putting the bonding screw on. Are we talking about a neutral bar in a subpanel or a grounding bar in a subpanel?

If the neutral bar, then it should be isolated which means NO bonding screw IF its fed by a 4-wire feeder. If its fed by a 3-wire feeder, then the neutral bar SHOULD have a bonding screw.

The bonding screw goes from the neutral to the case. Not sure what is meant by 3 wire or 4 wire feeder. My plan is to run 2 #3 hots 1 #6 neutral and 1 #6 ground through EMT. Does that count as 3 wires or 4 wires as I understand the ground wire is optional in this case but I plan to use a ground wire.

With that being said, subpanels in the same structure as the main were ALWAYS suppose to be 4-wire fed with an isolated neutral bar(no bonding screw). This is your situation so u should leave the bonding screw out.

This is definitely a sub panel in the same structure.

I cant imagine NC allowing 3-wire subpanels in the same structure as the main up til now. Perhaps youre talking about subpanels in DETACHED structures.

If thats the case, bonded neutral bars and 3-wire fed subs in detached structures were allowed up until the 2008 NEC code cycle. Perhaps this is what youre thinking of...

That may explain why the guy in CT wanted a bonding screw and what the electrician in NC was referring to. In CT the sub panel was in a detached garage off the main building. Here in NC we were discussing the possibility of a sub panel in the garage from the house.

Now to your question of what could happen.

If u leave the screw in, u setup the potential for neutral return current to flow on grounded metal pathways. If the neutral connection between the main and sub is lost or becomes high resistant, then neutral return current WILL flow on grounded metal on its way back to the main service neutral bar. U dont want that!!

Sounds like a positive way of flagging a bad neutral. In that case all 120V circuits would be degraded. But why would that not be an issue in a detached building?

Many Thanks for the reply. Very informative.

 

[wyliesdiesels]

3-wire is hot, hot, neutral.

4-wire is hot, hot, neutral, EGC(ground)

Detached buildings NOW require 4-wire as well.

U should remove the bonding screw in that panel.

If u used the EMT as the EGC and only had hot, hot, and neutral, it would still be 4-wire.

Heres a pic for u:

 

[600SL]

3-wire is hot, hot, neutral.

4-wire is hot, hot, neutral, EGC(ground)

Detached buildings NOW require 4-wire as well.

U should remove the bonding screw in that panel.

If u used the EMT as the EGC and only had hot, hot, and neutral, it would still be 4-wire.

Heres a pic for u:

Thanks for the post.

I do have one more question since this will only be a 240V sub panel is a neutral required. That would essentially make it 2 hots and a ground.

 

[laser3kw]

With that being said, subpanels in the same structure as the main were ALWAYS suppose to be 4-wire fed with an isolated neutral bar(no bonding screw). This is your situation so u should leave the bonding screw out.

AH-HA! thanks, that just cleared up that up. I was wondering the whole 3 wire, 4 wire thing.

 

[wyliesdiesels]

Thanks for the post.

I do have one more question since this will only be a 240V sub panel is a neutral required. That would essentially make it 2 hots and a ground.

Why are u doing a 240v only subpanel?

But obviously thats correct, a neutral is NOT required.

 

[600SL]

Why are u doing a 240v only subpanel?

But obviously thats correct, a neutral is NOT required.

I currently have a TIG welder and Just bought a Plasma cutter and about to get a MIG Welder and a 5 ton AC/Heater. Of course each requires a different size circuit and plug. TIG 100 Amp, Plasma 80 Amp, AC 50 Amp, MIG 30 Amp. The AC is the only thing that could possibly use 120 volts for the blower but that blower could be run from the main panel. I am converting my dedicated TIG conduit to a sub panel. Of course all this **** wont be run at once.

Since I only have 1 " conduit I am limited to 2 #2 and 2 #6. That puts it at about 96% of the maximum allowable fill with very stiff wire. It would be handy to have the 120 capability since I will have extra spaces but if I am unsuccessful at pulling all the wires removing the neutral would be a way out. I just recently up'd my hot wires to #2 from #3 because I found I can get a 110 AMP breaker.

 

[klassenl]

Pull the neutral even if you have no plans to use it right now. You may want it in the future.

 

[600SL]

Pull the neutral even if you have no plans to use it right now. You may want it in the future.

Thats hopefully the plan.

 

[larry4406]

wyliesdiesels - on the diagram you provided, it is labeled "sub-panel in same dwelling as service equipment" at the top. Below the sub panel there is a note "no ground required at sub-panel". I have this exact setup in sub-panels in my house.

I will be building a detached shed this summer and will run a 4-wire feeder to it but was planning to put a UFER ground in the footing and connect it to the ground (keeping neutral and ground separate). I assume this is proper?

 

[wyliesdiesels]

wyliesdiesels - on the diagram you provided, it is labeled "sub-panel in same dwelling as service equipment" at the top. Below the sub panel there is a note "no ground required at sub-panel". I have this exact setup in sub-panels in my house.

I will be building a detached shed this summer and will run a 4-wire feeder to it but was planning to put a UFER ground in the footing and connect it to the ground (keeping neutral and ground separate). I assume this is proper?

Yes if u have footings u should do a UFER grounding electrode.

Here is the diagram for a subpanel in a detached structure:

 

[600SL]

Pull the neutral even if you have no plans to use it right now. You may want it in the future.

Got them pulled today including the neutral.

 

[matt_i]

I was going to recommend going larger on the conduit. Save yourself stuffing to the max, which isn't bad if its all bent 90s or sweeps, but you try to jam #2 into around the "corner" of a 1" LB....not fun.

 

[Daveco]

A lot of good info in this thread

 

[600SL]

Got it in. Not to bad 2 #2 and 2 #6 in 1" EMT with 4 1/2 bends and about 50 ft. Kind of a maximum pull scenario. Makes me feel the conduit fill guidelines are right on.